L EARTH SCULPTURE OR THE ORIGIN OF LAND-FORMS BY JAMES GEIKIE, LL.D., D.C.L., F.R.S., ETC. U* MURCHISON PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF EDINBURGH ; FORMERLY OF H.M. GEOLOGICAL SURVEY AUTHOR OF "THE GREAT ICE AGE," "PREHISTORIC EUROPE," ETC. ILLUSTRATED wr THE UNIVERSITY, OF 122 NEW YORK G. P. PUTNAM'S SON.S LONDON JOHN MURRAY IQ08 COPYRIGHT 1898 BV G. P. PUTNAM'S SONS Ube Itntcherbocher fnccee, ew PREFACE A LTHOUGH much has been written, especially ** of late years, on the origin of surface-features, yet there is no English work to which readers not skilled in geology can turn for some general account of the whole subject. It is true that all geological text-books, and many manuals of geography, devote some space to its discussion, while not a few excellent treatises deal at large with one or more of its sub- divisions. Geological literature is also by no means poor in admirable popular monographs descriptive of the geology and geography of particular regions, in which the origin of their surface-features is more or less fully explained. But for those who may be de- sirous of acquiring some broad knowledge of the results arrived at by geologists as to the development of land-forms generally, no introductory treatise is available. Possibly, therefore, the present attempt to supply a deficiency may not be wholly unaccept- able. In a work addressed more particularly to non-spec- ialists, technical terminology should be employed as sparingly as possible, and I have consequently made scant use of those neologisms in which, unfortunately, 18975 iv PREFACE the recent literature of the subject too much abounds. Technical words and expressions cannot, however, be entirely dispensed with, but those which my readers will encounter have, as a rule, been long current, and few are likely to be unfamiliar. The materials used in the preparation of this book are for the most part from the common stock of geo- logical knowledge, and it has not been thought neces- sary, therefore, to burden the pages with references. Those who would pursue the subject further must consult the larger text-books of geology in English, French, and German, which usually indicate the more notable sources of information. The following works will also be found very helpful as guides and instruct- ors : Sir A. C. Ramsay's Physical Geology and Geography of Great Britain. Prof. A. H. Green's Physical Geology (chap. xiii.). Sir A. Geikie's Scenery and Geology of Scotland. Prof. E. Hull's Physical Geology and Geography of Ireland. Sir J. Lubbock's Scenery of Switzerland and the Causes to which it is Due. Dr. E. Fraas's Scenerie der Alpen. Major J. W. Powell's Canyons of the Colorado. MM. De la Noe and Emm. de Margerie, Les Formes du Terrain an admirable and well illustrated work, descriptive of the geological origin of land- forms. Prof. A. Penck's Morphologic der Erdoberflache a PREFACE v masterly review and classification of the surface-feat- ures of the earth, with a full discussion of their origin. This treatise is particularly rich in references to the literature ; the whole history of geological opinion on the subject of which it treats may therefore be gath- ered from its pages. Prof. A. de Lapparent's Lefons de Geographic Phys- ique a most instructive and comprehensive outline of geo-morphology. The second half of the work deals more particularly with geographical evolution, the special treatment of which does not come within the limits of my essay. This interesting subject has of late years been studied with great assiduity, especially by Prof. W. M. Davis and others in North America. The maps and sections, and the monographs, me- moirs, and reports of our own and other national geological surveys are storehouses of information and instruction in physiographical geology. Some of these works that deal more especially with denuda- tion and the relation of surface-features to geological structure have indeed become classical. Amongst these are Ramsay's notable paper, " On the Denuda- tion of South Wales and the Adjacent Counties of England " (Memoirs Geological Stirvey of England, vol. i., 1846) ; Heim's Mechanismus der Gebirgsbil- dung, etc. (which, although an independent work, was yet commenced under the auspices of the Swiss Geo- logical Commission) ; Dutton's "Tertiary History of the Grand Canon District " (Monograph II. of U. S. Geological Survey). vi PREFACE For the use of several illustrations (Figs. 8, 25, 26, 75, 78) from Major Powell's Canyons of the Colorado, I am indebted to his publishers, Messrs. Flood & Vincent. I am under similar obligations to the Coun- cil of the Geological Society for a section (Fig. 34) borrowed from my brother's paper on the North-west Highlands ; to Mr. Stanford for reproductions of il- lustrations (Figs. 77, 83, 87, 88) from my Outlines of Geology ; to Herr Tempsky, Vienna, for Figs. 41, 45, 56, from KirchhofFs Lander kunde des Erdteils Europa ; and to my friend, Mr. W. E. Carnegie Dickson, for the photographs reproduced on Plates I. and II. EDINBURGH, July i, 1898. CONTENTS CHAPTER I PAC INTRODUCTORY i Early views as to origin of Surf ace -features Rocks and Rock- structures Architecture of the Earth's Crust General evidence of Rock-removal. CHAPTER II AGENTS OF DENUDATION 18 Chemical composition of Rocks Epigene Agents Insolation and Deflation Chemical and mechanical action of Rain Action of Frost ; of Plants and Animals ; of underground Water ; of Brooks and Rivers Rate of Denudation Denudation and Sedimentation go hand in hand. CHAPTER III LAND-FORMS IN REGIONS OF HORIZONTAL STRATA 44 Various factors determining Earth Sculpture Influence of Geo- logical Structure and the Character of Rocks in determining the Con- figuration assumed by Horizontal Strata Plains and Plateaux of Accumulation. CHAPTER IV LAND-FORMS IN REGIONS OF GENTLY INCLINED STRATA 73 Escarpments and Dip-slopes Dip-valleys and Strike-valleys Forms assumed by a Plateau of Erosion Various directions of Es- carpments Synclinal Hills and Anticlinal Hollows Anticlinal Hills. vii viii CONTENTS CHAPTER V PACK LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DIS- TURBED STRATA . 92 Typical Rock-structures in Regions of Mountain-uplift General Structure of Mountains of Upheaval Primeval Coincidence of Un- derground Structure and External Configuration Relatively weak and strong Structures Stages in the Erosion of a Mountain-chain Forms assumed under Denudation Ultimate face of Mountain-chains. CHAPTER VI LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DIS- TURBED STRATA (continued") . . . . . .128 Structure and Configuration of Plateaux of Erosion Forms as- sumed under Denudation Mountains of Circumdenudation His- tory of certain Plateaux of Erosion Southern Uplands and Northern Highlands of Scotland Stages in Erosion of Table-lands. CHAPTER VII LAND-FORMS IN REGIONS AFFECTED BY NORMAL FAULTS OR VERTICAL DISPLACEMENTS 150 Normal Faults, general features of Their connection with Folds Their origin How they affect the Surface Faults of the Colorado region, and of the Great Basin Depression of the Dead Sea and the Jordan Lake Depressions of East Africa Faults of British Coal- fields Bounding faults of Scottish Highlands and Lowlands Fault- bounded Mountains General conclusions. CHAPTER VIII LAND-FORMS DUE DIRECTLY OR INDIRECTLY TO IGNEOUS ACTION .......... 173 Plutonic and Volcanic Rocks Deformation of Surface caused by Intrusions Laccoliths of Henry Mountains Volcanoes, Structure and Form of Mud-cones Geysers Fissure-eruptions Volcanic Plateaux Denudation of Volcanoes, etc., and resulting features. CHAPTER IX INFLUENCE OF ROCK CHARACTER IN THE DETERMINA- TION OF LAND-FORMS ....... 195 Joints in Rocks and the part they play in determining Surface- features Texture and Mineralogical composition of Rocks in rela- tion to Weathering Forms assumed by various Rocks. CONTENTS ix CHAPTER X PAGE LAND-FORMS MODIFIED BY GLACIAL ACTION . . . 212 Geological action of existing Glaciers Evidence of Erosion Origin of the Ground-moraine : its independence of Surface-moraines Infraglacial smoothing and polishing, crushing, shattering and plucking Geological action of Prehistoric Glaciers General evi- dence supplied by Ancient Glaciers of the Alps. CHAPTER XI LAND-FORMS MODIFIED BY GLACIAL ACTION (continued) . 232 Former Glacial conditions of Northern Europe Extent of the old Inland Ice Glacial character of Boulder-clay Central Region of Glacial Erosion and Peripheral Areapf Glacial Accumulation Fluvio- glacial deposits Loess, origin of its materials Glaciation of North America Modifications of Surface produced by Glacial Action. CHAPTER XII LAND-FORMS MODIFIED BY ^OLIAN ACTION . . . 250 Insolation and Deflation in the Sahara Forms assumed by Gran- itoid Rocks and Horizontal and Inclined Strata Reduction of Land- surface to a Plain Formation of Basins Dunes of the Desert Sand-hills of other regions Transport and Accumulation of Dust Loess, a dust deposit Lakes and Marshes of the Steppes. CHAPTER XIII LAND-FORMS MODIFIED BY THE ACTION OF UNDER- GROUND WATER . 266 Dissolution of Rocks Underground Water-action in Calcareous lands Karst-regions of Carinthia and Illyria Effects of Superficial and Subterranean Erosion Temporary Lakes Caves in Limestone Caves in and underneath Lava " Crystal Cellars " Rock-shelters Sea-caves. CHAPTER XIV BASINS 278 Basins due to Crustal Deformation Crater-lakes Dissolution Basins Lakes formed by Rivers ^olian Basins Drainage dis- turbed by Landslips Glacial Basins of various kinds ; as in Corries, Mountain-valleys, Lowlands, and Plateaux Ice-barrier Basins- Submarine Basins of Glacial Origin. CONTENTS CHAPTER XV PAGE COAST-LINES .315 Form and general trend of Coast-lines Smooth or Regular Coasts Influence of Geological Structure on various forms assumed by Cliffs Cliffs, cut in Bedded and in Amorphous Rocks Sea-caves Flat Coast-lines and Coastal Plains Indented or Irregular Coasts General trends of Coast-lines determined by form of Land-surface Subordinate Influence of Marine Erosion. CHAPTER XVI CLASSIFICATION OF LAND-FORMS 335 Plains of Accumulation and of Erosion Plateaux of Accumulation and Erosion Hills and Mountains : Original or Tectonic, and Sub- sequent or Relict Mountains Valleys : Original or Tectonic, and Subsequent or Erosion Valleys Basins Coast-lines. CHAPTER XVII CONCLUSION '.-' * 364 The study of the Structure and Formation of Surface-features prac- tically involves that of the Evolution of the Land. APPENDIX . . . . . . . . . 373 GLOSSARY "* . . . 375 INDEX . 387 LIST OF ILLUSTRATIONS FIGURE PAGE 1. Section of Horizontal Strata . . . . . ... . 7 2. Section across an Anticline ......... 9 3. Section across Normal Anticlines and Synclines 10 4. Section across Anticlines and Synclines with Inclined Axes . . 10 5. Section across Faulted or Dislocated Strata . . . . . n 6. Section across Unconformable Strata . . . . . .41 7. Section across a series of Alluvial Terraces . . . . .51 8. Section and Bird's-eye View of Colorado Plateau (Powell) . . 54 9. Diagrammatic Section across Colorado Plateau . . . .58 10. Diagrammatic Section showing Stages of Erosion by a River cutting through Horizontal Strata (after Captain Dutton) .... 62 11. Section across Suderoe (Far6e Islands) on a true scale ... 69 12. Map of an Island composed of Dome-shaped Strata .... 74 13. Section through the Island shown in Fig. 12 . . . -74 14. Section of River-valley ......... 75 15. Enlarged section of a portion of the Island shown in Fig. 12 . . 77 16. Diagram Map of Plateau of Erosion 78 17. Section across reduced Plateau of Erosion ..... 79 18. Longitudinal Section of River Course . . ... .80 19. Section of Escarpments and Outliers 84 20. Section across the Wealdean Area (Ramsay) 84 21. Section across Permian Volcanic Basin, Ayrshire .... 86 22. Synclinal Hills and Anticlinal Valleys 87 23. Escarpment Hills and Synclinal Hill 88 24. Section across West Lomond Hill and the Ochils .... 88 25. Synclinal Valley, West of Green River (Powell) .... 89 26. Anticlinal Ridge, Green River Plains (Powell) ..... 90 27. Isoclinal Folds .......... 93 28. Isoclinal Folds .......... 94 29. Isoclinal Folds 94 30. Overfold passing into Reversed Fault, or Overthrust ... 95 xi xii LIST OF ILLUSTRATIONS FIGURE PAGE 31. Reversed Fault ......... 95 32. Single Thrust-plane ......... 95 33. Section across Coal-basin of Mons (M. Bertrand) . . . .96 34. Section from Quinaig to Head of Glenbeg (Geol. Survey) . . 97 35. Synclinal Double-fold ......... 97 36. Anticlinal Double-fold 98 37. Diagram of Mountain Flexures 99 38. Diagram of Anticlinal Mountains ....... 105 39. Synclinal Valley shifting toward Anticlinal Axis .... 106 40. Section across the Swiss Alps (A. Heim) .no 41. Summit of Santis, East Side (A. Heim) . . . . . . ill 42. Section across the Schortenkopf, Bavarian Alps (E. Fraas) . in 43. Section across the Kaisergebirge, Eastern Alps (E. Fraas) . .112 44. Section across the Val d'Uina (Gumbel) . . . . . .112 45. Sichelkamm of Wallenstadt (Heim) ...... 112 46. Section across the Northern Limestone Alps (E. Fraas) . . . 113 47. Section across the Diablerets (Rene vier) . . . . . .113 48. Section across Dent de Morcles (Renevier) . . . . .114 49. Inversion and Overthrust in the Mountains South of the Lake of Wallenstadt (E. Fraas, after A. Heim) . . . ..114 50. Symmetrical Flexures of the Jura Mountains . . . . .115 51. Section across Western part of the Jura Mountains (P. Choffat) . 116 52. Section across part of the Sandstone-zone of the Middle Carpathians (Vacek) 116 53. Section across part of the Middle Carpathians (Vacek) . . .117 54. Section across the Appalachian Ridges of Pennsylvania (H. D. Rogers) . . . . . 118 55. Unsymmetrical Folds, giving rise to Escarpments and Ridges . .120 56. Structure of the Ardennes (after Cornet and Briart) .... 126 57. Diagrammatic Section across a Plateau of Erosion . . . .129 58. Section across portion of Southern Uplands, showing Old Red Sand- stone resting upon Plain of Erosion . . . . . .136 59. Section from Glen Lyon to Carn Chois (Geol. Survey) . . . 146 60. Section of Normal Fault . . . . , . . . .153 61. Normal Fault, with High Ground on Downthrow Side . . . 155 62. Normal Fault, with High Ground on Upcast Side .... 156 63. Faults in Queantoweep Valley, Grand Canon District (Dutton) . 158 64. Ranges of the Great Basin (Hinman, after Gilbert : length of section, 120 miles) 159 65. Section from the Mediterranean across the Mountains of Palestine to the Mountains of Moab (after M. Blanckenhorn) .... 161 66. Section across the Vosges and the Black Forest (after Penck) . . 164 LIST OF ILLUSTRATIONS xiii FIGURE PAGE 67. Section of Coal-measures near Cambusnethan, Lanarkshire, on a true scale . . . . . ...'.'. . . . 166 68. Section on a true scale across " Tynedale Fault," Newcastle Coal-field 168 69. Section across Great Fault bounding the Highlands near Birnam, Perthshire 169 70. Section across Great Fault bounding the Southern Uplands . . 170 71. Diagram Section across Horstgebirge . . . . . .170 72. Mountain of Granite . . . . . . . . . . 175 73. Plain of Granite overlooked by Mountains of Schists, etc. . . 176 74. Diagrammatic Section of a Laccolith showing Dome-shaped Eleva- tion of Surface above the Intrusive Rock (after G. K. Gilbert) . 177 75. View of Necks Cores of old Volcanoes (Powell) . . f : ~ . 188 76. Section of Highly Denuded Volcano, Minto Hill, Roxburgshire . 189 77. Diagrammatic Section across the Valley of the Tay, near Dundee . 190 78. View of Mesa Verde and the Sierra el Late, Colorado (Hayden's Re- port for 1875) . . . . . . . . . . 203 79. Wind Erosion : Table-Mountains, etc., of the Sahara (Mission de Chadames) . . . . . . . . . . . 254 80. Wind Erosion : Harder Beds amongst inclined Cretaceous Strata, Libyan Desert (J. Walther) 254 81. Wind Erosion : Stages in the Erosion and Reduction of a Table- mountain (J. Walther) ........ 255 82. Manganese Concretions weathered out of Sandstone, Arabah Mount- ains, Sinai Peninsula (J. Walther) 256 83. Formation of Sand-dunes ........ 259 84. Advance of Sand-dunes 259 85. Longitudinal Sections of Lake-basins on a true scale . . . 293 86. Sea-cliff cut in Horizontal Strata 319 87. Sea-cliff cut in Strata dipping Inland 320 88. Sea-cliff cut in Strata dipping Seaward ...... 320 89. Sea-cliff cut in Beds dipping Seaward 323 FULL-PAGE PLATES Plate I. Joints in Granite, Glen Eunach, Cairngorm (from a photograph by W. E. Carnegie Dickson) to face 200 Plate II. Weathering of Joints in Granite, Cairngorm Mountains (from a photograph by W. E. Carnegie Dickson) . . . to face 202 EARTH SCULPTURE CHAPTER I INTRODUCTORY EARLY VIEWS AS TO ORIGIN OF SURFACE-FEATURES ROCKS AND ROCK-STRUCTURES ARCHITECTURE OF THE EARTH'S CRUST GENERAL EVIDENCE OF ROCK-REMOVAL. WHEN geologists began to inquire into the origin of surface-features, they were at first led to believe that the more striking and prominent of these had come into existence under the operation of forces which had long ago ceased to affect the earth's crust to any marked extent. It is not hard to understand how this conception arose. The earlier observers could not fail to be impressed by the evidence of former crustal disturbances which almost everywhere stared them in the face. Here they saw mountains built up of strangely fractured, contorted, and jum- bled rock-masses ; there, again, they encountered the relics of vast volcanic eruptions in regions now practi- cally free from earth-throes of any kind. In one place ancient land-surfaces were seen intercalated at inter- 2 EARTH SCULPTURE vals among great successions of marine strata ; in other places, limestones, evidently of oceanic origin, were found entering into the framework of lofty mountains far removed from any sea. It was these and similar striking contrasts between the present and the past which doubtless induced the belief that the earth's crust, after having passed through many revolutions more or less catastrophic in character, had at last become approximately stable the occasional earthquakes and volcanic disturbances of recent times being looked upon as only the final manifestations of those forces which in earlier ages had been mainly instrumental in producing the varied configuration of the land. Mountains and valleys belonged to earth's Sturm und Drang period. That wild time had passed away, and now old age, with its lethargy and repose, had supervened. The tumultuous accumula- tions of stony clay, blocks and boulders, gravel and sand that overspread extensive areas in temperate latitudes were believed to be the relics of the last great catastrophe which had affected the earth's sur- face. Some notable disturbance of the crust, it was thought, had caused the waters of northern seas to rush in devastating waves across the land. When these diluvial waters finally retired, then the modern era began an era characterised by the more equable operation of nature's forces. But with increased knowledge these views gradu- ally became modified. Eventually, it was recognised that no hard-and-fast line separates past and present. INTRODUCTORY 3 The belief in world-wide, or nearly world-wide, cata- strophes disappeared. Geologists came to see that the fashioning of the earth's surface had been going on for a long time, and is still in progress. The law of evolution, they have found, holds true for the crust of the globe just as it does for the myriad tribes of plants and animals that clothe and people it. It is no longer doubted that the existing configuration of the land has resulted from the action of forces that are still in operation, and by observation and reasoning the history of the various phases in the evolution of surface-features can be unfolded. No doubt the evidence is sometimes hard to read in all its details, but its general bearing can be readily apprehended. The salient facts, the principal data, are conspicuous enough, and the mode of their interpretation is in a manner self-evident. In setting out upon our present inquiry, however, it is obvious that we ought, in the first place, to know something about rocks and the mode of their arrange- ment. We must make some acquaintance with the composition and the structure or architecture of the earth's crust before we can form any reasonable con- clusion as to the origin of its surface-features. Now, so far as that crust is accessible to observation, it is found to be built up of two kinds of rock, one set be- ing of igneous origin, while the other appears to con- sist mainly of the products of water action. These last are typically represented by such rocks as con- glomerate, sandstone, and shale, which are only more 4 EARTH SCULPTURE or less ancient sediments, formed and accumulated in the same way as the gravel, sand, and mud of existing rivers, lakes, and seas. Another common rock of aqueous origin is limestone, of which there are count- less varieties some formed in lakes, like the shell- marls of our own day ; others representing the calcareous ooze arid coral-reefs of ancient seas ; while yet others are obviously chemical precipitates from water surcharged with carbonate of lime. Now and again, also, we meet with rocks of terrestrial origin, such, for example, as many beds and seams of peat, lignite, and coal, which are simply the vegetable debris of old land-surfaces. To these land-formed beds we may add certain sandstones of wind-blown origin indurated sand-dunes, in short. The igneous rocks consist partly of lavas and frag- mental materials which have been ejected at the sur- face, as in modern volcanoes, and partly of formerly molten masses which have cooled and consolidated below ground. The former, therefore, are spoken of as volcanic, the latter as plutonic or hypogene rocks. As it is useful to have some general name for the rocks which owe their origin to the action of epigene agents (z. e., the atmosphere, terrestrial water, ice, the sea, and life), we may term these derivative, since they have been built up chiefly out of the relics of pre-ex- isting rocks and the cUbrisvi plants and animals. By- and-by we shall learn that igneous and derivative rocks have in certain regions been subjected to many remarkable changes, and are in consequence so IN TROD UCTOR Y 5 altered that it is often hard to detect their original character. These altered masses form what are called the metamorphic rocks. They are typically repre- sented by such rocks as gneiss, mica-schist, clay-slate, etc. The derivative rocks, with which in many regions igneous rocks are associated, occupy by far the larger portion of the land-surface, entering abundantly into the composition of low grounds and mountains alike. Most of these derivatives are sedimentary accumula- tions, and very many are charged with the remains of animals and plants. By noting the order in which such stratified deposits occur, and by comparing and correlating their fossils, geologists have been able to group them into a series of successive systems, the oldest being that which occurs at the bottom of the series. 1 The united thickness of the several systems probably exceeds twenty miles, but it must not be supposed that all these occur together in any one region. Many broad acres of the earth's surface are occupied by the rocks belonging to one system only. In other countries two or more systems may be present. Again, each individual system is of very variable thickness swelling out here, thinning off there : in some lands being represented by strata many thou- sands of feet in thickness, in others dwindling down to a few yards. In short, we may picture to ourselves each system as consisting of a series of larger and smaller lenticular sheets, irregularly distributed over 1 See Appendix for Table of Geological Systems. 6 EARTH SCULPTURE the earth's surface. The various systems thus fre- quently overlap, the younger stealing over the surface of the older so as often to bury these out of sight. The metamorphic rocks do not appear at the sur- face over such extensive areas as those just referred to. Nevertheless, they are widely distributed, and now and again overspread continuously vast regions. The enormous tract that extends from the Great Lakes of North America to the shores of the Arctic Ocean is almost entirely occupied by them. Another im- mense area of crystalline schistose rocks is met with in Brazil. The Highlands of Scotland, the Scandi- navian Peninsula, and .North Finland are in like man- ner largely composed of them, and the same is the case with many parts of Africa, Asia, and Australia. It is further noteworthy that similar rocks form the backbones of most of the great mountain chains of the globe. As already indicated, metamorphic rocks are of various origin, some of them being primarily of igneous and others of aqueous formation. Those which form the nuclei of the youngest mountain chains are sometimes of relatively recent age, while those occupying such broad tracts as Brazil, the Canadian uplands, etc., are of vast antiquity. Crys- talline schistose rocks, with associated granites and other igneous rocks, seem everywhere to underlie the sedimentary fossiliferous formations. Very often the latter are separated by a broadly marked line of demarcation from the schists, granites, etc., upon which they repose. In other cases the sedimentary rocks IN TROD UCTOR Y 7 become gradually altered as they are traced down- wards, until eventually they themselves assume the aspect of crystalline schists, penetrated here and there by granitoid igneous rocks. The origin of those ancient crystalline schists has been much discussed, but does not concern us here. Some geologists have maintained that the rocks in question represent the original cooled crust of the globe, while the majority consider them to be all metamorphic. It is enough for our present pur- pose to know that a pavement of such rocks appears everywhere to underlie the sedimentary fossiliferous formations. FIG. i. SECTION OF HORIZONTAL STRATA. The upper continuous line, A -J9, = surface of ground ; the lower continuous line, C-D, = sea- level; /, limestone,; JE, sandstones and shales. The great bulk of the derivative rocks being of sedimentary origin, it is obvious that they must have been at the time of their formation spread out in ap- proximately horizontal layers upon the beds of ancient lakes and seas. This we are justified in believing by what we know of the accumulation of similar sediments in our own day. The wide flats of our river- valleys, the broad plains that occupy the sites of silted- up lakes, the extensive deltas of such rivers as the Nile, the Po, the Amazon, the Mississippi, the narrow 8 EARTH SCULPTURE or wide belts of low-lying land which within a recent period have been gained from the sea, are all made up of various kinds of sediment arranged in gently in- clined or approximately horizontal layers. Now, over considerable areas of the earth's surface the derivative rocks show the same horizontal arrangement, a struct- ure which is obviously original. And this is frequently the case with younger and older sedimentary strata alike. Here, for example (Fig. i), is a section across a country, the superficial rock-masses of which are horizontally arranged. The upper line of the section (A-B) represents, of course, the surface of the ground, while the lower we shall take to be the level of the sea. The section thus shows the geological structure or arrangement of the rocks from the surface down to the level of the sea. The strata represented consist of a great series of sandstones and shales with one prominent bed of limestone (/) at the top. In this case we cannot doubt that the horizontal bedding is original that the strata were accumulated one above the other in the same order as we see them. Although such horizontal arrangements are of com- mon enough occurrence, and now and again charac- terise the sedimentary systems over wide areas, yet, as a general rule, strata tend to be inclined. In many regions the inclination, or dip, as it is termed, is some- times very high not seldom indeed the beds are seen standing on end, like rows of books in a library. This last appearance of extreme disturbance is not confined INTRODUCTORY 9 to the strata of any system ; nevertheless, it is more characteristic of the older than the younger systems. In the sequel we shall have to study these and other rock-structures more particularly, but for the present we need not do more than make some general acquaintance with them. A very common arrangement is shown in the next diagram (Fig. 2). Here the strata are arranged in the form of a truncated arch, or anticline. At X the A.