ZTbe mniversfts of Cbicaao UC-NRLF 11 15 fl STUDIES IN MINOR FOLDS A DISSERTATION SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL OF SCIENCE IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF GEOLOGY BY CHARLES ELIJAH DECKER THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS 1920 iERKELEY IBRARY N1VERSITY Of CALIFORNIA EARTH SCIENCES LIBRARY EXCHANGE ZTbe ZHmx>er8its of Gbicaao STUDIES IN MINOR FOLDS A DISSERTATION SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL OF SCIENCE IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF GEOLOGY BY CHARLES ELIJAH /DECKER THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS 1920 EARTH SCIENCES IISRARY COPYRIGHT 1920 BY THE UNIVERSITY OF CHICAGO All Rights Reserved Published September 1920 . .- , - Composed and Printed By The University of Chicago Press Chicago. Illinois. U.S.A. CONTENTS PAGE INTRODUCTION 3 TYPES or FOLDS 4 MINOR FOLDS IN ASSOCIATION WITH MAJOR FOLDS 16 FOLDS IN THE MIDST OF HORIZONTAL OR GENTLY DIPPING STRATA . 22 ORIGIN or FOLDS 48 NATURE AND ORIGIN or STRESSES 76 RELATION or THESE MINOR MOVEMENTS TO MAJOR MOVEMENTS . . 78 SUMMARY . 81 433086 LIST OF ILLUSTRATIONS PAGE FIG. i. DIAGRAM or A SYMMETRICAL ANTICLINE 5 FIG. 2. PHOTOGRAPH OF A SYMMETRICAL ANTICLINE 5 FIG. 3. DIAGRAM or AN UNSYMMETRICAL ANTICLINE 6 FIG. 4. PHOTOGRAPH or AN UNSYMMETRICAL ANTICLINE 6 FIG. 5. DIAGRAM OF AN OVERTURNED FOLD 7 FIG. 6. PHOTOGRAPH OF AN OVERTURNED FOLD 7 FIG. 70. DIAGRAM OF A RECUMBENT ANTICLINE 8 FIG. 76. DIAGRAM OF AN OVERTURNED FOLD WITH RECUMBENT TOP 8 FIG. 8. PHOTOGRAPH OF AN OVERTURNED FOLD WITH RECUMBENT TOP 8 FIG. 9. DIAGRAM OF A CLOSED ANTICLINE 10 FIG. 10. PHOTOGRAPH OF A CLOSED ANTICLINE 10 FIG. ii. DIAGRAM OF AN OPEN FOLD n FIG. 12. PHOTOGRAPH OF AN OPEN FOLD n FIG. 13. AN ERODED DOME IN THE ARBUCKLE MOUNTAINS 12 FIG. 14. END OF AN ERODED DOME IN THE ARBUCKLE MOUNTAINS 12 FIG. 15. DIAGRAM OF A SYNCLINE BETWEEN Two ANTICLINES 13 FIG. 16. PHOTOGRAPH OF A SYNCLINE BETWEEN Two ANTICLINES 13 FIG. 17. DIAGRAM OF A MONOCLINAL FOLD 15 FIG. 18. PHOTOGRAPH OF A MONOCLINAL FOLD 15 FIG. 19. DIAGRAM OF PART OF SOUTHWEST LIMB OF ARBUCKLE ANTI- CLINE 20 FIG. 20. SYNCLINE BETWEEN Two ANTICLINES IN ARBUCKLE MOUN- TAINS 22 FIG. 21. PLUNGING ANTICLINE IN ARBUCKLE MOUNTAINS 22 FIG. 22. SMALL INTRA-FORMATIONAL FOLD 38 FIG. 23. ANTICLINE HAVING Axis PARALLEL WITH VALLEY 39 FIG. 24. ANTICLINE AT NORTH EDGE OF MEADVILLE, PA 39 FIG. 25. UNSYMMETRICAL ANTICLINE NEAR GIRARD, PA 40 FIG. 26. ANTICLINE NEAR NORTH EAST, PA 40 FIG. 27. ANTICLINAL FOLD WITH UNERODED CREST. 42 FIG. 28. SMALL ANTICLINE ALONG ELK CREEK 42 FIG. 29. THRUST FAULT ALONG LITTLE ELK CREEK 44 FIG. 30. THRUST FAULT ALONG SIXTEEN MILE CREEK 44 viii LIST OF ILLUSTRATIONS PAGE FIG. 31. THRUST FAULT NEAR GIRARD, PA 45 FIG. 32. THRUST FAULT ALONG WALNUT CREEK 45 FIG. 33. UNSYMMETRICAL FOLD WITH Two THRUST FAULTS 46 FIG. 34. Two THRUST FAULTS WITH SMALL FOLD BETWEEN 46 FIG. 35. Two SMALL FOLDS IN OPEN COAL MINE 66 FIG. 36. SMALL FOLDS IN SHALE NEAR DUNKIRK, N.Y 66 FIG. 37. RIDGE ON TERRACE ABOVE FOLD ; 71 FIG. 38. FOLD DEFORMING A FORTY-THREE-FOOT TERRACE 71 FIG. 39. ANTICLINE WITH ERODED CREST NEAR PROSPECT, N.Y 72 FIG. 40. FOLD WITH ERODED CREST NEAR KINGSVILLE, OHIO 72 FIG. 41. FOLD WITH CREST UNERODED IN TWENTY-FOOT TERRACE... 