-"' C V FIG. 2. SECTION ACROSS AN ANTICLINE. The upper continuous line, A-B, = surface of ground ; the lower continuous line, C-D, = sea- level; X-Y, = vertical axis. beds are approximately horizontal, but from this point they dip on the right towards B, and on the left in the direction of A. Note further that the angle of inclination is the same on each side of the anticline ; in other words, the anticlinal axis (X-Y) is vertical. From A to B the distance we shall suppose is six miles. The succeeding section (Fig. 3) we shall take to be of equal length. Here we have a succession of anticlines, or saddle-backs, separated one from another by troughs, or sync 'lines, as they are termed. In other 10 EARTH SCULPTURE words, the strata are undulating. From these sections we learn that folds or undulations vary considerably in width. -In the region represented by Fig. 2 we have an area six miles in breadth, consisting of a thick series of strata disposed in the form of one sin- gle arch or anticline ; while in Fig. 3, representing * * * L D FIG. 3. SECTION ACROSS SYMMETRICAL ANTICLINES AND SYNCLINES. Upper continuous line, A-B, = surface of ground ; lower continuous line, C-A = sea-level ; a a, anticlines ; j j, synclines ; a x, s x, axes of folds. an equal area, the strata are folded into a series of several anticlines and synclines. In both regions the anticlines are symmetrical ; that is to say, their axes (a x, s x) are vertical. But folds or undulations may follow each other much more rapidly than is shown in the preceding section. In countries built up of steeply inclined FIG. 4. SECTION ACROSS UNSYMMETRICAL ANTICLINES AND SYNCLINES. Upper continuous line, A-B^ = surface of ground ; lower continuous line, C-Z>, = sea-level; a .r, s x, axes of folds. rocks, the undulations of the strata are more abrupt, and the axes of the folds are frequently inclined. In IN TROD UCTOR V ii Fig. 4, for example, most of the anticlines and syn- clines lean over to one side, and this to such a de- gree, that here and there upper beds are doubled un- der older beds of the same series of strata ; in other words, the order of succession appears to be inverted. From the fact that strata are generally inclined from the horizontal, and frequently curved and folded, it is obvious that they have been subjected to the action of some great disturbing force, for folding and FIG. 5. SECTION ACROSS FAULTY OR DISLOCATED STRATA. /, normal fault, inclined in the direction of downthrow. contortion may affect masses of strata many thousands of feet in thickness. Another evident mark of dis- turbance is furnished by the presence of dislocations, or faults, as they are technically termed, along the line of which the rocks have been shifted for, it may be, hundreds and sometimes even for thousands of feet. One of the simplest kind of faults is shown in 12 EARTH SCULPTURE the preceding illustration (Fig. 5). Here, as in preceding figures, the upper line (A-H) represents the surface of the ground. At f the strata are tra- versed by a fault, which has caused a vertical displace- ment of the beds to the extent of, say, 500 feet, for it is obvious that the coal and fireclay (8, 9), and the strata amongst which they lie on the left-hand side, were formerly continuous with the corresponding beds on the other side of the fault. From the facts now briefly set forth we may draw certain conclusions. In the first place, the extensive geographical range of the derivative rocks, most of which are of marine origin, must convince us that the greater portion of our continental areas has been un- der water. It is not to be understood, however, that all the land-surfaces occupied by sedimentary strata have been submerged at one and the same time. On the contrary, the several geological systems have been accumulated at widely different periods. This is a point, however, to which we shall return : for the present, we need only keep in view the prominent fact that the existing land-surfaces of the globe are composed most frequently of marine strata. There are apparently only two ways in which this phenom- enon can be accounted for, and these explanations come to much the same thing. Either the general level of the ocean has fallen, or wide areas of the sea- floor have been pushed up from below and converted into dry land. Both changes appear to have taken place. The bed of the sea has sunk from time to INTRODUCTORY 13 time to greater and greater depths, and has thus tended to draw the water away from the surface of what are now continental areas. But if the earth's crust under the ocean has subsided, it has also been elevated within what are now dry lands again and again. The folds and corrugations of the strata, and the numerous dislocations by which rocks of all kinds are traversed, clearly demonstrate that movements of the solid crust have taken place. Such crustal dis- turbances are probably in chief measure due to the fact that the earth is a cooling body. As the solid crust sinks down upon the cooling and contracting nu- cleus, it must occupy less superficial space. Hence its rocky framework becomes subjected to enormous tan- gential squeezing and compression to which it yields by bending and folding, by fracture and displacement. Obviously, then, the mysterious subterranean forces must have played an important part in the formation of earth-features. Disturbed rocks are of more fre- quent occurrence than strata which have retained their original horizontality. It is no wonder, there- fore, that for a long time the general configuration of the land was believed to have been impressed upon it by plutonic agency. Indeed, in the case of certain mountain chains, we cannot fail to see that the larger features of such regions often correspond to a con- siderable extent with the main flexures and displace- ments of the underlying rocks. In many elevated tracts, however, composed of highly disturbed and contorted strata, no such coincidence of surface-feat- 14 EARTH SCULPTURE ure and underground structure can be traced. The mountain ridges do not correspond to great swellings of the crust ; the valleys neither lie in trough-shaped strata, nor do they coincide with gaping fractures. Again, many considerable mountains are built up of rocks which are not convoluted at all, but arranged in horizontal beds. More than this, many plateaux and even lowlands are composed of as highly flexed and con- torted strata as are to be met with in any mountainous country. Evidently, therefore, crustal movement is not the only factor in the production of surface-features. The sections already given will serve to illustrate the general fact that underground structure and su- perficial configuration do not necessarily correspond. Thus in Fig. i we have a series of pyramidal mount- ains developed in horizontal strata. The slope of the surface, therefore, frequently bears no relation to the "lie" of the beds below. This is further illus- trated in the succeeding figures, where we find de- pressions at the surface, while the rocks immediately underneath show an anticlinal arrangement ; and, con- versely, where the strata are trough-shaped the sur- face-feature is not a depression but an elevation. In the case of the horizontal strata shown in Fig. i we have no difficulty in perceiving that the present surface is not that of original deposition. It is impos- sible that sedimentary deposits could have been piled up in the shape of great pyramids : obviously the beds were formerly continuous, as shown by the dotted lines. Clearly some " monstrous cantles " have been cut out IN TROD UCTOR Y 15 and removed. And the same is necessarily true of the folded strata. In each case (Figs. 2, 3, 4) masses of strata have disappeared ; the tops or backs of the anticlinal arches have been more or less deeply incised, and the material carried away. In subsequent pages it will be shown that the thickness of rocks thus re- moved can be proved to amount in many cases to thousands of feet. Not less striking is the evidence of rock-removal furnished by the phenomena of faults. At the sur- face there may be no inequality of level corresponding to that seen below (see Fig. 5). Obviously, how- ever, a considerable thickness of rock has vanished. Were the missing continuations of the strata to be replaced upon the high side of the fault, they would occupy the space contained within the dotted lines above the present surface AB. Such dislocations often interrupt the continuity of the strata in our coal- fields. In such regions we may traverse level or gently undulating tracts, and be quite unconscious of the fact that geologically we have several times leaped up or jumped down hundreds of feet in a single step. Nay, some rivers flow across dislocations by which the strata have been shifted up or down for thousands of yards, and in some places we may sit upon rocks which are geologically more than a thousand fathoms below or above those on which we rest our feet. Faults, then, afford clear evidence of the wholesale removal of rocks from the surface of the land. Such proofs of rock-removal can be appreciated by 16 EARTH SCULPTURE anyone, and will come frequently before us in the discussion that follows. There is another kind of evidence, however, leading to the same general con- clusion, which may be briefly touched upon at this stage of our inquiry. In this and other countries there are enormous masses of rock, often widely ex- tended, which have cooled and consolidated from a state of igneous fusion. Some of these, it is well known, have flowed out as lavas at the surface, while others were never so erupted, but have solidified at greater or less depths below ground. Among the latter is granite, a rock believed to be of deep-seated origin. Its plutonic character is evinced not less by its composition and structure than by its relation to the rock-masses that surround it. Every mass of granite, then, has cooled and consolidated, probably very slowly, and certainly at a less or greater depth in the earth's crust. When this rock is met with over a wide area at the actual surface, therefore, forming, it may be, great mountains or rolling and broken low- lands, we know that in such regions thick masses of formerly overlying rocks have been removed. The granite appears at the surface simply because the covering of rocks underneath which it cooled and solidified has been subsequently carried away. The occurrence at the surface of crystalline schists and other metamorphic rocks has a similar significance. Although the processes by which rocks become so highly altered are still more or less obscure, yet there can be no doubt that the metamorphism had taken INTRODUCTORY 17 place when the rocks affected were more or less deeply buried in the crust. While we may safely infer, from the general phe- nomena of geological structure, that earth-movements have shared in the production of surface-features, we must be convinced, at the same time, that some other factor has aided in the work of shaping out our lands. Earth-movements quite account for the folding and fracturing of strata, for the uplifting of great mount- ain masses, but they cannot have caused the general loss which these masses have sustained. We may conceive it possible that subterranean action may now and again have resulted in wide-spread shattering of rocks at the surface, but such action could not have caused the broken material to disappear. Further, when we bear in mind that the thickness of rock removed from the surface of the land is sometimes to be measured by many thousands of feet, or even yards, we see at once that subterranean action cannot have been directly implicated in the spoliation of the land. How, then, have anticlines been truncated ? What power has removed the strata from the high side of a fault ? What, in a word, has produced that trunca- tion and discontinuity of beds which is so common a feature of derivative rocks all the world over ? And how shall we account for the presence at the sur- face of deep-seated plutonic rocks and metamorphic masses ? When we have satisfactorily answered such questions we shall have solved the problem of the origin of surface-features. CHAPTER II AGENTS OF DENUDATION CHEMICAL COMPOSITION OF ROCKS EPICENE AGENTS INSOLA- TION AND DEFLATION CHEMICAL AND MECHANICAL ACTION OF RAIN ACTION OF FROST ; OF PLANTS AND ANIMALS ; OF UNDERGROUND WATER ; OF BROOKS AND RIVERS RATE OF DENUDATION DENUDATION AND SEDIMENTATION GO HAND IN HAND. T^HE present, geologists tell us, contains the key to * the past. If we wish to find out how rocks have been removed, and what has since become of them, we must observe what is taking place under the influence of existing agents of change. How, then, are rocks being affected at present ? We do not pro- ceed far in our investigation before we discover that they are everywhere becoming disintegrated. In one place they are breaking up into angular fragments ; in another, crumbling down into grit, sand, or clay. Brooks and rivers and the waves upon our coasts are constantly undermining them ; everywhere, in short, rocks are being assaulted and reduced. But in order to bring this fact more forcibly before the reader, it will be well to sketch, as briefly as may be, the general character of the warfare which is being waged against 18 AGENTS OF DENUDATION 19 rocks over all the land-surface, and to note the various results that flow from this incessant energy of the epigene or superficial agents of change. As these agents are often associated in their work, it is sometimes hard, or even impossible, to say which has played the most effective part in the demolition of rocks. Nevertheless, it will conduce to clearness if we endeavour to consider the operation of each by itself, so far, at least, as that is possible. Before doing so, however, we must glance for a moment at the general characters of rocks. We have already taken note of the fact that rocks are of various origin igneous, derivative, and metamorphic. It is now necessary to consider their composition and structure, for, according as these differ, rocks are variously affected by epigene agents, some yielding rapidly, others being more resistant. We need not go into detail. Their composition and structure may be de- scribed in the most general terms. For our purpose it will suffice to group them roughly under these four heads : Felspathic, Argillaceous, Silicious, and Cal- careous rocks. This is very far from being an exhaustive classification, but under these groups may be included all the rocks that enter most largely into the formation of the earth's crust. i. Felspathic Rocks. These rocks contain as their dominant constituent the mineral, or, rather, the family of minerals, known under the name of felspar. The group includes nearly all the igneous and most of the metamorphic rocks. The derivative rocks that 20 EARTH SCULPTURE come under the same head are of relatively small importance. The minerals entering most abundantly into the composition of the felspathic rocks are the felspars (aluminous silicates of potash, soda, and lime), various ferro - magnesian silicates, such as mica, py- roxene, hornblende, and olivine (aluminous silicates of magnesia, lime, iron-oxides, etc.), and quartz (silica, silicic acid). The crystalline igneous rocks occur either in more or less regular beds (lavas), interstrati- fied with derivative rocks, or they penetrate these in the form of irregular veins, dykes, sheets, or large amorphous masses. The lava-form rocks are often associated with beds of volcanic debris (tuff, etc.). Some igneous rocks are smoothly compact in texture, such as obsidian and pitchstone, which are simply varieties of volcanic glass ; others, such as basalt, consist-partly of glass and partly of crystalline in- gredients, and vary in texture from compact to coarse- grained ; yet others are built up wholly of crystalline substances, and may be fine-grained or very coarsely granular, as granite. The crystalline schists are equally variable as regards texture. They differ, however, from the igneous rocks in structure. While the latter are confusedly crystalline, the schists show a kind of streaky structure or pseudo-lamination, their constituent minerals being arranged in rudely alternate lenticular layers. Igneous rocks and schists are traversed by cracks and fissures which usually ramify irregularly in all directions. In many bedded igneous rocks (lavas), AGENTS OF DENUDATION 21 however, these cracks, or "joints," as they are termed, are somewhat more regular, being, as a rule, disposed at approximately right angles to the planes of bed- ding. In certain fine-grained rocks, such as basalt, the jointing is often very regular, giving rise to a prismatic columnar structure, as in the basalts of Staffa and the Giant's Causeway. The main fact, however, with which we are at present concerned is simply this : that all crystalline, igneous, and schistose rocks are traversed by cracks and fissures of one sort or another. It is further to be noted that these rocks, in common with rocks of all kinds, are more or less porous, and therefore liable to be permeated, however slowly, by percolating water. 2. Argillaceous Rocks. These rocks are composed chiefly of clay, but other ingredients are usually pre- sent. Some are soft, such as ordinary brick-clay ; others are of firmer consistency, and frequently show a fine fissile structure, as in common argillaceous shale ; yet others are hard, tough rocks, some of which are capable of being cleaved into thin plates, as roofing-slate. 3. Silicious Rocks. These might be described in general terms as gravel-and-sand rocks. The most abundant and widely distributed rocks of this class are the sandstones composed generally of grains of quartz (silica) cemented together by carbonate of lime, by iron-oxide, or other substance. Cementing material, however, is not always present, some sand- stones having been solidified by pressure alone. The 22 EARTH SCULPTURE gravel-rocks, or conglomerates, usually consist of rounded fragments of quartz or some hard silicious rock. But to this there are exceptions, the stones in some conglomerates consisting of calcareous or of felspathic rocks or of a mixture of many different kinds. A silicious sandstone which has been more or less metamorphosed is termed quartz-rock. 4. Calcareous Rocks. Under this head are grouped limestones of every kind. They vary in character from soft earthy marls and chalks to hard, granular, crystalline limestones and saccharoid marbles. Some are nearly pure carbonate of lime ; others contain larger or smaller percentages of quartz, clay, iron- oxide, and other impurities. The Argillaceous, Silicious, and Calcareous groups comprise the great bulk of the derivative rocks as well as a few metamorphic rocks. They are all origin- ally of aqueous or sedimentary origin, and generally occur, therefore, in beds or strata. Like the igneous rocks, they are more or less porous, although some especially the clay-rocks are much less permeable than others. In addition to the planes of lamination and stratification, which characterise most derivative rocks, there are other natural division-planes or joints which cut across the strata in directions more or less perpendicular to the bedding. More irregular usually are the joints which intersect hard slates and quartz- rock, these being divided generally much in the same way as schists and amorphous masses of crystalline igneous rock. AGENTS OF DENUDATION 23 There are not a few kinds of rock other than those now referred to, but they may be neglected as, from our present point of view, of relatively little import- ance. Amongst them are rock-salt, gypsum, coal and lignite, ironstones, and other ores. All these, doubtless, are very notable and valuable, but they are neither so abundant nor so widely distributed as the above-described groups ; in short, they occupy a very subordinate place in the architecture of the earth's crust. We have now to consider how the superficial or epigene agents attack and reduce rocks. And first, we may note that rocks at the surface are everywhere subject to changes of temperature warmed by day and during summer, cooled at night and during win- ter. Thus they alternately expand and contract, and this tends to disintegration, for the materials of which they are composed often yield unequally to strain or tension. This is particularly the case with many crys- talline felspathic rocks, such as coarse-grained granite, gneiss, and mica-schist built up, as these are, of min- erals that differ in colour, density, and expansibility. Even when a rock is homogeneous in composition, it is obvious that the heating and cooling of the surface must give rise to strain and tension. In countries where there is no great diurnal range of temperature, as in our own latitudes, any rock-changes due to this cause alone are hardly noticeable, since they are masked or obscured by the action of other and more potent agents. But in the rocky deserts of tropical 24 EARTH SCULPTURE and sub-tropical regions, bare of verdure and practi- cally rainless, the effects produced by alternate heat- ing and cooling are very marked. The rocks are cracked and shattered to a depth of several inches ; the surfaces peel off, and are rapidly disintegrated and pulverised. Wind then catches up the loose ma- terial and sweeps it away, leaving fresh surfaces ex- posed to the destructive action of insolation. More than this, the grit, sand, and dust carried off by the wind are used as a sand-blast to attack and erode the rocks against which they strike. In this manner cliffs and projecting rocks are undermined, and masses give way and fall to the ground, where, subject to the same grinding action, especially towards the base, they eventually assume the appearance of irregular blocks supported upon pedestals. Mushroom-shaped rocks and hills of this kind are common in all desiccated rocky regions. The transporting action of the wind, or " deflation," as it is termed, goes on without ceasing day and night and during all seasons ; and the result is seen in the deeply eroded rocks, enormous masses of which, it can be shown, have been thus gradually removed. The evidence of denudation is conspicuous, but its products have for the most part been carried away. In some places, as Professor Walther remarks of the Libyan Desert, are great walls of granite rising to heights of 6000 feet, but showing no slopes of dd- bris below, as would infallibly be present under tem- perate conditions of climate. In other places, again, AGENTS OF DENUDATION 25 are deeply excavated wadies containing no beds of gravel, grit, and sand, such as would not fail to show themselves had the depressions in question been formed by water-action alone. Everywhere, deep, cave-like hollows have been worn out in the rocks, and yet these hold no sediment or detritus, but are swept bare. The wind tends, in short, to transport all loose material from the scene of its origin to the borders of the desert. In latitudes like our own, insolation doubtless shares in the disintegration of rocks, but the most conspicu- ous agent employed in that work is rain. Rain 'is not chemically pure, but always contains some proportion of oxygen and carbonic acid absorbed from the atmo- sphere ; and after it reaches the ground organic acids are derived by it from the decaying vegetable and animal matter with which soils are more or less im- pregnated. Armed with such chemical agents, it attacks the various minerals of which rocks are com- posed, and thus, sooner or later, these minerals break up. The felspars and their ferro-magnesian associ- ates, for example, are decomposed the carbonic acid of the rain-water uniting with the alkalies and alkaline earths of those minerals to form carbonates, which are cafried away in solution. The silica set free by this operation is also to some extent removed, while the insoluble silicate of alumina, or clay, remains be- hind. Such insoluble materials are frequently stained yellow-brown or red, owing to the pressure of iron- oxides. In this way felspathic rocks gradually crum- 26 EARTH SCULPTURE ble down. Thus, granite, gneiss, basalt, and other rocks largely composed of felspar, usually show a weathered crust, which, according to the nature of the rock and the length of time its surface has been exposed, may vary from less than an inch up to many feet, or even yards, in thickness. Some granites, for example, are reduced to a kind of gritty clay which may be dug with a spade. Argillaceous and silicious rocks are not so readily affected by the chemical action of rain. Not infre- quently, however, when the grains of a sandstone are cemented together by some soluble substance, such as carbonate of lime, the rock will yield more or less read- ily to the solvent action of the water. All calcareous rocks, in short, tend to fall an easy prey. If they contain few or no impurities, they " weather " with little or no crust ; the rock is simply dissolved. Lime- stones, however, are seldom quite so pure as this, but are usually impregnated in a greater or less degree with quartz, clay, or other substance, which after the carbonate of lime has been removed remains behind to form a crust. The red and brownish earths and clays that so frequently overlie calcar- eous rocks, such as chalk and limestone, are simply the insoluble residue of masses of rock, the soluble portions of which have been dissolved and carried away by surface-water. In all regions where rain falls, the result of this chemical action is conspicuous ; soluble rocks are everywhere dissolving, while partially soluble rocks AGENTS OF DENUDATION 27 are becoming rotten and disintegrated. In limestone areas it can be shown that sometimes hundreds of feet of rock have thus been gradually and silently re- moved from the surface of the land. And the great depth now and again attained by rotted rock testifies likewise to the destructive action of rain-water perco- lating from the surface. This is particularly notice- able in warm-temperate, sub-tropical, and tropical latitudes, where felspathic rocks are decomposed not infrequently to depths of a hundred feet and more. In temperate and northern regions, the amount of rotted rock is rarely so great. The thicker rock- crusts of southern latitudes are supposed to be due to the larger supplies of organic acids derived from the more abundant vegetation. To some extent this is probably true. But there is another reason for the relatively meagre development of rotted rock in tem- perate and northern regions generally. Those re- gions, as we shall learn later on, have recently been subjected to glacial conditions. Broad areas of tem- perate Europe and North America have been scraped bare by ice-sheets, resembling those of Greenland and the Antarctic Circle. In more southern latitudes, the rotted rocks have escaped such abrasion and denudation, and hence it is not strange that we should find them attaining so great a thickness. The decom- posed rock-material encountered in the northern parts of Europe and America has been formed for the most part only since the disappearance of glacial condi- tions, while in southern regions rock-decay has gone 28 EARTH SCULPTURE on without interruption ever since those lands came into existence. The disintegrating action of rain in temperate and high latitudes is greatly aided by frost, and the same is the case in the elevated tracts of more south- ern latitudes. Rain renders the superficial portions of rock more porous, and thus enables frost to act more effectually ; while frost, by widening pores and fissures, affords readier ingress to meteoric water. Water freezing in soils and subsoils and in the inter- stitial pores and minute fissures of rocks forces the grains and particles asunder, and when thaw en- sues the loosened material is ready to be carried away by rain or melting snow and subsequently, it may be, by wind. The same process takes place on a larger scale in the prizing open of joints and the rending asunder of rocks and rock-masses. Hence in Arctic regions and at high levels in temperate and southern latitudes the wholesale shattering of rocks has pro- duced immense accumulations of angular debris. To such an extent has this action taken place, that in some countries the rocks are more or less completely buried in their own ruins. By-and-by so great do these accumulations become that frost is unable to get at the living rock. The loose fragments, how- ever, under which it lies concealed, are themselves shattered, crumbled, and pulverised, until they are in a condition to be swept away by wind or melting snow. By this means the solid rock again comes within reach of the action of frost, and so the work of AGENTS OF DENUDATION 29 disruption and disintegration continues. The great heaps or " screes " of rock-rubbish which cloak the summits and slopes of our mountains, and gather thickly along the base of precipice and cliff, have been dislodged by frost and rolled down from above, their progress downward being often aided by tor- rential rains, melting snow, and the alternate freezing and thawing of the saturated debris itself. Some reference has already been made to the indi- rect action of plants in the disintegration of rocks. The various humus acids, as we have seen, are power- ful agents of chemical change. Without their aid rain-water would be a less effective worker. The living plants themselves, however, attack rocks, and by means of the acids in their roots dissolve out the mineral matters required by the organisms. Further, their roots penetrate the natural division-planes of rocks and wedge these asunder ; and thus, by allow- ing freer percolation of water, they prepare the way for more rapid disintegration. Nor can we neglect the action of tunnelling and burrowing animals, some of which aid considerably in the work of destruction. There can be no doubt, for example, that worms, as Darwin has shown, play an important part in the form- ation of soil, which is simply rotted rock plus organic matter. We see, then, that the disintegration and decomposi- tion of rocks is a process everywhere being carried on from the crests of the mountains down to the sea, and in every latitude under the sun. No exposed 30 EARTH SCULPTURE rock-surface escapes attack. In parched deserts as in well-watered regions, in the dreary barrens of the far north as in the sunny lands of the south, at lofty ele- vations as in low-lying plains, the work of rock-waste never ceases. Here it is insolation that is the most potent agent of destruction ; there it is rain aided by humus and carbonic acids ; or rain and frost combine their forces to shatter and pulverise the rocks. In latitudes wh'ere frost acts energetically, the most con- spicuous proofs of rock-waste are the sheets and heaps of debris that are ever travelling down mountain- slopes, or gathering at the base of cliff and precipice. In lower latitudes the most impressive evidence of disintegration is the great thickness attained by rotted rock in positions where it is not liable to be readily swept away by running water. Hitherto we have been considering the superficial parts of rock, as these are affected by weathering. We are not to suppose, however, that the alteration of a rock ceases immediately underneath its crust. Rotted rock is not the only evidence of decay. In the case of felspathic rocks, it is found that some of the constituent minerals, more especially the felspars, usually show