73 FIG. 42. FOLD WITH CREST UNERODED IN Low TERRACE 73 FIG. 43. FAULT WITH TOP ERODED 75 FIG. 44. FAULT WITH TOP UNERODED 75 PLATE I. GENERAL MAP OF EASTERN LAKE REGION facing 3 PLATE II. GENERAL MAP OF OKLAHOMA facing 17 PLATE III. ISOBASES OF DEFORMED PENEPLAIN AND OF UPLIFT TO NORTHEAST facing 81 LIST OF TABLES TABLE PAGE I. PALEOZOIC FORMATIONS OF OKLAHOMA 18 II. PALEOZOIC FORMATIONS OF PENNSYLVANIA AND NEW YORK 25 III. LOWER DEVONIAN AND UPPER MISSISSIPPIAN FORMATIONS 27 IV. PORTAGE GROUP 28 V. TREND OF AXES IN LAKE REGION 80 VI. TREND OF AXES AT NORTHWEST EDGE OF APPALACHIANS . . 80 OUTLINE INTRODUCTION TYPES or FOLDS Symmetrical Anticline Unsymmetrical Anticline Overturned Anticline Recumbent Fold Closed Anticline Open Anticline Dome Syncline Isoclinal Folds Anticlinoria and Synclinoria Monoclinal Fold MINOR FOLDS IN ASSOCIATION WITH MAJOR FOLDS General Relations and Direction of Axes Minor Folds in the Arbuckle and Wichita Mountains of Oklahoma Igneous Rocks Sedimentary Rocks Structure of Rocks Types of Folds Summary FOLDS IN THE MIDST OF HORIZONTAL OR GENTLY DIPPING STRATA Location and Area Topography of the Area Lake Plain Upland Plain Stratigraphy Ordovician Formations Beekmantown Limestone Lowville Limestone Trenton Limestone Queenston Shale Upper Devonian Portage Group Huron Shale Girard Shales Chagrin Formation Chemung Formation Cleveland Shale Cattaraugus Formation STUDIES IN MINOR FOLDS Devono-Carboniferous Formations Riceville Shale Oswayo Formation Mississippian Formation Bedford Shales Pennsylvanian Formation Sharon or Olean Conglomerate Quaternary Deposits Pleistocene Post-glacial Geologic History Physiographic History Glacial Erosion Glacial Deposition Drainage Changes Glacio-Lacustrine Substage Post-glacial Changes Structure of the Rocks General Structure Local Structure General Types of Folds Intra-formational Folds Parallel Folds Transverse Folds Faults Unrelated to Folds Related to Folds Origin of Folds and Faults Igneous Activity and Heat from Molten Rock Rise in Temperature at Close of Glacial Period Pressure Due to Expansion of Ice Alteration of Iron Sulphides Weathering of Rocks Crystallization of Limestone Solution of Rocks beneath the Surface* Shrinkage by Compacting Soft Rocks Pressure of Valley Walls Relief from Compression Weight of Delta Landslides Pressure of Natural Gas Differential Movement Glaciation Drag of Icebergs STUDIES IN MINOR FOLDS 3 Tangential Compression Summary of Origin of Folds Age of Folds and Faults Pre-glacial Glacial Post-glacial NATURE AND ORIGIN OF STRESSES General Cumulative Stresses General Residual Stresses RELATION or THESE MINOR MOVEMENTS TO MAJOR MOVEMENTS Tilting to the Northeast Doming of Harrisburg Peneplain SUMMARY INTRODUCTION The data on which these studies are based have been secured from a narrow area south of Lake Erie, extending from Cleveland across northeastern Ohio, northwestern Pennsylvania, and into New York as far as Dunkirk (see Plate I) . A few folds on Lake Ontario were studied, both in northern New York and southern Canada. A number of inter- mediate points between Niagara and Dunkirk were visited, as were several others farther southeast in New York and westward from Cleveland to Sandusky, Ohio. A few folds were studied in the folded areas of the Arbuckle and the Wichita mountains of Oklahoma. In geological literature much attention has been given to structural studies, especially to folds and folded regions. However, the major folds have seemed so to overshadow the minor ones that the latter have been passed over, being considered relatively unimportant. Economic considerations have directed attention to some minor folds which received no consideration as having any importance from a structural standpoint. Of the folds of this type, some of the