traces of decomposition at depths of many feet or even yards below the weathered super- ficial portions. It is hard, indeed, to get a specimen of any such rock from the bottom of our deepest quar- ries which is perfectly fresh. Water soaks through interstitial fissures and pores, and finds its way by joints and other division-planes, so that chemical ac- i v m i v t ros | I T I op DEN UDA T1ON 3 1 tion, with resultant rock-decay, is carried on at the greatest depths to which water can penetrate. This underground water eventually comes to the surface again through similar joints, etc., opening upwards, and thus forms natural springs. All these springs contain mineral matter, derived from the chemical decomposition and solution of rock-constituents. Many, indeed, are so highly impregnated, that as soon as they are exposed to evaporation they begin to de- posit some of their mineral matter. Thus vast quan- tities of rock-material are brought up from the bowels of the earth. To such an extent is this the case in certain regions, that the ground is undermined and the surface not infrequently subsides. In countries where calcareous rocks largely predominate, acidulated water filtering down from the surface through fis- sures and other division-planes has often licked out a complicated series of tortuous tunnels and galleries. So far has this process been carried on in some re- gions that the whole rainfall finds its way into subter- ranean courses, and the entire drainage of the land is conducted underground. The dimensions attained by many well-known limestone caverns, and the great width and depth of the channels through which sub- terranean rivers reach the sea, help us to appreciate the amount of rock-material which underground water is capable of removing. When we add to this all the mineral matter leached out at the surface and carried away by streams and rivers, it is obvious that in course of time the land cannot fail to have been con- 32 EARTH SCULPTURE siderably modified by chemical action alone. In point of fact, it can be shown that from the surface of cer- tain regions hundreds of feet of various calcareous rocks have thus been gradually removed ; while in other cases the contour of the ground has been nota- bly affected by the collapse of underground channels and chambers. But if the results of the chemical action of meteoric water be most evident in places where calcareous rocks predominate, yet the thick- ness attained in other countries by the crusts of less soluble rocks shows plainly enough that the whole land-surface of the globe is affected by the same action. We may now consider the mechanical action of terrestrial water, by means of which the more or less insoluble residue of disintegrated rock is removed. Weathered rock is generally very porous, and is thus readily pulverised by frost. Some crusts crumble away as they are formed, while others adhere more persistently. On slopes and in mountain-regions generally, decomposed and disintegrated materials are seldom allowed to remain long in situ rain and melting snow soon sweep away the finer portions. Great thicknesses of rotted rock are, therefore, some- what exceptional in such places. Where, on the other hand, the land-surface is plain-like, or gently undu- lating, and the drainage sluggish, weathered materials are not so readily removed. Nevertheless, under the influence of rain alone, or of rain and melting snow, the products of rock-waste are everywhere travelling, AGENTS OF DENUDATION 33 slowly or more rapidly, according to circumstances, from higher to lower levels. In temperate latitudes, where the rainfall is distributed over the year, this transference of material is not so conspicuous as in countries where the rainfall is crowded into a short season. Even in our own country, however, one may observe how in gently undulating tracts rain washes the finer particles down the slopes and spreads them over the hollows. After exceptionally heavy or long- continued rain this process becomes intensified fine mud, silt, sand, and grit are swept into the brooks and streams, and the swollen rivers run discoloured to the sea. Similar floods often result from the melting of snow in spring. During such floods our rivers are generally more turbid than when they are swollen merely by heavy or continuous rain. When thaw en- sues weathered rock-surfaces crumble down, while superficial accumulations of disintegrated materials become more or less saturated by melting snow. To such a degree is this soaking sometimes carried, that the whole surface of sloping fields may be set in mo- tion. The soils creep, slide, and occasionally flow. Not infrequently also the subsoils and disintegrated rock-surfaces on steep inclinations collapse and slide into the valleys. Everyone, in short, is familiar with the fact that flooded rivers are invariably muddy, and that the mud or silt which discolours them has been abstracted from the land. In temperate lands of small extent like England the rivers are under ordinary conditions somewhat clear. 34 EARTH SCULPTURE But in continental tracts the larger rivers are always more or less turbid. This is due to many causes. Some rivers, for example, head in glaciers, and are thus clouded at their very origin. Others, again, cross several degrees of latitude, and traverse differ- ent climatic regions. Hence it will rarely happen that snow is not melting or rain falling in some part of a great drainage-area. Many rivers, again, after escaping from the mountains, flow through countries the superficial formations of which are readily under- mined and washed away, and thus the main stream and its affluents become clouded with sediment. It is in tropical and subtropical latitudes, of course, that the most destructive effects of rain are witnessed. During the wet season the rivers of such regions dis- charge enormous volumes of mud-laden water. We may conclude, then, that under the influence of atmospheric agents rocks are everywhere decomposed and disintegrated ; and, further, that there is a uni- versal transference from higher to lower levels of the materials thus set free. Now and again, it is true, there may be long pauses in the journey the materi- als may linger in hollows and depressions. Eventu- ally, however, they are again put in motion, and by direct or circuitous route, as the case may be, find their way into the rivers, and finally come to rest in the ocean. The river-systems of the world, then, are the lines along which the waste products of the land are carried seawards. But rivers are much more than mere transporters of sediment. Just as in desert AGENTS OF DENUDATION 35 lands wind employs disintegrated rock-material as a sand-blast, so rivers use their stones, grit, and sand as tools with which to rasp, file, and undermine the rocks over which they flow. In this way their chan- nels are gradually deepened and widened. Some of the transported material is held in solution, part is carried in mechanical suspension, and another portion is pushed and rolled forward on the bed. It is the solid ingredients, of course, that act as eroding agents. While much of the finer sediment finds its way into the drainage-system by the agency of rain and melt- ing snow, the coarser materials are derived chiefly from the destruction of the rocks that underlie or overhang the course of a river and its feeders. In temperate and northern latitudes natural springs and frost are responsible for much of the rock debris which cumbers the beds of streams, but much also is dis- lodged by the undermining action of the water itself. Rock-fragments when first introduced are more or less angular, but as they travel down stream they often break up into smaller pieces along natural cracks or joints, and the sharp corners and edges of these get worn away by mutual attrition, and by rasping on the rocky bed. In this manner the several portions gradu- ally become smoothed and rounded the process of abrasion resulting necessarily in the production of grit, sand, silt, etc. Thus in a typical river-course, consisting of mountain-track, valley-track, and plain- track, we note a progressive change in the character of the sediments as the river is followed from its 36 EARTH SCULPTURE source to the sea. In the mountain-track, where the course is steep and usually in a rocky channel, angular and subangular fragments abound, and the -detri- tus generally is coarse. In the valley-track, the inclin- ation of which is gentle, well-rounded gravel, with grit and sand, predominate, the latter becoming more plentiful as the plain-track is approached. In the plain-track the prevailing sediments are fine sand and silt. The amount of material removed by a river de- pends on the volume of the water, the velocity of the current, and the geological character of the drainage- area. Thus, the larger the river, other things being equal, the greater the burden of sediment. Again, a rapid current transports material more effectively than a gentler stream, while rivers that flow through lands whose rocks are readily eroded carry more sediment than rivers of equal volume and velocity traversing regions of more resistant rocks. Should a lake interrupt the current of a river, all the gravel, sand, and mud may be intercepted, and the stream will then issue clear and pellucid at the lower end of the lake, as the Rhone does at Geneva. The lake, in short, acts as a settling reservoir. By and by, how- ever, the lacustrine hollow becomes silted up and con- verted into an alluvial flat, through which the silt-laden water winds its way towards the ocean. Reaching that bourn, the current of the river is arrested, and its sediment thrown down. Should no strong tidal current sweep the coast, removing sediment as it ar- AGENTS OF DENUDATION 37 rives, the sea becomes silted up in the same way as the lake, and in time a delta is formed. The growth of the latter necessarily depends partly on the activ- ity of the river and partly upon the depth of the estuary and the action of waves and tidal currents. But if nothing interrupted the growth of a delta- were all the materials brought down by a river to ac- cumulate at its mouth it is obvious that the rate of increase of a delta would enable us to form an esti- mate of the rate at which the drainage-area of the river was being eroded. It is certain, however, that such conditions never obtain. Even in the quietest estuaries much of the sediment is carried away by the sea. The rate of delta-growth must be exceeded by that of fluviatile transport. Geologists, however, have adopted another method of estimating the loss sustained by the land. They can measure the amount of material held in solution, and of solid matter carried in suspension and rolled forward on the bed of a river. As might have been expected, the amount varies with the season of the year in each individual river, while different rivers yield very different results. But even in the case of the least active streams the transported material is much more considerable than might have been sup- posed. Hence one need not wonder that in spite of obstacles the deltas of many rivers advance seawards more or less rapidly. The delta of the Rhone, for ex- ample, pushes forward at the rate of about 50 feet annually, while that of the Po increases by more than 38 EARTH SCULPTURE 70 yards, and that of the Mississippi by 80 to 100 yards in the same time. It is sufficiently obvious that the material carried seawards by rivers must afford some indication of the rate at which the surface of the land is being lowered by subaerial action. Having ascertained the annual amount discharged by any individual river, we learn, at the same time, to what extent the drainage-area of that river is being denuded. In the case of the Mis- sissippi, for example, it has been calculated that the amount of sediment removed is equal to a lowering of the whole drainage-area by -g-^-^th of afoot. In other words, could we gather up all the material discharged in one year, and distribute it equally over the wide regions drained by that river and its tributa- ries, we should raise the land-surface by g^^th of a foot. That does not seem to be much, but at this rate of erosion one foot of rock will be removed from the Mississippi basin in 6000 years ; and the Mississippi is not so active a worker as many other rivers. An aver- age of many estimates of the similar work performed by rivers in all quarters of the globe shows that the rate at which drainage-areas generally are being low- ered is one yard in 8000 to 1 1,000 years. It must not be supposed that this erosion is equal throughout any drainage-area. As a rule, denudation will take place most rapidly over the more steeply inclined portions of the ground. On mountain declivities and hill slopes rock-disintegration and the removal of waste products will proceed more actively than upon low AGENTS OF DENUDATION 39 grounds and plains. The work of erosion will be carried on most effectively in the torrential tracts of streams and rivers. Indeed, we may say that it is in valleys generally that we may expect to find the most cogent evidence of erosion now in action. A little consideration will show that the estimates just referred to do not tell us all the truth concerning denudation. They show us only the amount of waste material which is swept into the sea. They afford no indication of the actual amount of rock-disintegration and erosion. Rock-rubbish gathers far more rapidly in mountain-regions than it can be removed by run- ning water. Indeed, over a whole land-surface rocks are disintegrated and debris accumulates from year to year. Nor is the amount of material brought down by a river to its mouth an index even to the activity of the river itself as a denuding and transporting agent. Enormous volumes of detritus are deposited in valleys or come to rest in lakes and inland seas. Hitherto we have been treating of the work done by the atmosphere and running water. Some refer- ence has also been made to frost as a potent disin- tegrator of rocks. But we have still to consider the action of glaciers in modifying the surfaces over which they flow. It can be shown that valleys have been widened and deepened, and broad areas more, or less remodelled, by flowing ice, so that glaciers must not be ignored in any general account of denuding agents. It will be more convenient, however, to leave them for the present ; for however interesting and import- 40 EARTH SCULPTURE ant their action may be, it is yet of minor consequence so far as the origin of surface-features as a whole is concerned. For similar reasons we may delay the con- sideration of marine erosion. The action of the sea upon the land is necessarily confined to a narrow belt, whereas that of the subaerial agents affects the whole surface of the land. We may take it that the denudation of the surface, rendered everywhere so conspicuous by the discon- tinuity of strata, has been effected mainly by the at- mosphere and running water. Other agents have, no doubt, played a part, but those just referred to must be credited with the chief share in the work of erosion. Such is the general conclusion to which we are led by the study of causes now in action. And observation and reflection combine to assure us that subaerial erosion has been equally effective during the formation of all the derivative rocks which enter so largely into the framework of the earth's crust. For these rocks are for the most part of sedimentary origin they tell us of ancient lakes, estuaries, and seas. All their materials have been derived from the degradation of old land-surfaces, partly no doubt by the sea, but in chief measure by subaerial agents. And the great thickness and extent attained by many of the geological systems enable us to form some idea of what is meant by denudation. What, for instance, shall we say of a system composed essentially of sed- imentary strata reaching a thickness of several thou- sand feet, and occupying an area of many thousand AGENTS OF DENUDATION 41 square miles ? Obviously, the materials of such a system have been derived from the waste of ancient lands. Mountain-masses must have been disinte- grated, and removed in the form of sediment, and gradually piled up, layer upon layer, on the floor of the sea. Every bed of sedimentary rock, in short, is evidence of denudation. Further, it has been ascertained that in the build- ing up of the various great geological systems the same materials have been used over and over again. Sediments accumulated upon the sea-bottom have subsequently, owing to crustal movements, entered FIG. 6. SECTION ACROSS UNCONFORMABLE STRATA. a , beds of sandstone, shale, etc. ; b <5, conglomerates and sandstone resting discordantly or unconformably upon a a ; u u, line of unconformity. into the formation of a new land-surface, and there- after, attacked by the epigene agents of change, have again been swept down to sea as gravel, sand, and mud. The history of such changes is easily read in the rock-structure known as unconformity. In the accompanying section (Fig. 6), for example, two sets of strata are shown the upper (b) resting discord- antly or unconformably upon the lower (a). The lower series of sandstones and shales is charged with the remains of marine and brackish-water organisms 42 EARTH SCULPTURE and of land-plants. The overlying strata (K) are like- wise of aqueous origin, and consist chiefly of con- glomerates and sandstones below, and of somewhat finer-grained sedimentary beds above. Like the older series (#), they likewise contain marine and brackish- water fossils. The beds (a) introduce us to an estu- ary, or shallow bay of the sea, into which sediment is carried from some adjacent land. The whole series has evidently been deposited in water of no great depth, as is shown by the character of the rocks and their fossil contents. And as the strata attain a thickness of more than 2000 feet, we must infer that during their accumulation the sea-floor was slowly subsiding, the rate of sedimentation probably keep- ing pace with the subsidence. In other words, the bed of the sea appears to have been silted up as fast as it sank, so that relatively shallow-water conditions persisted during the deposition of the land-derived sediments. Then a time came when the sea-floor ceased to sink and another movement of the crust took place, which resulted in the folding of the sedi- mentary strata and the conversion of the sea-bottom into dry land. The folded rocks were now subjected during some prolonged period to the action of the various subaerial agents of erosion, whereby the whole land-surface was eventually denuded and planed down. When the work of erosion had been so far completed, the entire region again subsided, and formed the bed of a shallow sea. Under these conditions the drowned land-surface became overspread in time with new ac- AGENTS OF DENUDATION 43 cumulations of sediment, derived from the degradation of adjacent areas that still continued above sea-level. The strata (b) are in point of fact largely composed of materials derived from the breaking up and disinte- gration of the underlying series (a), just as the latter have themselves been derived from the demolition of pre-existing rock-masses. After the formation of the upper series (ti) the region was re-elevated, and once more formed a land-surface, which has doubtless en- dured for a long period, seeing that much erosion has taken place, the horizontal beds having been greatly denuded, trenched, and furrowed, so that at the bottom of deep valleys the underlying older series has been laid bare and eaten into by running water. Such is the kind of tale which one may read almost everywhere. The very existence of sedimentary strata implies denudation of land-areas denudation and sedimentation go hand in hand. When we bear in mind that the average thickness of the sedimentary rocks which overspread so large an area of the dry lands of the globe cannot be less than 8000 or 10,000 feet, we cannot fail to be impressed with the magni- tude of denudation. And this impression will be deepened when we reflect that the bulk of the mate- rials entering into the composition of the derivative rocks has been used over and over again. The mere thickness of existing sedimentary strata, therefore, is very far indeed from being an index to the amount of erosion which has been effected since the deposition of the oldest aqueous strata. CHAPTER III LAND-FORMS IN REGIONS OF HORIZONTAL STRATA VARIOUS FACTORS DETERMINING EARTH SCULPTURE INFLUENCE OF GEOLOGICAL STRUCTURE AND THE CHARACTER OF ROCKS IN DETERMINING THE CONFIGURATION ASSUMED BY HORIZONTAL STRATA PLAINS AND PLATEAUX OF ACCUMULATION. TTITHERTO we have been considering erosion 1 1 from one point of view only. We glanced first at the general evidence of denudation as furnished by the abrupt truncation and discontinuity of strata, and by the appearance at the surface of rocks which could never have originated in that position. Then we dis- cussed the action of existing agents of change, and saw reason to conclude that the denudation every- where conspicuous must be the result of that action. Some reference has also been made to the fact that rocks are of various composition and consistency, and therefore tend to yield and crumble away unequally. It follows from this that denudation will be retarded or hastened according as the rocks succumb slowly or more rapidly to the action of eroding agents. Given an elevated plane-surface of some extent, composed 44 LAND-FORMS IN HORIZONTAL STRATA 45 of rocks of different degrees of durability, and it is obvious that such a surface must in time become irregularly worn away. The readily eroded rocks will disappear most rapidly, and thus by and by the plane-surface will be more or less profoundly modified and come to assume a diversified configuration. The relatively hard and resisting rocks will determine the position of the high grounds, while the low grounds will practically coincide with the areas occupied by the more yielding rock-masses. This we shall find holds true to a large extent of all land-surfaces. Nevertheless, existing configura- tions have not been determined solely by the min- eralogical composition of the rocks. There is yet another factor to be taken into consideration. The form assumed by a land-surface under denudation de- pends not only on the composition of rocks, but very largely on the mode of their arrangement. Certain rock-structures, as we shall learn, favour denudation, while others are more resisting. So dominant, indeed, has been the influence of geological structure in de- termining the results worked out by erosion, that without a knowledge of the structure of a country we can form no reliable opinion as to the origin of its surface-features. But even this is not all. We have likewise to con- sider the geological history of the land with a view to ascertain what appearance it presented when rains and rivers were just beginning the work of erosion. For it is obvious that the direction of the drainage must 46 EARTH SCULPTURE have been determined in the first place by the original inclination of the surface. Once more, we know that existing land-surfaces have often been disturbed by subterranean action, and that such action has not infrequently led to con- siderable modification of drainage-systems. It is remarkable, however, how persistent are great rivers in maintaining their direction. When it has been once fairly established, a large river may outlive many revolutions of the surface. River-valleys are not seldom older than the mountain-ridges which they sometimes traverse ; or, to put it in another way, new mountains may come into existence without deflecting the rivers across whose valleys they may seem at one time to have extended for the rivers have simply sawed their way through the ridges as these were being gradually developed. The history of the denudation of a land-surface is in truth often highly complicated and hard to read. Many factors have aided in determining the final re- sults of erosion, and it is not always possible to assign to each its proper share in the work. But we may truly say that the sculpture of the land the form it has assumed under denudation has been determined mainly by these three factors : (a) the original slope of the surface ; (K) the geological structure of the ground ; and (f) the character of the rocks. Both hypogene and epigene agents, therefore, have been concerned in the evolution of land-forms. In regions much disturbed by subterranean action within LAND-FORMS IN HORIZONTAL STRATA 47 relatively recent geological times, many of the most striking surface-features are obviously due to deforma- tion and dislocation of the crust. All such features, however, sooner or later become modified by epigene action, and thus it has come to pass that in countries which have existed as dry land for vast periods of time, undisturbed in the later stages of their history by crustal movement, the surface-features are such as only epigene action can account for. Original irregularities of the ground, the result of hypogene action, have been obliterated and replaced by an outline wholly due to denudation. The existence of fractured and folded strata enables us vividly to realise the fact that hypogene action has played a prominent part in the evolution of land-forms. Not only are many inequalities of the surface the di- rect result of that action, but even after such irregu- larities have been removed, the various positions assumed by the flexed and fractured rocks have largely determined the configuration subsequently worked out by the epigene agents of change. Thus both directly and indirectly crustal movements have had a large share in the production of surface-features. It is not necessary for our purpose to inquire into the causes of such movements. In the opinion of most geologists they are due to the secular cooling of the earth. As the nucleus cools it contracts, and the already cooled crust sinks down upon it. This move- ment necessarily results in the fracturing and wrinkling of the crust, which as it sinks is compelled to occupy 48 EARTH SCULPTURE a smaller superficial area. The deformation brought about in this way varies in extent. In some places the general subsidence of the crust has not been marked by much disturbance of the rocks ; the orig- inal horizontality of the strata has been largely pre- served. In other regions the reverse is the case, the strata having been everywhere folded and fractured ; and between these two extremes are many gradations. The various structures assumed by disturbed rock- masses show that crustal movements are of two kinds, horizontal and vertical. Folding and its accompany- ing phenomena are obviously the result of tangential pressure. Sometimes the strata are so folded as to present the appearance of a series of broad, gentle undulations. At other times the folds are pressed closely together and bent over to one side in the direction of crustal movement. In certain regions so great has been the horizontal thrust, that masses of rock, thousands of feet in thickness, have sheared under the pressure and travelled forwards for miles, older rocks being pushed forward bodily over younger masses. But besides such horizontal movements there are vertical movements of the crust, typically repre- sented by the dislocations known as normal faults. Normal faults are more or less vertical displacements, often of small amount, but not infrequently very great. Many are vast rents traversing the crust in some determinate direction, the rocks on one side of the fault having subsided for hundreds or even for thousands of feet. We may reserve for the present. LAND-FORMS IN HORIZONTAL STRATA 49 however, any further discussion of the rock-structures that result from hypogene action. All that we need at present bear in mind is the general fact that the crust of the earth is subject to deformation. We now proceed to inquire more particularly into the influence of geological structure and the character of rocks upon the development of land-forms. We shall therefore consider first the form assumed by lands built up of approximately horizontal strata. This is the simplest kind of geological structure : the tale it tells is not hard to read. We can follow it from first to last in all its details. But if we succeed in grasping what is meant by the denudation of hori- zontal strata, we shall have little difficulty in explaining the origin of surface-features in regions the geological structure of which is much more complicated. As common examples of horizontally bedded strata we may take the alluvial deposits that mark the sites of vanished lakes ; the terraces of gravel, sand, and silt that occur in river-valleys ; deltas, and raised beaches. Fluvio-marine deposits and raised beaches of recent age generally form low plains rising but a few feet or yards above sea-level. Their inclination is seawards, usually at so low an angle that they often appear to the eye level, or approximately so. This gently sloping surface is an original configuration, for it corresponds with the structure of the various under- lying deposits, the general inclination or dip of which is in the same direction as the surface. When that surface is approximately level denudation necessarily 50 EARTH SCULPTURE proceeds very slowly, although in time the action of rain alone will suffice to lower the general level. But however much raised beaches and deltas of recent age may have been modified superficially by subaerial denudation, we must admit that their most character- istic features are original, and due to the mode of their formation. The same holds true to a large extent of recent lacustrine and fluviatile deposits. The wide flats that tell us where lakes formerly existed, and the broad alluvial tracts through which streams and rivers meander, are, like deltas and raised beaches, plains of accumulation. It goes without saying, however, that many of these plains are more or less eroded, and have acquired an undulating, furrowed, and irregular surface. Some alluvial tracts, indeed, have been so cut up by rain and running water that, in the rough, rolling ground over which he toils, the traveller may find it hard to recognise the characteristic features of a plain. In a broad river-basin alluvial terraces and plains usually occur at various heights, marking successive levels at which the river and its tributaries have flowed while deepening their courses. The lowest terraces and flood-plains are, of course, the youngest, and show, therefore, least trace of subaerial erosion. As we re- cede from these modern alluvia and rise to higher levels, the terraces and plains become more and more denuded. The highest-lying river-accumulations, in- deed, may be so much eaten into and washed down LAND-FORMS IN HORIZONTAL STRATA 51 that only scattered patches may remain, and few or no traces of the original flat surface can then be recog- nised. Thus fluviatile terraces and recent alluvia all tend to become modified superficially, while at the same time they are undermined and cut into by streams and rivers. The plains of accumulation at present referred to belong to a recent geological age, and consist for the most part of incoherent deposits, such as gravel, sand, clay, silt, loam, and so forth. And it is worthy of note that the nature of the deposits has to some ex- FIG. 7. SECTION ACROSS A SERIES OF ALLUVIAL TERRACES. r, solid rocks ; i, oldest terrace ; 2, second terrace ; 3, third and youngest terrace ; 4, river and recent alluvial plain. tent influenced the denudation of the ground. Thus terraces and plains composed mainly of gravel tend to retain their original level surface, while similar flats of clay and loam of the same age as the gravel have frequently been furrowed and channelled to such an extent that the originally level surface has largely, or even entirely, disappeared. The reason is obvious, for clay and loam are somewhat impervious, while gravel is highly porous. Consequently rain falling on the surface of the latter is rapidly absorbed, and little or no superficial flow is possible. But in the case of the more impervious deposits rain is absorbed very 52 EARTH SCULPTURE sparingly, and naturally tends to produce inequalities as it seeks its way over the gently inclined surface. The origin and present aspect of such recent plains of accumulation are so obvious and so readilyaccounted for, that it is hardly necessary to do more than cite a few examples. Amongst the most notable are the great deltas of such rivers as the Mississippi, the Amazon, the Rhone, the Po, the Danube, the Rhine, the Niger, the Ganges, etc., and the broad flats and terraces which occur within the drainage-areas of the same rivers. The vast plains of the Aralo-Caspian area, and the far-extended tundras of Northern Siberia, are likewise examples of plains of accumulation, all of which belong to recent geological times. How- ever much some of these plains may have been furrowed and trenched by running water, we yet have no difficulty in recognising that the general form of the surface is due to sedimentation. The deposits of which they are built up have been laid down in approximately horizontal or gently inclined layers, and the even or level surface is thus simply an expression of the arrangement of the bedding. In a word, the geological structure has determined the configuration of the surface. But it is needless to say that horizontal strata are not confined to low levels, nor do they always consist of unconsolidated materials, like gravel, sand, and clay. Horizontal strata of such rocks as sandstone, shale, limestone, basalt, etc., enter largely into the composition of certain lofty plateaux and mountain- LAND-FORMS IN HORIZONTAL STRATA 53 regions. And they belong, moreover, to very differ- ent geological periods, some being of comparatively recent formation, while others date back to ages incalculably remote. One of the most interesting and instructive regions of the kind is the remarkable plateau of the Grand Canon district of Arizona and Utah. This plateau occupies an area of between 13,000 and 16,000 square miles, and is traversed by the Colorado River of the West, which follows a tortuous course tow- ards west-south-west through a succession of pro- found ravines or canons. The strata visible at the surface are approximately horizontal, and attain a thickness of many thousand feet. It may be said, therefore, that the prevalent plain-like character of the surface is an expression of the underground struct- ure that, in short, the Grand Canon district is a plateau of accumulation. This, in a broad sense, is doubtless true ; but when we come to examine the configuration and structure of the district more closely, we find reason to conclude that the original surface has been greatly modified by denudation. We learn, moreover, that the strata are not quite horizontal. The inclination is certainly gentle, but a slope of only one degree, if continued for a few miles, will result in a fall of several hundred feet. If a surface be in- clined at an angle of one degree, then for every eleven miles of distance it will lose 1000 feet of ele- vation. Now, in the Grand Caflon district the gen- eral inclination of the strata is towards north and 54 EARTH SCULPTURE o 1 1 O O , c 1 I s I o' 2 U. V o -3, - ? S w | c o Q -5 04 " -o 00 g, J f north-east, while the slope of the surface is in the opposite direction. Thus it comes to pass that strata which lie open to the day upon the south-west mar- gin of the plateau gradu- ally descend towards north and north-east, until, in a distance of 120 miles or thereabouts, they lie bur- ied at a depth of several thousand feet. It is not quite true, therefore, that in the Grand Canon dis- trict the form of the ground is an exact expres- sion of the underground structure. On the con- trary, the average slope of the surface is against and not with the average dip of the strata. Never- theless , it cannot be doubted that the general configuration of the re- gion its plateau-charac- ter has, in the first place, been determined by the approximately horizontal LAND-FORMS IN HORIZONTAL STRATA 55 disposition of the strata, and that it may be rightly termed a plateau of accumulation. A glance at the geological history of the district will show how far the plateau-character is original, and to what extent and by what means it has been subsequently modified. Reference has been made to the fact that the rocks composing the plateau are chiefly of aqueous origin, and approximately horizontal. Here and there in the bottoms of deep canons we get peeps at another set of rocks that form the pavement upon which the horizontal strata repose. With the history of these older underlying rocks we need not concern ourselves further than to note that they are of Pre-Cambrian and early Palaeozoic age. It is with the superincumbent masses that we have to deal. Those attain a vast thickness, and range in age from Carboniferous down to Eocene times. At the beginning of the Carboniferous Period the district formed a portion of the sea-floor, and similar marine conditions obtained during the deposi- tion of all the succeeding systems of strata down to the close of Cretaceous times. Throughout all that long succession of ages the sea would appear never to have been deep, although during the early part of the Carboniferous Period it was probably deeper than in subsequent times. When we consider that the marine sediments reach a united thickness of over 15,000 feet it may at first sight appear impossible that so thick a mass of materials could accumulate in a shallow sea. The explanation, however, is simple enough sub- 56 EARTH SCULPTURE sidence kept pace with sedimentation. Slowly and gradually the bed of the sea went down slowly and gradually it was silted up by sediments derived from the adjacent land. At last, towards the close of Cretaceous times, cer- tain new crustal movements began elevation ensued, and the sea finally retired from the district. An ex- tensive lake now occupied the site of the plateau- country, for a prolonged period, during which sediments were washed down as before from the neighbouring uplands, and gathered over the level surface of the Cretaceous marine strata until they had reached a thickness of 5000 feet or more. As these deposits appear likewise to have been laid down chiefly in shallow water, it may be inferred that the slow subsid- ence of the area which accompanied the accumula- tion of the underlying marine strata was repeated during the lacustrine period. The whole region, it will be understood, had been elevated at the close of Cretaceous times ; but the movement was differential, the greatest rise having been experienced by the uplands surrounding the la- custrine basin. Eventually the river, escaping over the lower lip of that basin, deepened the outlet and succeeded in draining the lake, which was then re- placed by an alluvial plain. At this stage the nearly level surface of the drained lake-bed sloped gently from east-north-east to west-south-west, and thus de- termined the direction of the primeval Colorado River and its larger tributaries, which headed then LAND-FORMS IN HORIZONTAL STRATA 57 as now in the high lands overlooking the basin. When these waters first began to wander across the alluvial plain, the slope of the surface and the inclina- tion of the underlying sedimentary strata doubtless coincided. But these conditions were ere long dis- turbed by successive movements of elevation, and the prevalent horizontality of the strata was modified. Here and there the beds were bent or flexed, and traversed by great fractures along which the strata became vertically displaced for thousands of feet. Yet, strange to say, none of these earth-movements succeeded in deflecting the main drainage of the dis- trict. The Colorado and its chief affluents continued to flow in the courses they had attained at the final disappearance of the great lake. It is clear, there- fore, that the bending and dislocation of the strata must have proceeded very slowly, for the rivers were able to cut their way across both flexures and faults as fast as these showed at the surface. Before the great lake had vanished some portions of the older marine strata had been elevated, and formed part of the land surrounding the basin. Here they were for a long period exposed to the erosive action of epigene agents, and must have suffered much loss. But all such denudation sinks into insig- nificance when we consider the magnitude of the erosion which has taken place since the great lake dried up. Fortunately, owing to the simple geologi- cal structure of the Grand Canon district, the amount of that erosion can be readily estimated. According EARTH SCULPTURE to Captain Dutton, the average thickness of strata removed from ^ an area of 13,000 to 15,000 square | miles cannot have been under 10,- 2 ooo feet. This may seem a startling conclusion, but it is based on evi- o % dence which cannot be gainsaid. 'o Throughout the major portion of the plateau-country horizontal t Carboniferous strata occupy the ?. surface. As these are followed | northward they gradually dip in J; that direction under younger strata | (Permian, Mesozoic, and Cainozoic | rocks), until they are buried at last f to a depth of 10,000 feet and more. j> Now Captain Dutton has shown 1 that this vast thickness of over- **f lying strata formerly extended ; throughout the whole Grand i Canon district. This is proved by I the fact that many outliers or relics > of the rocks in question still re- . , . , z mam, scattered at intervals over the broad surface of the Car- I boniferous strata. They form conspicuous table-shaped and pyr- 3 amidal hills, rising more or less a abruptly above the great Carbon- iferous platform. The accompany- ing diagram shows the general LAND-FORMS IN HORIZONTAL STRATA 59 relations of those isolated "buttes" and "mesas," as they are termed, to the underlying Carboniferous rocks and the strata at T, of which they are detached outliers. The dotted line (a-ti) indicates the level originally attained by the plateau. All the rock that formerly existed between a-b and the surface of the Carboniferous strata (C) has been denuded away. How has this enormous erosion been effected, and what are the more prominent features of the denuded area ? A low-lying plain of accumulation, such as a delta, cannot experience much erosion ; the surface is approximately level, or has only a very gentle in- clination, and any water flowing over it must be sluggish and ineffective. But conceive such a plain upheaved for several hundred feet, and it is obvious that the fall of the river to the sea will then be in- creased and its erosive action greatly augmented. It will therefore proceed to dig a deeper and wider course for itself. Now let us suppose that an ele- vated plain is traversed not by one main river only, but by numerous affluents, each with its quota of tributary streams. The running waters will continue to deepen their channels until the gradient by the pro- cess is gradually reduced to a minimum and vertical erosion ceases. The main river will be the first to attain this base-level a level not much above that of the sea. The plain-track will gradually extend from the sea inland until the same low gradient is attained throughout the whole course of the river. In time all the affluents with their tributaries will arrive at the same stage. 60 EARTH SCULPTURE But rivers do not only cut vertically ; they also un- dermine their banks and cliffs, and thus erode hori- zontally ; hence it follows that the valleys will be widened as well as deepened. The widening process may be greatly aided by the action of wind, rain, springs, and frost. Not infrequently, indeed, these agents play as important a part as the streams them- selves. Under the conditions now described an ele- vated plain will in course of time be cut up into more or less numerous segments, the upper surfaces of which will represent the original level of the land ; where the interval between two valleys is wide we shall have a broad, flat-topped segment ; where the interval is short the segment will be correspondingly restricted in size. In a word, the segments will vary in extent according to the multiplicity and intricacy of the valley-system. A word now as to the form of the slopes and cliffs bounding the valleys. We are dealing, it will be re- membered, with an elevated plain of accumulation. The horizontal strata, we shall suppose, are more or less indurated beds of conglomerate, sandstone, shale, and limestone. All rocks, as we have seen, are traversed by natural division-planes or joints, and these in the case of stratified rocks consist of two sets intersecting each other and the planes of bedding at approximately right angles. Horizontal strata are in this way divided up into rudely cuboidal, quadrangu- lar, or rectangular blocks. Joints are, of course, lines of weakness along which, when rocks are undermined, LAND-FORMS IN HORIZONTAL STRATA 61 they tend to give way. Thus when horizontal strata are cut into by rivers and undermined they break off at the joints, and vertical cliffs result. It does not often happen, however, that in a considerable series of strata all the beds are of quite the same character. Frequently some are relatively harder and unyielding, while others are softer and more readily reduced. Let us suppose that the uppermost bed cut into by the river is somewhat hard and difficult to grind through. In time the water saws its way down into the succeeding stratum, which we shall take to be a soft or easily eroded shale. In the overlying hard rock the river has been able to cut merely a narrow steep-sided trench. The shale, however, offers much less resistance to the vertical and lateral action of the water, and is thus rapidly intersected and washed away from underneath the superincumbent harder stratum. The latter, losing its support, then yields along its joint-planes, and a larger or smaller slice is detached from the wall of the cliff and falls in ruins. In this way the cliffs gradually retire as they are un- dermined in a word, the ravine is not only deepened but widened. Much of the rock debris dislodged from the cliffs falls into the river, and is gradually broken up and carried away ; but some comes to rest at the base, forming a talus, and thus retards the denudation of the shale. To the action of the river we must add that of other epigene agents, such as wind, rain, springs, and frost, under the influence of which the 62 EARTH SCULPTURE shale weathers away more rapidly than the overlying rock, and eventually forms a sloping stage upon which the debris derived from the receding cliffs continues to accumulate. Meanwhile, however, the river digs down through the shale and encounters, we shall suppose, another thick stratum of hard rock. Lateral erosion by the running water is now reduced to a minimum ; slowly the current saws its way down FIG. 10. DIAGRAMMATIC SECTION SHOWING STAGES OF EROSION BY A RIVER CUTTING THROUGH HORIZONTAL STRATA. (After Captain Button.) A, relatively hard rocks ; s, relatively soft strata ; r r, river at successive stages as valley is deepened and widened. vertically, just as it did in the uppermost unyielding bed, until it again reaches a second layer of shale. The undermining action is now repeated, and a sec- ond line of rock-wall begins to retreat in the same manner as the first. And so the process goes on with all the succeeding strata through which the river cuts, until it finally attains a minimum gradient and ceases to erode. But note that, while the deepening of the ravine proceeds, the cliffs never cease to retire. Each LAND-FORMS IN HORIZONTAL STRATA 63 individual layer of softer rock continues to waste away more rapidly than the harder bed above it. Thus eventually a river-valley appears bounded, not by vertical cliffs, but rather by a succession of hori- zontal tiers of precipitous faces, corresponding to the outcrops of the several strata of harder rock separated the one from the other by the longer or shorter slopes yielded by the shales. Finally, we may further note that the recession of the cliffs will be much influenced by the rate at which their basal portions are undermined. Each slice re- moved from a steep rock-face narrows the width and increases the inclination of the sloping stage above. Hence, as Captain Button has clearly shown in his admirable description of the Colorado Canons, the de- scent of debris from each stage is facilitated, while the weathering of the soft rocks and the undermining of the overlying harder beds are accelerated. Thus, curiously enough, as the same author remarks, the state of affairs at the bottom influences the rate of recession at the summit. When a river has reached its base-level and ceases to erode, the valley-slopes and cliffs, nevertheless, under the influence of weathering, continue to retire. The debris showered down from above now tends to accumulate below, and thus affords protection to the rocks against which it is banked. And the talus thus formed continues to rise higher and higher. The ex- posed strata above, however, having no such protec- tion, weather as before, each rock-tier retreating, but 64 EARTH SCULPTURE at a gradually diminishing rate. What form the ground will ultimately assume will largely depend upon climatic conditions. If the climate be moist and frost be active in winter, the sharp edges of the rock- tiers will be bevelled off, and the sloping surfaces will become heavily laden with debris and disintegrated rock-material, the further degradation and removal of which will be retarded by the growth of vegetation. Thus, in time, the sharp angles will tend to disappear, and a somewhat undulating slope will replace the more strongly marked features which the same rocks would have yielded under arid conditions. Let us now recall what was said as to the cutting up of our elevated plain into a multiplicity of flat- topped segments, and we shall see reason to conclude that these segments must be bounded by steep faces, the aspect of which will vary according to the nature of the strata and the character of the climate. If the climate be arid, and the strata consist of alternate hard and soft beds of variable thickness, the bound- ing walls of the segments may in some places be ap- proximately vertical, or they may show a succession of short cliffs with intermediate sloping stages. If, on the other hand, the climate be moist, those features will be more or less softened and modified. In the former case step-like profiles will abound ; in the latter the ground will likewise ascend in stages, but these will be less accentuated, and may even be in large part replaced by continuous slopes. Again, each flat- topped segment of the denuded area, eaten into on all LAND-FORMS IN HORIZONTAL STRATA 65 sides, will continually contract, the bounding cliffs and slopes retiring step by step until they eventually meet atop. The flat summit now disappears, and is replaced by a sharp crest, ridge, peak, or rounded top, as the case may be. Each diminishing segment, in short, ultimately acquires a more or less strongly pronounced pyramidal form. This, however, is not the final stage. Denudation continues pyramidal hills, dome-shaped heights, and crested ridges gradu- ally crumble down, until at last all abrupt and pro- minent irregularities of surface disappear, and the once elevated plain returns to its former state, that of a gently undulating or approximately flat stretch of low-lying land. The cycle of erosion is completed. Thus in the erosion of a plateau of horizontal strata we recognise the following stages : (i) The excava- tion of deep trenches by streams and rivers ; (2) the gradual sapping and undermining of cliffs, etc., the widening of valleys, and the consequent cutting up of the plateau into a multitude of flat-topped blocks or segments ; (3) the progressive contraction of the seg- ments, and their conversion into pyramidal or round- topped hills and crested ridges ; and (4) the continued reduction and lowering of the hills and final resolution of the plateau into a plain. This plain, in the hypothetical case we have been considering, is supposed to be at a level very little above that of the sea. But the minimum level to which a region tends to be reduced need not be at such a low elevation. The streams and rivers dis- 66 EARTH SCULPTURE charging into a great lake or inland sea cannot erode their valleys below the level of the quiet water which is the receptacle of their sediment. That surface becomes for them a base-level of erosion, and all their energies are employed in the task of reducing to that level the land over which they flow. Soon or late, however, the outlet of the lake will be deepened, the surface of the latter will fall, and the base-level will, of course, be lowered at the same time. But should a slow movement of elevation affect the lower end of the great lake, and thus, by counterbalancing the work of river erosion at its outlet, maintain the surface at approximately the same level for a pro- longed period of time, then denudation may eventually succeed in reducing to that base-level all the lands that drain into the lake. The lake might be entirely silted up, but so long as the movement of elevation persisted, and the river (at the former outlet of the lake) continued to saw its way down as rapidly as the ground was upheaved, the old base-level of erosion would be maintained. We may now return to the Grand Canon district and the question of its erosion. During the progress of the great denudation the interior spaces of the dis- trict, according to Captain Button, " occupied for a time the relation of an approximate base-level of erosion." The whole region has been greatly ele- vated, but this upheaval was not effected all at one time. On the contrary, in place of one single con- tinuous movement a succession of uplifts has taken LAND-FORMS IN HORIZONTAL STRATA 67 place, each separated from the other by a period of repose. It was during one of these prolonged pauses that enormous sheets of strata, averaging some 10,000 feet in thickness, were gradually broken up and re- moved from the surface of the Carboniferous rocks, while the latter themselves were planed down to a flat expanse. This Carboniferous platform served for a long time as a base-level of erosion. The horizontal masses under which it lay buried were first deeply incised by the Colorado River and its affluents and their countless tributaries. The strata thus became broken up into innumerable separate blocks or seg- ments, which, little by little, were reduced in size and most of them eventually demolished. But before the last remaining " buttes " and " mesas " could be re- moved a great change supervened. A general up- heaval of the entire area for several thousand feet took place, and the base-level to which the district had been so largely reduced was destroyed. The gradients of all the rivers now increased, and the velocity of the currents was correspondingly aug- mented, with the result that the erosion of ravines and caftons recommenced. It is beyond the purpose of these pages to trace further the history of the Grand Cafton district. But those who wish to have an adequate conception of what is meant by river erosion would do well to con- sult Captain Button's work. From it they will learn how the Colorado River has, within a very recent geological period, dug out a valley " more than 200 68 EARTH SCULPTURE miles long, from 5 to 1 2 miles wide, and from 5000 to 6000 feet deep." From our present point of view the chief lesson which we derive from a study of the Grand Canon district is simply this : that horizontally arranged strata tend under the action of epigene agents to form flat-topped mesas and pyramidal hills and mountains. The contours of those prominent features and the detailed sculpturing of cliffs and rock-terraces will depend largely upon the character of the strata out of which the hills and mountains are carved, and also to a great extent upon the climate. In a dry elevated tract like that of the Canon district the influence exerted by the petrological character of the strata in determining the detailed features of the ground is everywhere conspicuous. In other regions where moister climatic conditions prevail this influ- ence, although never absent, is yet not so strongly marked. In the foregoing discussion the configuration as- sumed by horizontal strata has been dealt with in such detail that it is not necessary to cite more than a few other examples to show that wherever the same geological structure occurs denudation has resulted in the production of similar land-forms. The lonely group of the Faroe Islands, lying about half-way between Scotland and Iceland, are the relics of what at one time must have been a considerable plateau. They extend over an area about seventy miles in length from north to south, and nearly fifty miles in width from east to west. The original LAND-FORMS IN HORIZONTAL STRATA 69 plateau could not have been less than 3500 square miles in extent. But as the islands have everywhere experienced excessive marine erosion, it is certain that the plateau out of which they have been carved formerly occupied a much wider area. The geological structure of the islands is very simple. They are built up of a great succession of basalts with thin intervening layers of tuff (volcanic dust, etc.) arranged in ap- proximately horizontal strata. The islands are for the most part high and steep, many of them being mere mount- ain-ridges that sink abruptly on one or both sides into the sea. The larger ones show more diversity of surface, but possess very little level land. All have a mountainous character, and, owing to the similarity of the rocks and their arrangement, exhibit little variety of feature. They form as a rule strag- gling, irregular, flat-topped masses, and sharper ridges, that are notched or broken here and there into a series of isolated peaks and truncated pyramids. Sometimes the mountains rise in gentle acclivities, but more generally they show steep and abrupt slopes, which in several instances have inclinations of 25 to 70 EARTH SCULPTURE 27 or even 30. In many places they are yet steeper, their upper portions especially becoming quite pre- cipitous. They everywhere exhibit a well-marked terraced character ; precipices and long walls of bare rock rise one above another, like the tiers of some cyclopean masonry, and are separated usually by short intervening slopes, sparsely clothed with grass and moss, or sprinkled with tumbled rock-rubbish. The coasts are usually precipitous, many of the islands having only a few places where a landing can be effected. Not a few are girt by cliffs, ranging in height from 200 or 300 feet up to 1000 feet, and even in some places exceeding 2000 feet. The best-defined valleys are broad in proportion to their length. Fol- lowed up from the head of a sea-loch, they rise some- times with a gentle slope until in the distance of two or three miles they terminate in a broad amphitheatre- like cirque. In many cases, however, they ascend to the water-parting in successive broad steps or terraces. Each terrace is cirque-shaped, and framed in by a wall of rock, the upper surface of which stretches back to form the next cirque-like terrace, and so on in succession until the series abruptly terminates at the base, it may be, of some precipitous mountain. Occasionally the neck between two valleys running in opposite directions is so low and flat that it is with difficulty that the actual water-parting can be fixed. In such cases we have a well-defined hollow, bounded by precipitous, terraced hill-slopes, crossing an island from shore to shore. Were the land to be slightly LAND-FORMS IN HORIZONTAL STRATA 71 depressed such hollows would form sounds separating adjacent islands, while the valleys that head in cirques would form sea-lochs. There can be no doubt, in- deed, that the existing fiords of the Faroes simply occupy the lower reaches of land-valleys, and that the sounds separating the various islands from each other in like manner indicate the sites of long hollows of the character just described. In a word, the islands are the relics of a plateau of comparatively recent geological age, for the rocks date no further back than Oligocene times. All the land-features are the result of subaerial erosion guided and determined by the petrological character and horizontal arrangement of the strata. The precipitous cliffs of the coast-line owe their origin, of, course, to the undermining action of the sea, the rocks ever and anon giving way along the well-marked vertical joint-planes. In Great Britain horizontal strata occupy no broad areas. But wherever they put in an appearance they yield the same surface-features. Thus in the north-west Highlands we have the striking pyrami- dal mountains of Canisp, Suilven, and Coulmore, carved out of horizontal red sandstones of Pre-Cam- brian age. In Caithness, again, we have the peaked and truncated pyramids of Morven, Maiden Pap, and Smean, hewn out of approximately horizontal Old Red Sandstone strata. Ingleborough is another good example of a pyramidal mountain having a similar geological structure. Many illustrations are likewise furnished by the horizontal strata of other lands. 72 EARTH SCULPTURE Thus pyramidal and more or less abrupt hills, the precipitous sides of which are defined by vertical joints, are common in the horizontally bedded " Quadersand- stein " of Saxon Switzerland. So again in the region of the Dolomites, whenever the strata are horizontal the mountains carved out of them tend to assume pyramidal forms. In a word, we may say that all the world over the same geological structure gives rise to the same land-forms. River-courses hewn in horizontal strata will vary somewhat in form according to the nature of the rocks and the character of the climate. In regions built up of relatively unyielding rocks, or of alterna- tions of these and less resisting beds, the valleys tend to be trench-like, and the mountain-slopes are more or less abrupt. But under the influence of rain, springs, and frost these harsh features are toned down, river-cliffs are benched back, and abrupt ac- clivities are replaced by gentler slopes. Should the strata consist of soft materials throughout, there will be a general absence of harsh features ; round-topped hills and moderate valley-slopes will characterise the land. Nevertheless, whether the strata be " hard " or "soft," thick-bedded or thin-bedded, or show alterna- tions of many different kinds, and whether the climate be arid or humid, equable or the reverse tropical, temperate, or arctic the same general type of surface- features can always be recognised. CHAPTER IV LAND-FORMS IN REGIONS OF GENTL Y INCLINED STRATA ESCARPMENTS AND DIP-SLOPES DIP-VALLEYS AND STRIKE-VAL- LEYS FORMS ASSUMED BY A PLATEAU OF EROSION VARIOUS DIRECTIONS OF ESCARPMENTS SYNCLINAL HILLS AND ANTI- CLINAL HOLLOWS ANTICLINAL HILLS. THE most characteristic land-forms met with in regions where the strata are inclined in some general direction are escarpments and dip-slopes, the former coinciding with the outcrops, and the latter with the inclination or dip of the strata. In such regions some streams and rivers not infrequently flow in the direction of dip, and thus cut across the escarpments, while others may traverse the land along the base of the escarpments. The origin of these phenomena is not hard to trace. Let us suppose that some wide tract of horizontal strata has been elevated along an axis so as to form a considerable island. If the movement of elevation were slowly effected the sea would doubtless modify the land-surface as it arose, but for simplicity's sake we shall ignore such action, and suppose that the new-born land exists as an elongated island, the sur- 73 74 EARTH SCULPTURE face sloping away at a low angle on either side of a somewhat flattened axis. (Fig. 12.) At first, then, the surface coincides with the underground structure a dome-shaped land formed of dome-shaped strata. (Fig. 13.) It is obvious that the drainage will be in FIG. 12. MAP OF AN ISLAND COMPOSED OF DOME-SHAPED STRATA. The strata are inclined in the direction of the arrows. the direction of the dip of the strata all the main rivers will take the quickest route to the sea. But as we cannot suppose that the surface of the new-made land would be without some irregularities, the streams and rivers would not actually follow straight courses. FIG. 13. SECTION THROUGH THE ISLAND SHOWN IN FIG. 12. Slopes of surface coincide with arrangement of strata. On the contrary, it could not but happen that one stream would eventually join another, and in this way many might become tributaries of one or more large rivers. Thus we should have certain courses cut in the general direction of the dip, while others joining these would in some places go with the inclination of LAND-FORMS IN GENTL Y INCLINED STRA TA 75 the strata, and in other places would traverse that at various angles. The strata consist, we shall suppose, of " hard " and " soft " rocks limestones, sandstones, shales, etc., and they are well jointed at right angles to the planes of bedding. Thus, while the strata dip seaward, one set of joints is inclined at a high angle in the opposite direction the other set cutting the strata in the direction of the dip. Now so long as the streams follow the dip it is obvious that they will tend to form trench-like valleys the rocks will be undermined and give way along vertical joint-planes. FIG. 14. SECTION OF RIVER- VALLEY. The valley coincides in direction with the " strike " of the strata, i. <., it trends at right angles to the dip or inclination ; 1 />', Lower and Upper Devonian; C7, Carboniferous Limestone; O, Cretaceous; T, Overfold and thrust-plane. Devonian and Carboniferous strata turned upside down above the thrust-plane. approach each other, while in the intervening space the strata are arched into a great anticline. The beds within the anticline, it will be observed, are much compressed below, while they open out above. This is known as fan-shaped structure. Reverse faults and thrust-planes have been referred to, but it must be noted that normal faults also now and again occur in complicated regions. The former, as we have seen, are the result of horizontal, the latter of vertical movements of the crust. Reversed faults, therefore, are almost entirely restricted to regions I \ 98 EARTH SCULPTURE where the rocks are more or less steeply inclined and contorted. Normal faults, on the other hand, occur under all conditions of rock-structure traversing alike horizontally arranged strata and inclined and folded beds of every kind. So much, then, for the general types of structure met with among highly folded strata. So far as our present knowledge goes, complex folding, such as is FIG. 36. ANTICLINAL DOUBLE-FOLD. seen in true mountains of uplift, has resulted from horizontal movement in one direction. This is shown by the manner in which most of the more closely compressed and steeper folds of a mountain-chain tend to lean over one way. Under the influence of an irresistible horizontal thrust the strata find relief by folding, and the crust bulges upwards, the flexured rocks naturally bending over in the direction of least resistance. The resulting structure may be shown diagrammatically as in Fig. 37. In this diagram only LAND-FORMS IN HIGHLY FOLDED STRATA 99 folds are represented ; in many cases, however, the rocks are not merely flexed, folded, and contorted, but dislocated and displaced. Frequently, indeed, they have yielded to the intense pressure by shearing, and slice after slice, hundreds or even thousands of feet in thickness, has been pushed forward and piled one on top of the other. Although the closer folds tend as a rule to lean over in the direction of crustal movement, yet occasionally they are inclined in the opposite direction, thus giving rise to the well-known FIG 37. DIAGRAM OF MOUNTAIN FLEXURES. The arrow shows the direction of thrust. fan-structure seen in the anticlinal double-fold, Fig. 36. Now and again, too, the folds may open out, and so form symmetrical flexures with vertical axes, or normal anticlines and synclines. The cause of such variations in the folding of the strata is an in- teresting question, but does not concern us here. When a tract of highly disturbed rocks has been ex- posed to erosion for a very prolonged period, it is usually hopeless to attempt to reconstruct the original configuration of the ground, save in a very general way. The primeval land-forms that may have re- sulted from crustal deformation have been entirely remodelled or removed by denudation. But there TOO ^^ EARTH SCULPTURE are many regions where similar extensive deformation has taken place at a relatively recent geological date, and where, therefore, time has not sufficed for the obliteration of all surface-features due to crustal dis- turbance. In the younger mountain-chains of the world, underground structure and orographical fea- tures to a certain extent coincide. The study of these mountains, therefore, enables us to realise the conditions that formerly obtained in tracts of highly complicated structure, from which, under l tinued erosion, all trace of the original configu of the ground has vanished. Not only so, but the havoc wrought by epigene action upon even the youngest of our mountains shows us how and by what means the complicated mountain-chains of earlier days have gradually been reduced. For, just as lands built up of horizontal and gently inclined strata have experienced various degrees of erosion, thus enabling us to trace the successive stages through which such lands must pass, so regions of highly com- plex structure present us with various phases of denud- ation. And thus, by comparing one tract with another, we may spell out the whole story ; and in the degraded relics of former mountain-systems we read the fate that must eventually overtake the proud- est elevations of the present. The study of the land-forms assumed by highly flexured strata should naturally begin with the exam- ination of some young mountain-chain. But even the youngest of such mountains has already under- LAND-FORMS IN HIGHLY FOLDED STRATA 101 gone much erosion, and its struSwre is often ex- tremely complicated. To examine any one system in detail, and to follow the whole process of its denuda- tion, would be a laborious work, far beyond the limits of our present inquiry. All that we desire is to ascer- tain if we can how far geological structure and oro- graphical configuration coincide during the period of a mountain's infancy and early youth, and by what means its original form becomes modified and event- * remodelled. For this purpose we may profitably our study by considering first some hypothetical We shall suppose, then, that under tangential pressure the horizontal strata of some region have bulged up and become folded along a given line or zone. Under such conditions great faults and thrust- planes would be likely enough to occur ; but for the sake of simplicity we shall ignore these, and fix our attention only on the flexing and folding. We shall suppose further that our mountain-chain is the result of one prolonged continuous earth-movement. How, then, will the elevation of the strata affect the sur- face ? Will the complex folding of the rocks give rise to similar intricate deformations of the surface ? This does not necessarily follow, for, were the move- ment of elevation very slow and protracted, the grad- ually rising surface might be so continually reduced by denudation that underground structure and exter- nal form would rarely or never correspond. But, on the other hand, were the rate of elevation in excess of the rate of erosion, the larger folds of the strata 102 EARTH SCULPTURE might be expected to give rise to similar undulations at the surface. It is very doubtful, however, whether the latter would ever be as strongly pronounced as the former ; for at great depths the folds would be pressed closely together, while they would naturally tend to open out upwards into broader undulations. Hence, deeply buried rock-masses might be intensely flexed and folded, while the surface might show only a more or less pronounced bulging. The infant mountain might appear as merely one single long swell or undulation, with smooth slopes, declining at no great angle to the low grounds. Or there might be a series of two or more such undulations. The study of existing mountain-chains, however, leads to the belief that in some cases at least very considera- ble deformation of the surface has accompanied mountain-making, all the larger folds of the strata being probably at first represented above ground by corresponding ridges and depressions. We do not know whether the elevation of a moun- tain-chain was ever suddenly effected. So far as we can judge from the evidence supplied by geological structure, it would seem as if the horizontal move- ments of the crust had been gradual and protracted, and often interrupted by long pauses. There is little reason to doubt, however, that during the growth of a mountain-chain sudden snapping of rocks under pressure must have occurred frequently enough, and that earthquakes of greater or less intensity must have accompanied the upheaval. If such has been LAND-FORMS IN H1GHL Y FOLDED STRA TA 103 the case, it would follow that the surface might be very considerably affected rocks might be shattered and weakly constructed ridges shaken down so that the anticlinal ridges of a mountain-chain might well have presented, even in the days of its infancy, a broken and ruptured surface. But, to return to our hypothetical mountain-chain, we shall suppose this consists of a series of parallel ridges which attain their greatest elevation along a line or axis not far removed from the thrust-side of the chain. From this axis the ridges decline gradually in importance in the direction of earth-movement, and eventually die out in a series of gentle undula- tions. Each of the ridges, we shall suppose, coin- cides with an anticline, and each of the intervening hollows with a syncline. In a word, we shall take the surface to be a more or less exact expression of the geological structure, the undulations of the ground, however, being less pronounced than those of the strata at considerable depths. The diagram (Fig. 37, page 99), will represent a section across such a chain. It will be observed that all faults and possible intru- sions of igneous rock are neglected. In any series of stratified rocks some are sure to be more porous than others, while all will be traversed by joints or cracks approximately at right angles to the bedding-places. This, then, we shall take to be the case with the rocks of which our young mountain- chain is composed ; and we shall suppose that the parallel ridges extend in a linear direction for many io 4 EARTH SCULPTURE miles, gradually declining in elevation towards both ends of the chain. With these conditions of surface, it is obvious that drainage will take place in the di- rection of the great longitudinal valleys or synclinal troughs, while a set of transverse streams will flow down the slopes of the anticlinal ridges. Many of these will thus become tributary to the rivers making their way along the axial hollows. All the rivers in course of time must cut into the rocks, but it is obvi- ous that the transverse streams will be of a torrential character, and will tend therefore to carve out nar- rower, deeper, and straighter channels than the larger rivers can excavate in the less inclined, broad axial depressions. Immense quantities of rock-material will be swept down from the anticlinal ridges to accumulate in heaps and sheets in the synclinal troughs, or to be swept away more readily, according as the gradients of the latter are gentle or steep. Erosion, in short, will be carried on most actively upon the anticlinal mountains. This would naturally follow, whatever the character of the geological struc- ture might be, for the erosive action of running water increases with the gradient. But in all cases denudation is hastened or retarded according as the rock-structure is weak or strong. If, therefore, the mountains of our hypothetical chain be more weakly built than the parallel synclinal troughs, the former will tend to be reduced more rapidly than the latter. This can be shown diagrammatically as in Fig. 38, p. 105. Here we have a section across two LAND-FORMS IN HIGHLY FOLDED STRATA 105 anticlinal mountains and a synclinal valley. The strata consist of a series of more or less porous sandstones separated by intervening layers of impermeable clay- rocks. Moreover, they are jointed, and the joints traversing the anticlines tend to open out upwards, while the reverse is the case with those cutting the synclines. Some of these joints may be shrinkage- cracks which came into existence during the slow con- solidation of the strata, perhaps long before the latter were flexed and folded. But a large proportion no doubt would be produced while the rocks were being bent and doubled up. In whatever way formed, joints are readily permeated by meteoric water, which finds its way down from the surface and soaks into FIG 38. DIAGRAM OF ANTICLINAL MOUNTAINS Pervious strata (stippled) and impervious layers (thin lines) ; //, joints, cutting strata at right angles ; v, valley ; s j, springs coming out at junction of pervious and impervious beds. the porous strata below. Constantly augmented from above, the water thus imbibed is forced to percolate through the porous beds in the direction of the dip. Hence wherever these beds are truncated (as in the valley) the water comes out at the surface as natural springs. Thus in the illustration springs appear at s s, where permeable sandstones are underlaid by im- io6 EARTH SCULPTURE permeable clay-rocks. The effect of these springs is not hard to understand. They tend to undermine the sandstones, and as the dip of the strata is towards the valley, rock-falls and landslips must continue to take place until the anticline is reduced. Anticlinal mountains separated by a synclinal trough are thus in a state of unstable equilibrium. Sapped and un- dermined by rain, frost, and springs, their existence FIG 39. SYNCLINAL VALLEY SHIFTING TOWARDS ANTICLINAL Axis. a, synclinal valley ; 5 a II S I a >* Q 8 O H Pti o ./ of syn- II* EARTH SCULPTURE * =3 to - 15 O 13 W3 C 8 1 2 9 <: clinal troughs forming conspicuous mount- ains, while the intermediate anticlines cor- respond for the most part with valleys and depressions. If it be true, therefore, that the denuda- tion of young mountains, such as the Alps and the Carpathians, has been guided and determined to a large extent by geological structure, we ought to meet with still stronger evidence of a like kind in mount- ain-ranges of greater antiquity. The mountain-systems we have been consider- ing are of Csenozoic age ; they are among the latest great upheavals of the world. We see in the Appalachian Chain of North America a very much older system, for it came into existence about the close of Palaeozoic times. Being of such enormous antiquity, the Appalachians ought to give evidence of correspondingly great denuda- tion. All the weak geological structures should have collapsed and disappeared ages ago ; the heights ought not to coin- cide with anticlines. The accompanying section across a portion of the chain in Pennsylvania shows that this has actually happened, symmetrical synclines having as usual developed into hills, while anticlines have been degraded. Similar evidence might be adduced from LAND-FORMS IN HIGHLY FOLDED STRATA 119 many other regions, but enough has been advanced to show that in the process of erosion and denudation of mountains of uplift, anticlines, as compared with synclines, are essentially weak structures. When the flexures are symmetrical the synclines tend to be carved into hills, but when the axes are inclined the strata often give rise to a series of prominent escarp- ments or to a succession of ridges with intervening hollows, the escarpments and ridges corresponding to the outcrops of the more resistant rocks. (Fig. 55.) Comparing mountain-chain with mountain-chain, we find, as might have been expected, that the oldest mountains, if they are the least prominent, are at the same time the most stable. They have endured so long that much of their primeval elevation has been lost ; the weakly built structures have been demolished, and only the stronger now remain. Great rock-falls and landslips are therefore seldom heard of among such mountains. It is quite otherwise with the younger uplifts of the globe. The valleys of the Alps, the Caucasus, the Himalayas, the Cordilleras, and other chains of relatively recent age are cumbered with chaotic heaps of fallen rock-masses. From time to time peaks and whole mountain-sides collapse and slide into the valleys ; and this rapid degradation will continue until every weak structure has been removed. The hills and mountains of our own country have long since passed through this phase of unstable equilib- rium. In the younger mountain-chains of the globe underground structure and superficial configuration 120 EARTH SCULPTURE still to a certain extent coincide, but in the more ancient and therefore more highly denuded mountain- systems such coincidence is of very rare occurrence. Anticlinal mountains built up of porous and relatively impermeable strata are restricted to regions of recent uplift, and have no long life before them. We have seen that in the case of plains and plateaux of accumulation the original surface of the ground is an expression of the geological structure, the general direction of their drainage-systems being determined FIG. 55. UNSYMMETRICAL FOLDS, GIVING RISE TO ESCARPMENTS AND RIDGES. h h, hard beds ; J s, soft beds. by the average inclination of the strata. The same is no doubt to a large extent true of regions of mount- ainous uplift ; the shape of the surface and the direction of the streams and rivers must at first have been determined by the arrangement or architecture of the rocks. But while it is comparatively easy to realise the conditions that obtained in a plateau-coun- try during the early stages of its existence, it is very much harder to picture to ourselves the general aspect which a mountain-chain must have presented at the time of its upheaval. We are justified by the evidence LAND-FORMS IN HIGHLY FOLDED STRATA 121 in believing that the larger inequalities of the surface must often have coincided with corresponding flexures and other deformations of the strata. But we need not suppose that all the convolutions, fractures, and displacements now laid bare in precipice and gorge actually appeared as such at the surface. Laboratory experiments have shown that a great deal of flexing, folding, contortion, and displacement may take place underground, while the surface simply swells up or bulges. And that may quite well have been the case with many mountain-chains. Yet we cannot ignore the possibility or probability that folding and displace- ment of strata may sometimes have resulted in whole- sale rupture and confusion at the surface. We need not wonder, therefore, if we sometimes find it hard to account for certain vagaries in the drainage-systems of mountain-chains. Even the youngest of these chains has experienced so much denudation, that it is often impossible to realise the surface-conditions which may have determined the initial directions of the rivers. The longitudinal watercourses doubtless follow the axial arrangement of the strata, some of them occupying structural hollows (synclines), while others run along the backs of anticlines, or follow the outcrops of relatively softer rocks. The origin of certain transverse river-courses is harder to under- stand. Some of these may cut across a succession of great ridges ; they break through the mountains in such a way as to suggest that they are perhaps follow- ing a line of fracture. Most commonly, however, this 122 EARTH SCULPTURE is certainly not the case. Sometimes it can be shown, as already indicated, that a transverse stream has simply eaten its way back into the heart of the mountain-ridge, which it has eventually breached or 11 gapped," and so worn down as to encroach upon the drainage-area of some adjacent longitudinal valley. Transverse streams working back in this way have not infrequently captured longitudinal rivers, which thus appear to mysteriously forsake their own valley in order to break through a mountain-ridge. Perhaps most of the sudden changes in direction of Alpine rivers are illustrations of this system of capture. It is possible, however, as some geologists have sup- posed, that certain transverse river-courses may have been determined by the presence of a series of minor crustal folds, arranged at right angles to the main or longitudinal flexures of a mountain-chain. But we know so little of the actual conditions of surface that obtained when such a chain was being upheaved, that we must often be content to remain in ignorance of the causes that may have led to the sudden deflection of a river across a mountain-ridge. When we bear in mind, however, that the present lines of drainage can agree only in a general way with those that came into existence at the birth of a chain that many anticlinal arches, now laid bare and deeply eroded, may never have shown at the original surface it is not hard to understand how certain transverse river-courses may have come to intersect a succession of ridges. In many cases such courses may really indicate the LAND-FORMS IN HIGHLY FOLDED STRATA 123 primeval inclination of the ground, the rivers having cut their way at first without any reference to deeply buried structures, which were only to be exposed later on during the general process of denudation. Although we may vainly endeavour to trace the history of all the river-courses of a mountain-chain, we need be in no doubt as to the ultimate fate of the mountains themselves. It is more difficult certainly to discover the various stages in the erosion of a mountain-system than in that of a plateau of accu- mulation ; but we are assured that all elevated lands, whatsoever their origin, tend to be lowered to their base-level. Should that base-level be steadfastly maintained, mountains and plateaux alike must event- ually be reduced to the condition of plains of erosion. But the modifications of the surface of a mountain- region developed during the process of erosion are infinitely complex. This is due partly to the very varied composition of the rocks, and partly to the complicated geological structure. The surface-features of a denuded plateau of accu- mulation have a general sameness ; there is little variety in the form of the hills and mountains all are more or less pyramidal. In regions of gently inclined and undulating strata the features due to erosion are more diversified, and this diversity be- comes greater as the dips of the strata increase and change rapidly in direction. The foothills that flank the base of so many mountains of uplift are com- posed very often of symmetrically folded strata, but as i2 4 EARTH SCULPTURE we pass inwards to the main chain the folds become steeper and unsymmetrical, and the structure is rendered still more complex by vast overthrusts and shearing-planes. As the structural complexity increases, and the rocks are thrown and twisted into every possible position, the surface-features are con- stantly, changing, so as to show, often within narrow limits, every variety of cliff and ridge and peak. We see then that it is geological structure chiefly that determines the form of the ground ; and since the inclination, the folding, and the shearing of rocks must be attributed to crustal movement, it is clear that hypogene action has played a most important part in the formation of mountains. We may say with truth that all true mountain-ranges owe their origin to deformation of the crust. But the shape which they ultimately assume is solely the result of erosion. It is hypogene action which provides the rough blocks ; it is by epigene action that these are subsequently carved and chiselled, the forms of the sculptured masses being determined by the nature and structure of their materials. In regions of recent uplift, the pro- cess of sculpturing, although considerably advanced, has not yet sufficed to obliterate the original or primeval shape of all the masses. But in elevated tracts of great antiquity the land-blocks have been entirely remodelled. In the general lowering of the surface by denudation, mountain-masses have been removed, and what were formerly depressed areas now often appear as dominant elevations. Mountains LAND-FORMS IN HIGHLY FOLDED STRATA 125 of recent uplift are characterised by steep profiles, by peaks and knife-edged aretes ; the structures are often unstable, and yield readily to the agents of erosion, so that rock-falls and landslips are constantly taking place. In regions of ancient uplift, on the other hand, the profiles are generally softer; peaks and sharp-crested ridges are of less frequent occur- rence, weak structures have disappeared, and the degradation of the mountains does not advance so rapidly. The levelling process, however, though slower, is quite apparent. The valleys are widened and deepened, the mountains crumble down, and, should the base-level of erosion be retained, the whole area will eventually be flattened out and resolved into a plain of erosion. Such then are the several stages through which a region of mountain-uplift must pass. First comes the stage of youth, when the surface configuration corre- sponds more or less closely with the underground structure. Next succeeds the stage of middle-life, when such coincidence is all but obliterated, when the valleys of youth have been exalted and its mountains have been laid low. Last comes old age and final dissolution, when the whole region has been reduced to its base-level. But the decay of a mountain-chain does not always proceed without interruption. Not infrequently the base-level is disturbed ; new hori- zontal movements of the crust take place, and bulging- up of the region is accompanied by further folding and fracturing of the strata. The mountain-system 126 EARTH SCULPTURE renews its youth. On the other hand, the old base- level may be destroyed by subsidence of the crust, and the mountains, partially or wholly drowned, may in time become largely buried under new accumula- tions of sediment. Re-elevation taking place, erosion recommences, and the degradation of the region is resumed. In the structure of not a few mountain- FIG. 56. STRUCTURE OF THE ARDENNES (after Cornet and Briart). MM, the existing surface ; the light-shaded area above this level represents the rock-masses removed by denudation. The Silurian rocks at the base of the section are indicated by thin white lines. Above these, on the left-hand side of the section, between C and M^ come Devonian conglomerate, sandstone, shale, and limestone ; next in succession follow the Carboniferous strata at and above M ; A A^ B B, C C, are dislocations. chains we may read the history of many such vicissi- tudes. So completely have some mountains been removed LAND-FORMS IN HIGHLY FOLDED STRATA 127 by denudation, that without some knowledge of geo- logical structure we should never have divined their former existence. An instructive example is furn- ished by the Carboniferous tracts of Belgium and Northern France. The structure of these regions shows that formerly a considerable range of moun- tains extended between Boulogne and Aix-la-Cha- pelle. At or towards the close of Carboniferous times a great earth-movement, acting in a direction from south to north, buckled up the strata, and these, yielding to the pressure, snapped across, and exten- sive overthrusting followed along the line referred to, the Carboniferous beds being inverted and overlaid by Devonian strata. The mountains of upheaval which thus came into existence attained a great elevation, the higher parts of the range reaching probably not less than 16,000 or 18,000 feet. The section (Fig. 56) will show how completely the sur- face has been remodelled, how mountains of elevation have been replaced by a plain of erosion. CHAPTER VI LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DISTURBED STRATA (continued} STRUCTURE AND CONFIGURATION OF PLATEAUX OF EROSION FORMS ASSUMED UNDER DENUDATION MOUNTAINS OF CIR- CUMDENUDATION HISTORY OF CERTAIN PLATEAUX OF ERO- SION SOUTHERN UPLANDS AND NORTHERN HIGHLANDS OF SCOTLAND STAGES IN EROSION OF TABLE-LANDS. IN our last chapter we considered the history of a mountain-chain, following that history from the stage of youth to old age and final dissolution. This last we recognised in the plain of erosion. We have next to trace the subsequent history of such a plain. The geological structure of many mountain-chains, as already indicated, reveals the fact that these are often the result of more than one uplift. After having been for long ages subjected to erosion, and even to sub- sequent subsidence and sedimentation, the same region has again yielded to lateral crush, and new series of folds and thrust-planes have come into existence. But the crust does not always yield in this particular fashion. Not infrequently relief from pressure is ob- tained by widespread bulging-up of the surface, one or more broad swellings with perhaps corresponding 128 LAND-FORMS IN HIGHLY FOLDED STRATA 129 broad depressions appear, instead of an intricate ar- rangement of more or less closely compressed folds. We may for convenience' sake speak of the latter as resulting from axial uplift, and of the former as due to regional uplift, even although it be obvious that in most wide regions of uplift there must be an axis or line of maximum movement. Now it can be shown that one and the same region has not infrequently experienced both kinds of uplift. Axial uplifts have in time been succeeded by regional uplifts ; for again and again we encounter ancient FIG 57. DIAGRAMMATIC SECTION ACROSS A PLATEAU OF EROSION. Isoclinal folds. plains of erosion occurring at various levels above the sea, their geological structure showing clearly that they have replaced old mountains of complicated structure. Such elevated plains may be termed plat- eaux or table-lands of erosion, to distinguish them from plateaux of accumulation or deposit. The characteristic feature of the latter, it will be remem- bered, is the general coincidence of the surface with the underground structure, while the former shows no such correspondence. The structure of a table- land of erosion may thus be represented as in Fig 57. Many such table-lands are recognised in Europe, the Highlands and Southern Uplands of Scotland 130 EARTH SCULPTURE and the Scandinavian plateaux being good examples. Ancient plateaux of the kind are all more or less de- nuded, trenched, and furrowed by valleys to such an extent that the plateau character is often somewhat obscured. For no sooner is a plain of erosion up- lifted than a new cycle of erosion begins. The di- rection of the drainage is determined, in the first place, by the slope of the ground, and this we can readily understand may be somewhat diversified. The surface may be canted either in one direction only, or in more than one, for the crustal movement is un- likely to be equal in amount throughout the whole region of uplift. Hence, the primeval rivers may all flow in one particular direction, or they may trend to various points of the compass. However that may be, it is certain that in course of time they must gradually deepen their valleys, and the plateau must eventually come to be cut up very much in the same way as a plateau of accumulation. But the mountains of circumdenudation resulting from this process will differ considerably in character from those carved out of horizontal strata. The varying structure of the rocks will necessarily influence erosion, and thus lead to a greater diversity of form. Should the strata be steeply inclined, and this will usually be the case, then it is obvious that the harder masses must come in time to project beyond the more readily reduced rocks with which they are associated. The general surface of the plateau will thus tend to assume a cor- duroy configuration, the long ridges coinciding with LAND-FORMS IN HIGHLY FOLDED STRATA 131 the outcrops of the " harder rocks," while the inter- vening parallel hollows will correspond with the out- crops of the more yielding strata. In short, the land-features evolved by denudation will have a gen- eral resemblance to those produced in a region of slightly inclined and gently undulating formations. But owing to the very varied character of the rocks; and their more complicated structures, the surface- features of a plateau of erosion will be more pro- nounced and much more irregular. In such a region the larger rivers, being frequently of primeval origin, will often be found to cut across mountain-ridge after mountain-ridge, and to follow courses more or less transverse to the corduroy surface. Others may keep closely to the outcrops, and run in the direction of the "strike" or trend of the strata, while some may take now one route and now another. The original surface of the plateau will generally be indicated by the direction of the main drainage-lines or principal rivers, while the subsequent slopes due to erosion will usually be manifested by the course of tributary streams. During the progress of denudation, how- ever, many modifications of the drainage will be brought about. Cases of the capture of principal rivers by lateral streams working their way back or across the strike can hardly fail to occur, and these and other changes may render the original drainage- lines obscure and hard to trace. To such an extent have many ancient plateaux of erosion been denuded, so deeply have they been i 3 2 EARTH SCULPTURE trenched, that their surface has become resolved into a truly mountainous region, wherein all the elevations are mountains of circumdenudation, the tops of which are the only remaining relics of the original plateau- surface. Such mountains, owing generally to the durability of their rocks and the strength of their structure, are not so readily demolished as the moun- tains in a range of recent uplift. They may not often emulate these in height and grandeur, their profiles may as a rule be less wild and irregular ; but such is not always the case. When a plateau of erosion stands at a great elevation, the mountains carved out of it are apt to rival the boldest and most abrupt of Alpine heights. Such abrupt slopes and the profound valleys that intervene are the result of relatively rapid and powerful vertical erosion. But when a plateau has only a moderate elevation, the configuration of its mountains tends to be less abrupt, and to approximate in character to that attained by a true mountain-chain during the period of its maturity, when all weak structures have been demolished and the surface no longer coincides with the folds of the strata. And this is just what might have been expected, when it is borne in mind that in each case the fundamental geological structure is the same. A mountain-chain is composed mainly of highly flexed and folded rocks. Subjected to erosion, the whole region is remodelled and eventually reduced to a base-level. But the rock- structure remains ; the plain of erosion is composed, just as the mountains were, of highly flexed and folded LAND-FORMS TN HIGHLY FOLDED STRATA 133 rocks. When that plain is uplifted en masse to form a plateau it is obvious that epigene action must tend to evolve out of the plateau mountains and ridges which, in their form and alignment, will closely re- semble those that existed over the same area before the old plain of erosion had come into existence. The rocks and rock-arrangements, being the same in both cases, must under denudation tend to produce a sim- ilar configuration. No doubt there might be certain contrasts, but these would not be due so much to geological structure as to changes in the character of the rocks. The planing away of great mountain- masses might well expose quite a different series of rocks, and these, when the region was again uplifted and carved into hill and valley, would doubtless weather differently from the rock-masses under which they formerly lay buried. But the general geological structure remaining the same, mountains and ridges would necessarily be developed along the old lines. We may now consider the structure of certain plat- eaux of erosion which there is every reason for be- lieving existed at one time as plains plains which had previously replaced mountain-systems. A good example is ready to our hand in the Southern Uplands of Scotland that belt of high ground which is drained by the Clyde, the Doon, and other streams flowing north-west, and by the Cree, the Dee, the Nith, and the Annan flowing south-east. The north-east sec- tion of the region is traversed by the Tweed, with an easterly to north-easterly course ; while the extreme i 3 4 EARTH SCULPTURE south-west portion is watered by the Stinchar, flowing in a south-west direction. The whole area drained by those rivers and streams might be described as a broad undulating plateau, furrowed and trenched by narrower and wider valleys. The mountains are some- what tame and monotonous flat-topped elevations with broad, rounded shoulders and smooth grassy slopes. The rocks composing the region consist for the most part of greywackes and shales, the former being usually hard greyish-blue rocks arranged in beds of variable thickness. They are much more abundantly developed than the shales which are asso- ciated with them, although now and again the latter attain considerable importance. The strata usually dip at high angles, often approaching the vertical, and, the same beds coming again and again to the surface, it is obvious that we are dealing here with a vast suc- cession of steeply inclined and closely pressed anti- clinal and synclinal folds. In many natural exposures, as on the coast and in the valleys, the intensely folded character of the strata is clearly revealed. Obviously the strata have been squeezed together, and affected in precisely the same way as the rocks of the Alps. Frequently, indeed, we find that overthrusting has taken place, the rocks having yielded to tangential pressure by shearing. The general trend or " strike " of the strata is from south-west to north-east, while the dip is sometimes north-west, sometimes south-east, changing now and again very rapidly, at other times remaining constant for long distances. In the former LAND-FORMS IN HIGHLY FOLDED STRATA 135 case the folds are not infrequently approximately symmetrical ; in the latter they are necessarily un- symmetrical. In a word, the geological structure is that which characterises all mountains of elevation like the Alps. Nor can we reasonably doubt that when the folding and fracturing took place the crust bulged up and a series of superficial ridges and hol- lows a true mountain-chain came into existence. That was a very long time ago, however, for the up- lift dates back towards the close of Silurian times. Then followed a protracted period of denudation, during which our mountains of folded rocks must have passed through the various stages of adolescence, maturity, and old age. Much of the region was re- duced to the condition of a low plain, diversified in part by swelling hills of less and greater height. All this work had been accomplished, and the degraded hills were continuing to crumble aw r ay, when the whole region was once more uplifted, and so converted into a table-land or plateau with an undulating surface. This movement of elevation had been completed, and renewed erosion had furrowed and trenched the plateau to some extent, before the beginning of Old Red Sandstone times, for the lowest or bottom beds of the Old Red Sandstone series here and there oc- cupy valleys carved out of the underlying Silurian greywacke and shale. To what extent the plateau was submerged during the Old Red Sandstone period we cannot tell. Probably the submergence was great- est over the north-east portion of the region, for it is '36 4 1 1 *" 1' ll ^ ' B 1 < is e! ^ p \ g ^^ o \ p i K . 5p rf| ^2 D o / ^^ 2 2 1 ,1 1 a oj ! C/2 ^ 1X4 ^ O J^ .% EARTH SCULPTURE in that quarter that we meet with the most extensive and continu- ous accumulations of Old Red Sandstone rocks. Be that as it may, we know that some time before the succeeding Carbon- iferous period re-elevation en- sued and a new cycle of erosion was inaugurated, during which the Old Red Sandstone rocks and the underlying Silurian strata were more or less pro- foundly denuded. Thereafter followed an epoch of renewed subsidence on a more extensive scale, when much of the plateau was drowned in the Carbonifer- ous sea, and marine sediments of that age were distributed over areas which had probably never been overflowed by the waters of Old Red Sandstone times. Judging from the present dis- tribution of the Carboniferous strata, it seems likely that the plateau was, as before, more deeply submerged towards north- east and south-east than in other directions. So far as we can tell, the region has never since been LAND-FORMS IN HIGHLY FOLDED STRATA 137 depressed below the sea, but in succeeding Permian and Triassic times long stretches of inland lakes or seas penetrated into the heart of the plateau, occupy- ing hollows which were certainly in existence during the preceding Carboniferous period. Such, without going into details, is a general out- line of the chief changes which have taken place in the Southern Uplands of Scotland. A plateau which came into existence towards the end of the Silurian period might well be expected to show a highly de- nuded aspect. It is true that during Old Red Sand- stone and Carboniferous times it was considerably depressed, and so escaped much erosion, but in the intervals separating those stages denudation must have been in active progress, as it has continued to be since the final disappearance of marine conditions. No doubt much rock has been removed from the whole surface of the region in question. Not only have wide and deep valleys been excavated, but the broad-backed hills and mountains can hardly fail to have been greatly reduced in height. It is still pos- sible, however, to trace the general configuration of the original surface. The average slope of the plateau appears to have been towards the south-east. This is indicated by the direction of the principal rivers the Annan, the Nith, the Ken, and the Cree. It is fur- ther shown by the distribution of the Old Red Sand- stone and later geological formations. Thus strata of Old Red Standstone and Carboniferous age oc- cupy the Merse and the lower reaches of Teviotdale, 138 EARTH SCULPTURE and extend up the valleys of the Whiteadder and the Leader into the heart of the Silurian uplands. In like manner Permian sandstones are well developed in the ancient hollows of Annandale and Nithsdale. Along the northern borders of the Southern Uplands we meet with similar evidence to show that even as early as the Old Red Sandstone period the ancient plateau along what is now its northern margin was penetrated by valleys that drained towards the north. But the main water-parting then, as now, lay not far south of this northern margin * ; in other words, the surface of the ancient plateau, a few miles back from its northern boundary, sloped persistently towards the south-east. Now the strike or general trend of the strata throughout the whole of these Uplands is south-west and north-east. We cannot doubt, there- fore, that when the ancient plain of erosion was up- lifted, and so became a plateau, the surface would be marked by many more or less well-defined ridges and hollows, probably none very prominent, but all hav- ing a north-east and south-west trend. The average slope of the surface being towards south-east, the 1 Many modifications of the drainage have been effected which cannot be re- ferred to here. It may be pointed out, however, that the head-waters of the Nith flow towards the north until they reach the broad Nithsdale, whence the drainage is directed south-east, so that Nithsdale may be said to cut right across the Uplands from north-west to south-east. This is probably a case of capture, the Nith, working back, having gradually invaded the northern drainage-area and captured such streams as the Afton and the Connel. The Clyde and the Doon are the only rivers of any size which have preserved their north-westerly course, and the head-waters of the former have just escaped capture by the Tweed. LAND-FORMS IN HIGHLY FOLDED STRATA 139 flow of the principal rivers would follow that direction, they would cut their channels across the outcrops of the strata. But the " corduroy "character of the plateau would now and again lead to occasional de- flections, while some streams and rivers would be conducted for long distances parallel to the strike of the strata. In a word, two sets of principal valleys would tend to be formed, namely, transverse and longi- tudinal valleys. Examples of the former have already been cited, such as the Cree, the Ken, and the Nith, and amongst the better-known longitudinal valleys may be mentioned those of the Teviot, the Ettrick, and the Yarrow. But a glance at any good map of the region will show that all the more important streams have a tendency to flow either in a transverse or a longitudinal direction, while many run now in one of these directions and now in the other. The Southern Uplands thus prove to be merely a highly eroded plateau. Their geological structure shows that towards the close of Silurian times the greywackes and shales were buckled up, folded, and faulted, and doubtless appeared at first as a range of true mountains of elevation. Thereafter followed a prolonged period of erosion, interrupted, it is true, at successive stages by partial submergence, but result- ing finally in the demolition of the old mountains of elevation and the conversion of the tract into a plain of erosion. Then came a final regional uplift, when that plain was converted into a plateau, which still exists, but in a highly denuded and eroded condition. 140 EARTH SCULPTURE The Northern Highlands of Scotland might be cited as another plateau of erosion with a somewhat similar geological history. There, as in the south, there is evidence to show that vast earth-movements resulted, towards the close of Silurian times, in the formation of great mountains of elevation. The thrust-planes visible in the north-west part of that region are on a much more extensive scale than those met with in the Southern Uplands. Probably the mountains of elevation which appeared over the site of the present Highlands were loftier and bolder than the pre-Devonian heights of Southern Scotland. They may quite possibly have rivalled the Alps in grandeur, for the folding and general disturbance of the rocks are quite as remarkable as the confusion seen in the mountains of Switzerland. We may well believe that when the Highland mountains first up- rose, their external form and internal structure would more or less closely coincide. No sooner had they come into existence, however, than the usual cycle of erosion would commence, and it is certain that after a prolonged interval they were to a large extent re- duced to their base-level much of the formerly ele- vated area acquiring the character of a plain of erosion. Subsidence next ensued, and that plain became grad- ually overspread with sediment, several thousand feet of Old Red Sandstone strata being deposited on the planed and abraded surface of the ancient rocks. At a subsequent date the whole region was uplifted and converted into dry land, forming a plateau country, LAND-FORMS IN HIGHLY FOLDED STRATA 141 which, so far as we know, has never since been com- pletely submerged, although it may well have ex- perienced many oscillations of level. It is out of that ancient plateau that the Highland mountains have been carved. The original surface- slope is, as usual in such cases, indicated partly by the direction of the principal drainage-lines and partly by the summits of the mountains, which decline in eleva- tion as they are followed outwards in the direction of the chief lines of drainage. Again, the main water- partings separating the more extensive drainage-areas of the country mark out in like manner the dominant portions of the same old plateau-land. The water- parting of the North-west Highlands runs nearly north and south, keeping quite close to the western shore, so that nearly all the drainage of that region flows inland. The average inclination of that section of the Highlands is therefore easterly, towards Glen- more and the Moray Firth. In the region east of Glenmore the land slopes in the directions followed by the rivers Spey, Dee, and Tay. These two regions the North-west and the South-east Highlands- are separated by the remarkable depression of Glen- more, running through Lochs Linnhe, Lochy, and Ness, and the further extension of which towards north-east is indicated by the straight coast-line of the Moray Firth as far as Tarbat Ness. This long de- pression marks a line of fracture and displacement of very great geological antiquity. The old plateau of the Highland area was fissured and split in two, that 142 EARTH SCULPTURE portion which lay to the north-west sinking along the line of fissure to a great but unascertained depth. 1 Thus the waters that flowed down the slopes of the north-west portion of the fractured plateau were dammed by the long wall of rock that rose upon the south-east side of the fissure, and compelled to flow off to north-east and south-west along the line of dis- placement. The erosion thus induced sufficed in course of time to hollow out Glenmore and all the mountain-valleys that open upon it from the west. The dominant portion of the ancient plateau east of the great fault is approximately indicated by a line drawn from Ben Nevis through the Cairngorm and Ben Muich Dhui Mountains to Kinnaird Point. North of that line the drainage is towards the Moray Firth ; east of it the rivers discharge to the North Sea ; while an irregular winding line, drawn from Ben Nevis eastward through the Moor of Rannoch, and southward to Ben Lomond, forms the water-parting between the North Sea and the Atlantic, and probably marks approximately another dominant area of the fractured table-land. The geological structure of the Highlands agrees so far with that of the Southern Uplands, that the dominant "strike" of the strata is south-west and north-east. This, therefore, is the trend of the flexures and folds and of all the larger normal faults and great 1 It is probable that movements have taken place again and again at different periods along this line of weakness, and these movements may not always have been in one direction. LAND-FORMS IN HIGHLY FOLDED STRATA 143 thrust-planes. Now such a structure would naturally determine the disposition of the surface-features worked out by erosion. Before the beginning of the Old Red Sandstone period, the pre-existing mount- ains of uplift had been largely degraded to a base- level. Much of the region, in other words, had been converted into a plain of erosion, which subsequently became depressed and buried under thick accumula- tions of sediment, derived in chief part from the de- nudation of such parts of the Highland area as still remained in the condition of dry land. After the deposition of the Old Red Sandstone the whole region was elevated en masse, and converted into a plateau or table-land. The surface of that plateau would doubtless be somewhat undulating and diversified. Probably the " stumps " of the highly denuded mount- ains, which had supplied materials for the formation of the Old Red Sandstone, still formed dominant areas. But wide regions had been planed down, and these would be marked by a kind of " corduroy " structure parallel lines of escarpment and ridges with intervening hollows, corresponding to the suc- cessive outcrops of " harder " and "softer" rocks. The regions overspread by the Old Red Sandstone, on the other hand, would be approximately level, sloping gently, however, towards the north, north- east, and south-east. We may, therefore, conceive the surface of the ancient Highland Plateau to have been from the first more irregular than that of the Southern Table-land. The primeval rivers would 144 EARTH SCULPTURE doubtless follow the average slopes of the plateau, and would thus sometimes cross the outcrops at all angles, and sometimes flow in the direction of the strike for longer or shorter distances. The great de- pression on the line of the Caledonian Canal, although partially rilled with the sediments of Old Red Sand- stone times, probably still formed a well-marked feature at the surface of the plateau when this was first uplifted. And the same may well have been the case with many other lines of fracture. In short, although the average slope of the ground determined the general direction of the drainage, the corrugated and often much diversified surface of the plateau must have led to endless deflection of the water-flow. Again, as erosion proceeded, and the valleys were cut deeper and deeper, many modifications of the drainage would naturally arise, cases of the " capture" of one stream by another having been of common occurrence. It is not, however, with the history of such changes that we have to do, but rather with the character of the existing valleys and mountains which have been carved and chiselled out of the ancient plateau. Of the valleys it may be said in general terms that they are all valleys of erosion. Many have been hollowed out along the outcrops, and are thus longitudinal, while others have been cut out across the " strike," and to this extent are transverse. Some of the former are of primeval antiquity : they correspond in direction not only with the strike of the strata, but with what seems to have been the original slope of the plateau, LAND-FORMS IN HIGHLY FOLDED STRA TA 145 the valley of the Spey being the most conspicuous ex- ample. The transverse valleys, represented typically by Glen Garry and the valley of the Tay, are obvi- ously also of great age, since they in like manner in- dicate the general slope of the plateau in the regions where they occur. A large proportion of the longi- tudinal valleys that drain into these transverse valleys are in all probability of subsequent origin, although some of them may have been outlined at as early a date as the latter. Although none of the longitudinal valleys can be described as synclinal, they may all nevertheless be termed structural, inasmuch as they coincide with the strike of the rocks. So likewise we may term Glenmore a structural hollow, since it occurs along a line of fracture ; and the same is the case with Glen Docherty and Loch Maree. These lines of fractures no doubt showed at the surface of the plateau when it wa's first uplifted, and so determined the di- rection of drainage and erosion. But all the valleys as we now see them are valleys of erosion, their di- rection having been determined sometimes by the average slope of the plateau, sometimes by the geo- logical structure. The mountains of the Highlands are likewise monu- ments of erosion, owing their existence as such some- times to the relative durability of their materials, sometimes to their geological structure, or to both causes combined. They are all, without exception, subsequent or relict mountains. Thus, in the follow- ing section from Glen Lyon to Carn Chois we see 146 EARTH SCULPTURE that the present configuration of the surface does not coincide with the complicated underground structure. It is the same, indeed, throughout all the Highland area. Take a section across any portion of that region, and you shall find that the more continuous " ranges " are developed along the outcrops they are, in short, escarpment mountains. So great has been the erosion, however, within such " ranges," that their alignment usually becomes obscured, and we are con- fronted by confused groups of mountains, drained by streams flowing in every possible direction. " Any Ckii FIG. 59. SECTION FROM GLEN LYON TO CARN CHOIS. (Geol. Survey.} m t mica-schist, etc. ; A limestone ; gr, greywacke, etc. ; j, amphibolite schist ; g, granite ; <, diorite ; /, fault. wide tract of the Highlands," as we have elsewhere remarked, "when viewed from a commanding posi- tion, looks like a tumbled ocean, in which the waves appear to be moving in all directions. One is also impressed with the fact that the undulations of the surface, however interrupted they may be, are broad ; the mountains, however much they may vary in their configuration according to the character of the rocks, are massive and generally round-shouldered, and often somewhat flat-topped ; while there is no great dis- parity of height amongst the dominant points of any individual group. Let us take, for example, the knot LAND-FORMS IN HIGHLY FOLDED STRATA 147 of mountains between Loch Maree and Loch Tor- ridon. There we have a cluster of eight mountain- masses, the summits of which do not differ much in elevation. Thus in Llathach two points reach 3358 feet and 3486 feet ; in Beinn Alligin there are also two points reaching 3021 feet and 3232 feet respect- ively ; in Beinn Dearg we have a height of 2995 feet ; in Beinn Eighe are three dominant points, 3188 feet, 3217 feet, and 3309 feet. The four masses to the north are somewhat lower, their elevations being 2860 feet, 2370 feet, and 2892 feet. The mountains of Lochaber and the Monadhliath Mountains exhibit similar relationships ; and the same holds good with all the mountain-groups of the Highlands. One cannot doubt that such relationship is the result of denudation. The mountains are monuments of erosion ; they are the wreck of an old table-land, the upper surface and original height of which are approximately indicated by the summits of the vari- ous mountain-masses and the direction of the princi- pal rivers. If we in imagination fill up the valleys with the rock-material which formerly occupied their place, we shall in some measure restore the general aspect of the Highland area before its mount- ains began to be shaped out by Nature's saws and chisels." A table-land of erosion, long exposed to denuda- tion, must obviously pass through the same phases as a plateau of accumulation. The elevated plain of complicated geological structure is first traversed by 1 48 EARTH SCULPTURE rivers, the courses of which are determined by the average slope of the land. As valleys are deepened and widened, and the whole surface comes under the influence of the epigene agents, new tributary streams continue from time to time to make their appearance, and eventually a perfect network of drainage-lines is established. Wherever the rocks yield most readily to erosion hollows are formed, and many of these will necessarily coincide with the outcrop or strike of the strata. Longitudinal valleys thus tend to be developed. As denudation proceeds, the capture of streams by rivers and of rivers by streams often takes place, and the hydrographic system becomes more or less modified, but the general direction of the chief lines of drainage remains unchanged. Eventually transverse rivers are found cutting across mountain-ridge after mountain-ridge, the latter hav- ing only been developed after the rivers had come* into existence. With the deepening and widening of the main valleys, and the continual multiplica- tion of subsidiary hollows by springs, torrents, and streams, the whole plateau eventually becomes cut up into irregular segments of every shape, form, and size a rolling mountain-land. Waterfalls, rapids, and other irregularities have now disappeared from the courses of the older rivers and streams, except, it may be, towards their heads, where more or less numerous feeders are busy cutting their way back into the mountains. Should the base-level be main- tained, the process of denudation must continue until LAND-FORMS IN HIGHLY FOLDED STRATA 149 the rolling mountain-land is finally reduced and re- solved once more into a plain of erosion. It is seldom, however, that a cycle of erosion is allowed to pass through all its stages. The study of many ancient plateaux has shown that the base-level is not infrequently disturbed sometimes by eleva- tion, at other times by depression. Long before the eroded plateau has been completely reduced, subsid- ence may ensue, and the drowned land may then become buried under vast accumulations of marine sediments. Should the region be once more up- heaved and converted into dry land, streams and rivers will again come into existence, and flow in directions determined by the slopes of the surface. Thus ere long another hydrographic system will be developed which may differ entirely from its prede- cessor, both as regards direction and arrangement. As the rivers cut their way down through the super- imposed marine strata they will eventually reach the buried land-surface, across which they will run with- out any reference to the former configuration. Should the base-level remain unchanged, a time will come when the overlying marine strata will be entirely removed, but the direction and general arrangement of the river-system acquired when the land was new- born will be maintained. Thus the direction of many transverse rivers, which in ancient plateau-lands are found cutting across mountains of every shape and disposition, have not infrequently been determined by the surface-slope of overlying masses, almost every vestige of which has since disappeared. CHAPTER VII LAND-FORMS IN REGIONS AFFECTED BY NORMAL FAULTS OR VERTICAL DISPLACEMENTS NORMAL FAULTS, GENERAL FEATURES OF THEIR CONNECTION WITH FOLDS THEIR ORIGIN HOW THEY AFFECT THE SUR- FACE FAULTS OF THE COLORADO REGION, AND OF THE GREAT BASIN DEPRESSION OF THE DEAD SEA AND THE JORDAN LAKE-DEPRESSIONS OF EAST AFRICA FAULTS OF BRITISH COAL-FIELDS BOUNDING FAULTS OF SCOTTISH HIGHLANDS AND LOWLANDS FAULT-BOUNDED MOUNTAINS GENERAL CONCLUSIONS. IN Chapter III. a short account was given of the dislocations or fractures by which rocks are frequently traversed. These, as we saw, are of two kinds normal faults and reversed faults or over- thrusts. The latter have been sufficiently referred to in connection with the appearances presented by highly flexured strata, amongst which, indeed, they are most usually encountered. Normal faults of vari- ous importance may likewise often be seen travers- ing areas of disturbed and contorted rocks. When such is the case, however, the larger of these faults not infrequently prove to be of later date than the flexures and thrust-planes. The latter are the result 150 VERTICAL DISPLACEMENTS 151 of former horizontal movements of the crust ; the normal faults, on the other hand, are vertical dis- placements due to later movements of direct subsid- ence. It will be understood, therefore, that reversed faults or overthrusts are practically confined to regions of highly flexed and contorted strata, while normal faults traverse every kind of geological structure. The latter, however, are certainly best displayed in areas of horizontal and moderately inclined strata, while they often form lines of separation between these and contiguous areas of highly disturbed rock-masses. The amount of downthrow of normal faults is very variable. Sometimes it does not exceed a few feet or yards, in other cases it may reach thousands of feet, so that strata of vastly different ages may be brought into juxtaposition. The smaller faults usu- ally extend for very short distances, while the larger ones may continue for hundreds or even thousands of miles. The course of great faults is usually approximately straight, but not infrequently it is curved. Very often they are accompanied by a series of smaller parallel dislocations ; and now and again, in place of one great fault, with accompanying minor dislocations, we may find a series of more or less closely set parallel minor faults. When the down- throw of all these minor faults is in one and the same direction, the result is practically the same as if there had been only one major dislocation with a large downthrow. Another fact may be noted : faults, especially large ones, often split up, as it were, into 152 EARTH SCULPTURE two or more. A major fault may begin as a mere crack or fracture, with little or no accompanying rock-displacement. But as it continues the amount of downthrow gradually increases until a maximum is reached, after which the displacement usually de- creases until finally the fault dies out. In not a few cases, however, the degree of downthrow varies very irregularly. Frequently faults are intimately connected with folds and flexures. This is shown at once by the fact that large dislocations very often trend in the same direction as the strike of the strata. Now and again, indeed, when a large fault can be followed to the end, it is found gradually to die out in a fold or flexure. In other words, what is a fault in one place is represented elsewhere by a flexure. It is not hard to see how that should be. Strain or tension must obviously be set up along the margin of a sinking area. If, for example, subsidence should take place within an area of horizontal strata, the horizontal position of the rocks along the margin of the sinking area will be interfered with. The pull or drag of the descending mass will cause the strata of the adjacent relatively stable area either to bend over or snap across. Should the movement be slow and pro- tracted, the rocks will probably at first yield by bending ; but as the movement continues they will eventually give way, and a fold will thus be replaced by a fracture. Towards either end of such a fault, therefore, we should expect it to die out into a simple VERTICAL DISPLACEMENTS 153 flexure or monoclinal fold. Probably most normal faults are in this way preceded by folding, except in cases where they have been more or less suddenly produced. Although normal faults may be looked upon as the result of direct subsidence, it is obvious that in some cases they may well have resulted from movements of elevation. During the slow uplifting of a broad plateau strain and tension will come into play along the margin of the rising area. Folds will thus be formed, and these will be replaced eventually by frac- tures and displacements. The resulting structure I FIG. 60. SECTION OF NORMAL FAULT. will thus be practically the same as if the folding and faulting had been produced by a movement of subsid- ence. Thus in Fig. 60 the fault / might have been caused either by the direct subsidence of the strata at x or by the elevation of the strata at a. There is reason to believe that some large faults have resulted from crustal movements continued through long periods of time. The rock-displace- ments may have been very slowly and gradually ef- fected, or the movement may have been more rapid, but interrupted again and again by longer or shorter tuses. Or, again, the rate of movement may have 154 EARTH SCULPTURE varied from time to time, and occasionally it may even have been sudden and catastrophic. But such evi- dence as we have would lead us to infer that vertical displacements, whether the result of downward or of upward movements, have not been more rapidly ef- fected than horizontal deformations. No doubt a sudden dislocation of the crust of large extent would show directly at the surface. But somewhat similar results would follow if the dislocation, without being quite sudden, were yet to be developed more rapidly than the rate of superficial erosion and denudation. Cases of the kind are well known, and to some of these reference will presently be made. It is with faulted rocks, however, as with folded mountains : when movement has ceased the Inequalities caused at the earth's surface tend to be reduced and greatly modified. The epigene forces are untiring in their action, so that in course of time areas of direct sub- sidence tend to become filled up and the surrounding high-lying tracts to be worn down. To such an ex- tent has this taken place, that in the case of certain great faults of high geological antiquity no inequality at the surface indicates their presence, and it is only by studying the geological structure that we are able to ascertain that such dislocations exist. Bearing in mind the activity of the denuding agents, we might expect that normal faults of geologically re- cent date should show most prominently at the surface. And this to a large extent is doubtless true. Never- theless, as we shall learn by-and-by, there are certain VERTICAL DISPLACEMENTS 155 faults of prodigious antiquity which still cause very marked inequalities at the surface. These often form the boundaries between highlands and lowlands. In such cases, however, the disparity of level is due not so much to vertical displacement, as to the fact that the lowlands are usually composed of less enduring materials than those which enter into the framework of the adjacent highlands. When a fault of great age traverses strata of much . the same consistency (say sandstones and shales), the rocks on either side of the dislocation, we find, have been planed down to FIG. 61. NORMAL FAULT, WITH HIGH GROUND ON DOWNTHROW SIDE. the same level. Thus in the low-lying coal-fields of Scotland the gently undulating surface gives no in- dication of the presence of the numerous dislocations which have been detected underground. Downthrows of hundreds of feet give rise to no superficial inequal- ities. It is only when one of these faults has brought relatively hard and soft rocks into juxtaposition that a marked surface-feature results. And in this case the hard rock invariably rises above the level of the soft rock, no matter on which side of the dislocation it happens to lie. Thus in Fig. 61 the hard rock a forms an eminence, although it is on the downthrow side of the fault, simply because it has withstood denud- 156 EARTH SCULPTURE ation more effectually than the soft rock (6). In Fig. 62, again, it is obvious that the high ground at x owes its origin to the presence of the relatively hard rock (/). To this matter, however, we shall return in the sequel. Meanwhile we must consider, first, the appearances presented in regions where vertical movements of the crust have taken place within relatively recent times/ The Colorado Plateau affords some excellent examples of simple folds and normal faults of com- paratively recent age. These have often profoundly affected the surface, lines of cliffs and bold escarp- ments rising along the high side of each dislocation. FIG. 62. NORMAL FAULT, WITH HIGH GROUND ON UPCAST SIDE. The plateau, in short, has been split across by well- marked normal faults, some of which can be followed for hundreds of miles. Yet the strata on both sides of such dislocations are of much the same character and consistency. Here, then, it might be supposed that the fracturing and displacement had been sud- denly effected. There is striking evidence, however, to show that such has not been the case. Although some of the faults referred to have a downthrow of several thousand feet, yet they have had no effect in disturbing the course of the Colorado River, which tra- verses the faulted region. The same, as we have seen, VERTICAL DISPLACEMENTS 157 holds true with regard to the flexures of that area. It is obvious, in a word, that the process of flexuring and faulting has proceeded so slowly that the river has been able to saw its way across the inequalities as fast as these appeared. But while the rate of river erosion has equalled that of crustal movement, the denudation of the plateau outside of the river- courses has not. Deformation and dislocation of the plateau have thus given rise to marked surface-feat- ures. Yet even in the case of these relatively young faults we find that the features determined by them have been very considerably modified by denudation. In the following section, for example, we see three FIG. 63. FAULTS IN QUEANTOWEEP VALLEY, GRAND CANON DISTRICT. (Button. ) faults of 1 300 feet, 300 feet, and 800 feet displacement respectively traversing the same series of strata, and yet giving rise to marked inequalities at the surface. The dotted lines, however, show to what an extent these features have been modified by denudation. There is an obvious tendency of the escarpments and cliffs to become benched back as they retreat, so that they do not show the abrupt character which they would have possessed had no superficial waste accom- 158 EARTH SCULPTURE panied and succeeded the crustal movements. (See Fig. 63.) In the Great Basin that extends between the bold escarpment of the Sierra Nevada, on the one hand, and the Wahsatch Mountains on the other, we encounter another series of large faults, which have deter- mined the leading features of the region. It would appear that the area of the Great Basin formerly attained a considerably greater elevation than at pre- sent. Towards the close of Tertiary times the whole of this area, including the adjacent Sierra Nevada and the Wahsatch Mountains, was upheaved in the form of a broad arch. The crust thus subject to ten- sion yielded by cracking across, and a system of long parallel north and south fissures was formed. In other words, the broad arch was split into a series of oblong blocks many miles in extent. When the movement of elevation ceased and subsidence en- sued, the shattered crust settled down unequally between the Sierra Nevada in the west and the Wah- satch Mountains in the east. The amount of dis- placement along the margins of the Great Basin is very great ; the fault at the base of the Sierra, for example, is estimated to be not less than 15,000 feet, while that which severs the Basin from the Wahsatch Mountains is also very great. The numerous parallel ranges that diversify the surface of the Great Basin itself are simply oblong crust-blocks, brought into position by normal faults. Being of so recent an age, they have suffered comparatively little modification. VERTICAL DISPLACEMENTS Nevertheless, they do not fail to show the tool-marks of epigene action everywhere escarpments are retreating, and one can see that already vast masses of rock have been removed from the sur- face. The accompanying dia- gram (Fig. 64) will serve to give a general idea of the geological structure of the Basin ranges. There is no reason to believe that the crustal movements above referred to were sudden or cata- strophic in character. Probably they were no more rapid than those which have affected the plateau of the Colorado. We are not without evidence of similar recent dislocations in the Old World, and there as elsewhere they give rise to more or less pronounced surface-feat- ures. One of the most interest- ing examples is seen in the great depression that extends north- wards from the Gulf of Akabah by the Wady el Arabah, the Dead Sea, the valley of the Jor- dan, and Lake Tiberias. This long hollow would appear to --5 160 EARTH SCULPTURE have come into existence at or about the close of Tertiary times. It is everywhere bounded by normal faults or by steep monoclinal folds, the one kind of structure passing into the other. Before this depres- sion came into existence the region it now traverses appears to have been a broad continuous plateau, built up of ancient crystalline and Palaeozoic rocks below, and approximately horizontal strata of Meso- zoic age above. At what particular date this plateau of accumulation first appeared, and how long it re- mained undisturbed, we cannot tell. Possibly the movement of subsidence to which the Dead Sea owes its origin may have coincided with the upheaval that resulted in the formation of the plateau. However that may have been, the latter was eventually tra- versed by a series of monoclinal folds and parallel faults, and between these the great depression of the Jordan came into existence. The Mesozoic strata of the plateau retain their approximately horizontal po- sition close up to the depression along its eastern margin, while the descent from the west is much less abrupt. But this is only broadly true. When the region is more closely investigated, the relatively gen- tle dip of the strata along the west side of the depres- sion is found to be interrupted again and again by more or less sharp monoclinal folds and by normal faults, the presence of which is betrayed at the sur- face by corresponding sudden changes in the form of the ground. In other words, the descent from the plateau on the west is often by a series of broader VERTICAL DISPLACEMENTS 161 and narrower terraces and escarp- ments, running parallel with the trend of the great hollow. The western margin of the Dead Sea, for example, is determined by a vertical displacement, similar in character to, but not so extensive as, that which bounds it on the east. The section (Fig. 65) will serve to illustrate the geological structures referred to. The flexures and faults of this interesting region do not date beyond the close of the Tertiary period, and consequently there has not been sufficient time to allow of a complete modification of the surface by epigene action. The most conspicuous features of the district are determined by folds and fractures under- ground structure and surface- configuration to a large extent coincide. But everywhere also we observe the evidence of ero- sion and denudation. Great sheets of rock have been grad- ually removed from the surface, which is seamed and scarred by innumerable ravines and water- tf 1^1 162 EARTH SCULPTURE courses, many of these being now dry and deserted. According to Professor Suess, the Jordan depres- sion continues north between the Lebanon and the Anti-Lebanon, through the valley of the Nahr el Asi (the Orontes) to near Antioch. The same geologist is further of opinion that the great trough of the Red Sea and most of the lacustrine hollows of East Africa are in like manner due to direct subsidence of the crust, the probability being that they and the Jordan depression all belong to one and the same system of crustal deformation. It is noteworthy that the de- pressed areas of Africa lie in zones or belts having an approximately meridional direction, that they are not margined or surrounded by mountain-ranges, but are sunk in broad plateaux, and, moreover, are accom- panied by abundant evidence of volcanic action. The troughs are mostly broad, and vary much and con- stantly in height above the sea, so that they are obvi- ously not the result of erosion. In many places they are flanked on both sides by abrupt declivities com- parable in character to those that overlook the Dead Sea. In some cases, however, steep bluffs and cliffs are confined to one side of a depression only. In short, we have in East Africa the same' phenomena which confront us in Palestine. The earth's crust in all those regions has evidently yielded to strain or tension by snapping across and subsiding. In place of one simple normal fault, however, we see a com- plex system of parallel dislocations and flexures, the folded and shattered rocks having settled down un- VERTICAL DISPLACEMENTS 163 equally, while molten matter and loose ejecta issued here and there in less or greater abundance along the chief lines of rock-disturbance. Similar geological structures on a smaller scale may be seen nearer home, and are well exemplified in the region of the Vosges and the Black Forest. These opposing mountains are the counterparts of each other, being built up of the same rocks, arranged in very much the same way. The basement rocks are granite and crystalline schistose rocks, which are overlaid by a series of Mesozoic strata. In the Vosges the dip of these strata is westerly, while the corresponding rocks in the Black Forest are inclined towards the east. Between the two ranges, as every- one knows, lies the basin of the upper Rhine, a basin which, like that of the Jordan, has been determined by a number of parallel normal faults. The Meso- zoic strata in the region surrounding the two ranges attain a thickness of at least 5000 feet, and there can be no doubt that these originally extended from west to east across what is now the basin of the Rhine. This is shown by the simple fact that the strata in question occupy that basin. (See Fig. 66, p. 164.) Doubtless the Mesozoic rocks were originally deposited in ap- proximately horizontal positions. Subsequently the sea retreated from the area, and a wide land-surface probably an elevated plain or plateau occupied its place. Eventually, in early Tertiary times, the region was subjected to crustal movements, and traversed from south to north by a series of dislocations, with i6 4 EARTH SCULPTURE downthrows in opposite di- rections. As a result of these displacements the Rhenish basin came into existence, while the rock-masses along its margins were pushed up to form the ranges of the Vosges and the Black Forest. The crustal move- ments referred to appear to have been continued down to 3 post-Tertiary times, and have probably not yet J ceased, the frequent earth- Q *~ quakes experienced in the neighbourhood of Darm- stadt being perhaps an indication of progressive subsidence along lines of dislocation. It is interest- I oo ing to note that these crustal movements have been ac- companied from time to time by volcanic action. The well-known Kaiserstuhl near Freiburg, for example, is the skeleton of what must have been a very consider- able volcano. The evidence that subsid- VERTICAL DISPLACEMENTS 165 encc in the Rhenish basin has continued into the post-Tertiary period is so striking that it may be briefly referred to here. Deep borings have shown that the Pleistocene deposits in the valley of the Rhine in Hesse occupy a profound hollow, surrounded on all sides by older rocks, the bottom of the basin being 270 feet deeper than the lowest part of its rim at Bingen. These deposits, however, are not lacus- trine, but fluviatile. Hence we must infer that fluv- iatile deposition has kept pace with the crustal movement. As the bottom of the Rhine valley has slowly subsided, the river has flowed on without interruption, continuously filling up the gradually deepening basin with its sediment. This is only another example of the fact that movements of the crust, whether of elevation or depression, have often >roceeded so slowly that they have been unable to lodify the direction of streams and rivers. While we recognise the influence of earth-move- Lents in determining the form of the surface in the egion under review, it is obvious that much rock- laterial has been removed. The presence of the 'esozoic strata in the basin of the Rhine shows that :hese must formerly have extended continuously over :he adjacent tracts. Yet they have since been largely lenuded away from the higher parts of the Vosges ind the Black Forest, so that the underlying crystal- line rocks have been laid bare, and now appear at the surface over considerable areas. When we turn our attention to regions of highly i66 EARTH SCULPTURE dislocated rocks, where the crustal displacements are of much greater antiquity than those we have just been considering, the surface-features, we find, have often been so modified by denudation that the posi- tion and even the very existence of normal faults can be determined only by close observation. In other cases, however, they give rise to marked features at the surface. The following section across a portion of the Lan- arkshire coal-field is drawn upon a true scale. The section traverses several normal faults, the largest FIG. 67. SECTION OF COAL-MEASURES (ON A TRUE SCALE) NEAR CAMBUSNETHAN, LANARKSHIRE. being a displacement of 350 feet, yet there is no feat- ure at the surface to indicate its presence. It is only by studying the geological structure that the existence of such dislocations can be discovered. The strata of the region in question are of much the same consistency throughout, and have therefore yielded equally to the various agents of erosion. Thus all inequalities of surface which may originally have resulted from faulting have been smoothed out. It is doubtful, however, whether such relatively small faults ever did show at the surface. The amount of displacement effected by them usually diminishes up- wards, so that the highest coal-seams are hardly dis- VERTICAL DISPLACEMENTS 167 located to such an extent as those which occur at lower levels. Many small faults, indeed, die out up- wards altogether. And when we remember that the rocks now exposed at the surface were formerly covered by enormous sheets of strata which have since been removed by denudation, it is not hard to believe that even some of the larger faults of our coal-fields may actually have died out before the original surface of the Carboniferous strata was reached. Some normal faults, however, are so very extens- ive the amount of displacement is so very great that we must believe they did reach the earth's surface at the time of their formation. Yet where these faults traverse strata having much the same charac- ter, they produce no inequalities of level at the sur- face. A good example is the Tynedale fault of the Newcastle coal-field, which has a downthrow in some places of 1 200 feet, and yet its existence is not be- trayed by the configuration of the ground. (See Fig. 68, p. 1 68.) Great normal faults, however, usually do show more or less conspicuously at the surface. This is due to the fact that by their means areas of soft and hard rock are often brought into juxtaposition. Many ex- amples might be cited from Great Britain. Thus in Scotland the Central Lowlands, consisting largely of relatively soft rocks, have been brought against the harder rocks of the Highlands on the one hand, and those of the Southern Uplands on the other. A i68 EARTH SCULPTURE line drawn from Stonehaven in a south-west direction to the Clyde near Helensburgh is at once the geo- logical and geographical boundary of Highlands and Lowlands, while a similar line extending from Dun- bar to the coast of Ayrshire near Girvan forms the corresponding boundary of the Lowlands and the GosforH* S.E. Btirn. IAIN Co* FIG. 68. SECTION ON A TRUE SCALE ACROSS " TYNEDALE FAULT," NEWCASTLE COAL-FIELD. Southern Uplands. The lines in question are great dislocations, having in places downthrows of 5000 to 6000 feet. But there can be no doubt that the in- equalities at the surface are due not so much to the amount of vertical displacement as to the different character of the rocks on opposite sides of the faults. This is well shown by the fact that the disparity level along a line of dislocation varies with the char- VERTICAL DISPLACEMENTS 169 acter of the rocks which are brought into juxtaposi- tion. Thus, when soft sandstone, as in Strathmore, abut against hard crystalline rocks, the latter rise more or less abruptly above the former the line of demarcation between Highlands and Lowlands is S. FIG. 69. SECTION ACROSS GREAT FAULT BOUNDING THE HIGHLANDS NEAR BlRNAM, PERTHSHIRE. A, " hard " grits and shales ; j, relatively " soft " sandstones, etc. Demarcation between Highlands and Lowlands well marked. strongly pronounced. But when, as between the val- leys of the Earn and the Teith, the hard igneous rocks of the Lowlands are brought against the crys- talline schists of the Highlands, the geographical boundary of the two regions is not nearly so well marked the Highland mountains seem to merge gradually into the Lowland hills. And the same phenomena are conspicuously displayed along the margin of the Lowlands and the Southern Uplands. In a word, it is obvious that while the position of the boundaries that separate the Lowlands from the mountain-areas to north and south has been deter- mined by normal faults, the existing configuration is the result of long-continued and profound denudation. The accompanying sketch sections (Figs. 69, 70) will serve to illustrate the foregoing remarks. 170 EARTH SCULPTURE Normal faults, as we have seen, have often deter- mined the boundaries between lowlands and high- lands. Not infrequently, indeed, it can be shown ww. FIG. 70. SECTION ACROSS GREAT FAULT BOUNDING THE SOUTHERN UPLANDS. A, " hard " greywackes, etc.; /", " hard " igneous rocks and overlying conglomerate c. Demarcation between Uplands and Lowlands not well marked. that the dominance of certain mountains is due rather to the sinking down of adjacent low-lying tracts than to bulging up of the crust within the mountain-areas B FIG. 71. DIAGRAM SECTION ACROSS HORSTGEBIRGE. <*, granite, gneiss, etc., forming the " Horst" ; , stratified rocks of relatively late age, resting upon a, dropped down along lines of dislocation ff; 0, outlier of , showing that the strata b were formerly continuous between A and B. themselves. Such mountains are, of course, bounded by faults, and are known to German geologists as Horste or Rumpfgebirge, the Harz being a good ex- ample. The Horste of Middle Europe are composed for the most part of crystalline schists and Palaeozoic rocks, more or less highly flexed and disturbed. The VERTICAL DISPLACEMENTS 171 mountains usually rise somewhat suddenly above the surface of the relatively undisturbed and approxi- mately horizontal Mesozoic strata of the adjacent low grounds, and for a long time it was supposed that these strata in the immediate vicinity of the Plorste were littoral deposits. Such, however, is not the case. They are of relatively deep-water origin, and, before faulting supervened, may have covered much of the high lands which now overlook them. It is obvious, in short, that the Horste represent portions of the crust which have maintained their position ; they are mount- ains which testify to a former higher crustal level ; the surrounding tracts have broken away from them, and dropped to a lower position. Probably enough has now been advanced to show that normal faults have had no inconsiderable share in determining surface-features. This, as might have been expected, is most conspicuous in regions of re- cent crustal deformation and fracture, where epigene action has not had time to effect much modification. In cases of very ancient fracture and displacement, lowever, the surface-features, as we have seen, are y greatly modified, and if well-marked disparity of level is still often met with along lines of dislocation, this is mainly due to the fact that rocks of unequal endurance have been brought into juxtaposition. In case of very considerable displacement it will usu- illy happen, indeed, that crystalline schists, plutonic rocks, or hard Palaeozoic strata will occur upon the ligh side and relatively softer strata on the low side 172 EARTH SCULPTURE of the fault. However prolonged and intense epigene action may have been, such a fault will nevertheless cause a marked feature at the surface, so long as the general surface of the land remains considerably above the base-level. But when the latter is approached denudation will eventually cease on the low side of the fault, while material will continue to be removed from the high side, and the disparity between the two will thus tend gradually to disappear. In short, the irregularities of surface determined by the presence of faults pass through the same cycle of changes as all other kinds of geological structure. Should the base-level remain undisturbed epigene action must eventually reduce every inequality, no matter what its origin may have been. Again, were such a reduced land-surface to be re-elevated and converted into a plateau, the lines of dislocation that happened to separate areas of hard rock from regions of soft rock would once more determine the boundaries between high and low ground. The surface of the soft rocks would be lowered most readily, while the more durable hard rocks would come to form elevations. In a word, the features that obtained before the land was reduced to base-level would, under the influence of denudation, tend to re-appear. CHAPTER VIII LAND-FORMS DUE DIRECTLY OR INDIRECTLY TO IGNEOUS ACTION PLUTONIC AND VOLCANIC ROCKS DEFORMATION OF SURFACE CAUSED BY INTRUSIONS LACCOLITHS OF HENRY MOUNTAINS VOLCANOES, STRUCTURE AND FORM OF MUD-CONES GEY- SERS FISSURE-ERUPTIONS VOLCANIC PLATEAUX DENUD- ATION OF VOLCANOES, ETC., AND RESULTING FEATURES. IN preceding pages we have had frequent occasion to refer to igneous rocks. These, as we have seen, may be broadly grouped under two heads Plu- tonic rocks and Volcanic rocks. The former have cooled and solidified at a less or greater depth below the surface ; the latter, on the other hand, have been extruded at or near the surface. No hard and fast line, however, can be drawn between these two groups. All plutonic rocks are indeed intrusive they have solidified below ground ; but the same is true of the sheets and dikes which traverse a volcano, and which, along with the bedded lavas and tuffs they traverse, are properly described as of volcanic origin. It will be understood, then, that the term plutonic is restricted to intrusive rocks which have consolidated at rela- tively great depths, while the term volcanic includes 173 174 EARTH SCULPTURE all igneous rocks which enter or have entered into the formation of a volcano, or which have evidently proceeded from any focus or foci of eruption. It is needless to say that we can know nothing by direct observation of the conditions and phenomena which attend the intrusion of deep-seated plutonic rocks. But so many of these have been laid bare by denudation, their composition and their relation to sur- rounding rock-masses have been so carefully studied, that geologists have learned much concerning igneous action of which but for denudation they must have remained largely ignorant. They have ascertained, for example, that such lavas as rhyolite, andesite, and basalt have their deep-seated equivalents in the plu- tonic granites, syenites, and gabbros. That is to say, we know that the same molten mass solidifies at great depths as granite or other wholly crystalline rock, and at the surface as rhyolite or other semi-crystalline lava. In short, plutonic rocks and their volcanic equivalents have practically the same chemical composition. An acid lava comes from an acid magma, a basic lava from a basic magma. Hence it is inferred that many plu- tonic rocks now exposed by denudation may have been the deep-seated sources from which ancient lavas have proceeded. On the other hand, there is reason to believe that many plutonic masses may never have had any such volcanic connections. But whether or no a given plutonic mass be the deep-seated source of some long-vanished volcano or volcanoes does not concern us here. We have sim- LAND-FORMS DUE TO IGNEOUS ACTION 175 ply to recognise the fact that its exposure at the surface is the direct result of profound denudation. Whether its intrusion had any effect in deforming the surface we cannot tell. Probably, in cases where none of the material was extruded to the surface by contemporaneous volcanic action, there may have been some bulging up of the ground. Deformation of the crust, in short, may quite well have accom- panied the subterranean movements of great masses of molten matter. But so long a time has elapsed since the granites and other highly crystalline plutonic rocks were intruded so enormous has been the thick- ness of rock removed from above them that such intrusion cannot be said to have had any direct effect in the production of existing surface-features. It is quite true that many hills and mountains are com- posed largely or even exclusively of plutonic rocks ; 5 FIG. 72. MOUNTAIN OF GRANITE. g, granite sending veins into schists, etc., O). The schists have been more readily lowered by erosion than the granite. but that is simply owing to the fact that these rocks are usually more durable than the rocks through which they rise. When, as not infrequently happens, plutonic masses are of less durable consistency and 176 EARTH SCULPTURE construction than the rocks that surround them, the latter invariably dominate and overlook the former. Thus while granite often forms prominent mountains (Fig. 72, p. 175), not infrequently it is found occupy- ing low tracts flanked by mountains of schist, slate, or other rock. (Fig. 73.) FIG. 73. PLAIN OF GRANITE OVERLOOKED BY MOUNTAINS OF SCHISTS, ETC. f, granite ; j, schists, etc. The granite has been more readily lowered by erosion than the surrounding schists. We must conclude, then, that whatever effect may have been produced at the surface by the intrusion of the more ancient plutonic rocks of England and other countries, such superficial effects, if any, have long since disappeared. The present configuration of the ground occupied by such rocks is wholly the result of epigene action. But when we consider the phenomena of more recent intrusions of igneous rock, we find reason to conclude that these have not only had a direct effect at the surface, but that this effect has not yet in all cases been removed by denudation. The ground has bulged up, and the swelling of the surface is still conspicuous. Among the most re- LAND-FORMS DUE TO IGNEOUS ACTION 177 markable examples known are the laccoliths or lac- colites (stone cisterns) of the Henry Mountains (southern Utah), which have been described by Mr. Gilbert. In that region molten rock, instead of ascending to the surface and building up mountains by successive eruptions, has stopped at a lower hori- zon, insinuated itself between the strata, and opened for itself a chamber by lifting all the superior beds. (See Fig. 74.) Proceeding from a laccolith are in- to. 74. DIAGRAMMATIC SECTION OF A LACCOLITH SHOWING DOME-SHAPED OVATION OF SURFACE ABOVE THE INTRUSIVE ROCK. (After G. K. Gilbert.) /*, pipe or conduit ; sA, sheet ; d d, dikes. :rusions of the same kind of igneous rock (trachyte), some of which (sheets) have squeezed themselves be- tween adjacent beds, while others (dikes) traverse the strata at less or greater angles. These remarkable rocks have been intruded in a great series of strata ranging in age from Carboniferous to Cretaceous, amongst which they are irregularly distributed, some 178 EARTH SCULPTURE appearing in the Carboniferous, some in the Jura- Trias, and others in the Cretaceous. From the low- est to the highest laccolith the range is not less than 4000 feet, those which are above not infrequently overlapping those which lie below. " Their horizon- tal distribution is as irregular as the arrangement of volcanic vents. They occur in clusters, and each cluster is marked by a mountain. In Mount Ellen there are perhaps thirty laccolites ; in Mount Holmes there are two ; and in Mount Ellsworth one. Mount Pennell and Mount Hillers have each one large and several small ones." The highest of these mountains attains an elevation of over 11,000 feet, rising some 5000 feet above the plateau at its base. The strata of which that plateau is built up are approximately horizontal, and appear at one time to have been cov- ered by some thousands of feet of Tertiary deposits, the nearest remains of which occur at a distance of thirty miles from the Henry Mountains. Mr. Gilbert is of opinion that the laccolites were most probably intruded after the deposition of the Tertiary strata, and before their subsequent removal by erosion. The whole structure of the Henry Mountains shows that the actual surface was affected by those intru- sions, the horizontal strata being arched upwards so as to form dome-shaped elevations, rising prominently above the general level of the plateau. The laccoliths are all of considerable size, the smallest measuring more than half a mile, and the largest about four miles in diameter. The mountains formed by them LAND-FORMS DUE TO IGNEOUS ACTION 179 consist of a group of five individuals separated by low passes, but having no definite range or trend. The subsequent erosion of these mountains, Mr. Gilbert remarks, has given the utmost variety of exposure to the laccoliths. In some places these are not yet un- covered, and we see only the arching strata which overlie them, the strata being cut across by only a few dikes or traversed by a network of dikes and sheets. In other places denudation has partly bared the laccoliths or even completely exposed them, so that their original form can be seen. In yet other places the bared laccolith itself has been attacked by the elements, and its original form more or less changed. It is even quite possible that occasionally laccoliths may have been entirely demolished, and that some of the truncated dikes now visible at the surface may mark the old fissures or conduits through which such vanished laccoliths were injected. From the evidence just referred to, it is obvious that intrusions of igneous rock, if of sufficient thick- ness, are capable of warping the surface, and of form- ing more or less considerable elevations. But as erosion tends to reduce all such upheavals more or less rapidly, it is only those of relatively recent age that can retain any trace of their original configura- tion. All masses of intrusive rock of great geological antiquity, which now form hills and mountains, do so in virtue of their greater resistance to the action of epigene agents. They may have arched up the rocks underneath which they formerly lay buried, and so 1 80 EAR TH SCULP TURE produced more or less prominent elevations at the surface, but such primeval land-forms have been en- tirely removed the features now visible are the direct result of erosion and denudation. Of true volcanic rocks it is not necessary to say much. Their eruption at and near the surface gives rise to hills and mountains of accumulation, the gen- eral aspect and structure of which are sufficiently fa- miliar. The typical volcano is a truncated cone, built up usually of successive lava-flows and sheets of loose ejecta. At the summit is the central cup, or crater, marking the site of the vertical funnel, or throat, through which the various volcanic products find passage to the surface. These are naturally arranged round the focus of eruption in a series of irregular sheets, beds, and heaps, which dip outwards in all directions. It is this disposition of the materials which gives its characteristic form to a volcano. The upper part of the cone inclines at an angle of 30 to 35, but this steep slope gradually decreases until towards the base the inclination may not exceed 3 or 5. In a typical volcano, therefore, the internal geological structure and the external configuration coincide the mountain with its graceful outline is the direct result of subterranean action. It is obvi- ous, however, that the quaquaversal arrangement of the lavas and tuffs is a weak structure. Many cones, it is true, are braced and strengthened by dikes and other protrusions of molten rock, which consolidate in the cracks and fissures that often traverse a volcanic LAND-FORMS DUE TO IGNEOUS ACTION 181 mountain in all directions. But, although such in- trusions may delay, they cannot prevent the ultimate degradation of a volcano which has ceased to be active. Active and dormant or recently extinct volcanoes differ in form, to some extent, according to the pre- valent character of their constituent rocks, and the manner in which these have been heaped up. Some cones consist of cinders, or other fragmental ejecta, with which no lava may be associated. Not infre- quently, again, such cones have given vent to one or more lava-flows. From small cinder-cones, show- ing a single coutie, to great volcanoes built up of a multitudinous succession of lavas and sheets of frag- mental materials, there are all gradations. The smaller cones are often the products of a single eruption ; while the larger cones owe their origin to many successive eruptions, between some of which there may have been prolonged periods of apparently complete repose. The beautiful symmetry of the typical cone is often disturbed. This is due some- times to the shifting of the central focus of eruption ; sometimes to the escape of lava and ejecta from lateral fissures opening on the slopes of the mountain. Not infrequently, also, the symmetry of a growing cone is liable to modification by the action of the prevalent wind, the loose ejecta during an eruption falling in greatest bulk to leeward. Tuff-cones and cinder-cones range in importance from mere inconsiderable hills to mountains approach- 1 82 EARTH SCULPTURE ing or exceeding 1000 feet in height. In the typical cinder-cone the crater is small in proportion to the size of the volcano ; it is simply an inconsiderable depression at the summit of the cone. Occasionally, however, we meet with large crateral hollows, mostly now occupied by lakes ringed round by merely an insignificant ridge of fragmental materials. Some- times, indeed, such large hollows show no enveloping ring whatsoever. Extensive craters* of this kind are believed to be the result of explosive eruptions, and it is quite possible, or even probable, that their width has been considerably increased by subsequent cav- ing in of the ground. Cinder-cones and tuff-cones vary in form according to the character of their con- stituent materials. When coarse slags and scoria? or pumice predominate, the sides of the cone may have an inclination of 35, or even of 40. When the materials are not quite so coarse, the angle of slope is not so great ; it diminishes, in short, as the ejecta become more finely divided, so as sometimes not to exceed 15. Just as there are cones composed chiefly or exclu ively of fragmental materials, so there are volcano built up of one or of many successive lava-flows, with which loose ejecta may be very sparingly associated, or even sometimes absent altogether. Lava-cones likewise vary in shape and size according to the nature of their component rocks. Some form abrupt hills of no great height ; while others are depressed cones, attaining a great elevation and sloping at a es : LAND-FORMS DUE TO IGNEOUS ACTION 183 very small angle, so as to occupy wide tracts. The abrupt cones consist chiefly of the more viscous lavas which have coagulated immediately round the focus of eruption. The depressed cones, on the other hand, are built up of the more liquid lavas, which flow out rapidly, and reach relatively greater distances from the focus of eruption. Not infrequently the cones formed by the out welling of very viscid lava show no crater the lava coagulates around and above the vent. In other cases the top of the abrupt dome-shaped cone is blown out by escaping gases, and a crater-shaped hollow is thus formed. The volcanoes of the Hawaiian Islands present the grand- est examples of the eruption of liquid lavas. Hawaii itself is made up of five volcanic mountains, ranging in height from some 4000 feet up to nearly 14,000 feet. All these are depressed cones. Mauna Loa (13,675 feet), for example, has a broad, flattened summit, sunk in which is the great cauldron-like crater, some 3^ miles in length by \\ in width, and 800 feet deep. From the lip of this crater the mount- ain slopes outwards at an angle of 3, which gradu- ally increases to 7, the diameter of the mountain at its base being not less than 30-40 miles. But composite cones, built up of lava and loose ejecta, are of far more common occurrence than cones composed of lava alone. To this class belong most of the better-known volcanic mountains. Their general characters have already been outlined in the short description we have given of a typical volcano. 1 84 EARTH SCULPTURE It remains to be noted that many composite vol- canoes show a cone-in-cone structure. During some paroxysmal eruption the upper portion of a volcano may be destroyed shattered and blown into frag- ments. Or, as a result of long-continued activity, the mountain becomes partially eviscerated, and the upper part of the cone eventually caves in, and a vast cauldron is formed, after which a protracted period of repose may ensue. When the volcanic forces again come into action a younger cone, or it may be several such cones, gradually grow up within the walls of the old crater. The younger cones may rise in the middle of the great hollow, or they may be eccentric, as in the case of Vesuvius, which has grown up upon the rim of the large crater of Monte Somma. Of comparatively little importance from our pre- sent point of view are mud-volcanoes. Some of these owe their origin to the escape of steam and hot water through disintegrated and decomposed volcanic materials, either tuff or lava, or both. They are usually of inconsiderable size, many being mere monticles, while others may exceed 100 feet in height. They show craters atop, and have the general form of tuff-cones. Their origin is obvious. The mud is- simply flicked out as it bubbles and sputters, and the material thus accumulates round the margins of the cauldron, until a cone is gradually built up. Other so-called mud-volcanoes have really no connection with true volcanic action, but owe their origin to the LAND-FORMS DUE TO IGNEOUS ACTION 185 continuous or spasmodic escape of various gases, such as marsh-gas, carbonic acid, sulphuretted hydro- gen, etc. The mud of which they are chiefly com- posed is saline, and usually cold. Now and again, however, stones and debris may be ejected. These "volcanoes" (variously known as salses, air-volcanoes, and maccalubas) usually form groups of conical hill- ocks like miniature volcanic cones. Here also may be noted, in passing, the sinter-cones formed by those eruptive fountains of hot water and steam which are known under the general term of geysers. When the geyser erupts on level, or approximately level, ground, the sinter tends to assume a dome-shape ; when, on the other hand, the springs escape upon a slope, the silicious deposits are not infrequently ar- ranged in successive terraces. All the volcanic eruptions to which we have been referring have proceeded from isolated foci. Some volcanoes are quite solitary, others occur in irregular groups, while yet others appear at intervals along a given line. These last are obviously connected with great rectilinear or curved dislocations of the earth's crust ; not a few of the former, however, apparently indicate the sites of funnels or pipes which have been simply blasted out by the escape of elastic vapours. There is yet another class of volcanic eruptions which have played a prominent part in geological history, although they are not now so common. These are the fissiire or massive eruptions, of which the best ex- amples at the present time are furnished by Iceland. 1 86 EARTH SCULPTURE Lavas, usually of the more liquid kind, well out some- times simultaneously from more or less numerous vents situated upon lines of fracture, or from the lips of the fissures themselves. Usually such floods and deluges of lava are not accompanied by the dis- charge of any fragmental materials. Sheet after sheet of molten rock has been discharged in this manner so as to completely bury former land-surfaces, filling up valleys, submerging hills, and eventually building up great plains and plateaux of accumula- tion. The basalt-plains of Western North America, which occupy a larger area than France and Great Britain, are the products of such massive eruptions, the lavas reaching an average thickness of 2000 feet. The older basalts of Iceland, the Faroe Islands, the Inner Hebrides, and Antrim are the relics of similar vast fissure eruptions. And of like origin are the basaltic plateaux of Abyssinia and the Deccan in India. The volcanic phenomena of the Hawaiian Islands have also much in common with fissure or massive eruptions. The forms assumed by the materials accumulated at the surface by subterranean action are all more or less distinctive and characteristic. Hills, mountains, plains, and plateaux, which owe their origin directly to volcanic activity, agree in this respect, that their internal structure and external form coincide. Even the most perfectly preserved examples of volcanic ac- cumulation, however, are seldom without some trace of the modifying influence of epigene action. The LAND-FORMS DUE TO IGNEOUS ACTION 187 shape of a volcanic cone, for example, during its period of growth is subject to modification. Wind affects the distribution of loose ejecta, while rain and torrents sweep down materials, and gullies and ravines furrow the slopes of the mountain. The ravages thus caused continue to be repaired from time to time so long as the volcano remains active. But when its fires die out and the mountain is given over to the undisputed power of the epigene agents, the work of degradation and decay proceeds apace. The rate of this inevitable destruction is influenced by many circumstances by the nature and structure of the materials, for example, and the character of the climate. Thus, cones built up of loose scoriae are likely to endure for a longer time than cones com- posed of fine tuff and hardened mud. Rain falling upon the former is simply absorbed, and consequently no torrents scour and eat their way into the flanks of the cones, while tuff- and mud-cones are more or less rapidly washed down and degraded. Again, a com- posite volcanic mountain of complicated structure, the product of several closely associated vents, but- tressed and braced by great pipes of crystalline rock and an abundant series of larger and smaller dikes, is better able to withstand the assaults of epigene agents than a cone of simpler build. Sooner or later, however, even the strongest volcanic mountain must succumb. Constantly eaten into, sapped, and under- mined, it will eventually be levelled. In regions of extinct volcanoes we may study every i88 EARTH SCULPTURE stage in the process of demolition. Isolated cones and groups of cones crumble away, until all the lavas and tuffs ejected from the old vents may have disap- peared, and the only evidence of former volcanic action that may remain are the basal portions of the dikes that proceeded from the foci, and the solid cores with which the latter were finally plugged up. (See Fig. 75.) As these cores usually consist of more FIG. 75. VIEW OF NECKS = CORES OF OLD VOLCANOES. (Powell.) durable materials than the rocks they pierce, they tend to form somewhat abrupt conical hills. It goes without saying that such extreme cases of denudation are met with only in regions where volcanic action has for a long time been extinct. Excellent exam- LAND-FORMS DUE TO IGNEOUS ACTION 189 pies on a relatively small scale are furnished by the so-called " Necks" of Scotland, of which the accom- panying section (Fig. 76) shows the general phe- nomena. Similar structures occur in many parts of Europe and North America. Mini* Hill s FIG. 76. SECTION OF HIGHLY DENUDED VOLCANO. MINTO HILL, ROXBURGHSHIRE. .A^, throat or neck of volcano plugged up with ejectamenta, angular and subangular stones, grit, dust, etc. ; S, Silurian rocks ; Z>, Old Red Sandstone strata. Frequently the products of great volcanic eruptions of vast geological antiquity have been largely pre- served, owing to their subsequent burial under sedi- mentary accumulations. Many of the hill-ranges of Central Scotland, for example, are built up of lavas and tuffs. These are the relics of volcanoes which came into existence in Palaeozoic times, and after erupting molten and fragmental materials for longer or shorter periods, eventually died out, becoming sub- merged and covered with sedimentary accumulations to depths of several thousand feet. Subsequent ele- vation of the region brought these sediments under the operation of the agents of erosion, and in time great thicknesses were removed, so that ultimately the ancient volcanic rocks were again laid bare and in their turn exposed to denudation. But if the lat- 1 9 o EARTH SCULPTURE IW& Wf w c l 1 Me S * * Q s 1 i < 1 w " H .S w *r S3 " j 1 H ; " o t I nkk S 2 H ' I'M -J V i%; ; il ', . ii Hi V ^ & a W c w X g 4," ^\n III < S-J \\