most noteworthy are those in Pennsylvania, West Virginia, and Oklahoma for oil and gas, Wisconsin for lead and zinc, Michigan and Wisconsin for iron, and Australia for gold. It is the purpose in this paper to illustrate by diagrams and photo- graphs a series of types of minor folds; to illustrate and study briefly a few minor folds in their relation to major ones; to illustrate a larger number of small folds in the midst of horizontal or gently dipping strata, showing their characteristics, methods of origin, age, and relation to faults; and finally, to connect these minor deformations, in so far as 4 STUDIES IN MINOR FOLDS possible, with larger movements, and show their significance, as indi- cating the presence of compressional stresses in the rocks in the interim between the great periods of deformation and mountain-building. The writer makes grateful acknowledgment for direction and help to Professor R. D. Salisbury, under whose supervision this study has been pursued, to Professor T. C. Chamberlin for helpful suggestions, to Professor R. T. Chamberlin for valuable criticisms and suggestions, to Frank Gahrtz for work on figures and maps, and to W. E. Coon for field assistance. TYPES OF FOLDS The most common type of fold is the anticline. Gilbert has defined it as one in which the strata dip in two directions away from the axis. 1 Anticlines are extremely variable in form, so that a number of distinct types have been recognized. These types are symmetrical, unsymmet- rical, overturned, recumbent, open, and closed folds, besides the com- pound form the anticlinorium. Symmetrical anticline. A symmetrical anticline is a fold whose axial plane is vertical and on which the dip at corresponding points on the two limbs is equal. A diagram of a symmetrical fold is shown in Figure i. In this figure cd is a line in the axial plane which divides the anticline into two equal and symmetrical limbs. The fold is rep- resented as having the crest eroded and restored by the dashed lines. Examples of folds of this general type were found near Westfield, New York; North East, Erie, Girard, and Meadville, Pennsylvania; and Andover, Conneaut, Kingsville, and Painesville, Ohio. A fold of this type which, however, is not perfectly symmetrical, is shown in Figure 2. It crosses the bed of Walnut Creek 5 miles south of Erie, Pennsylvania. The dip to the west is 14 and to the east is 1 7. The axis trends N. ioW. The beds vary in thickness from 2 to 7 inches, and consist chiefly of blue shale with some sandy beds interspersed. The stream has eroded the crest of the fold, cutting deeply into the axis forming the basin for the pool in the foreground. The loose material on top of the fold is shingle, contributed largely by the tributary which enters at the left of the crest. A more perfectly symmetrical fold is shown in Figure 31, in which the dip is 8 in each limb. Among the other folds of this type mentioned above, some of those near Conneaut and Painesville are more gentle and open, while some near Westfield, Andover, and Meadville are smaller and more closed. 1 G. K. Gilbert, Amer. Jour. Sci., 3d Ser., XII (1876), 21. STUDIES IN MINOR FOLDS 5 Unsymmetrical anticline. An unsymmetrical anticline is a fold in which the axial plane is inclined and in which the strata dip more steeply in one limb than in the other. A diagram of a fold of this type is shown in Figure 3. In this figure the line cd marks the inclined axial plane which divides the fold into two very unsymmetrical parts. The dip in FIG. i. Diagram of a symmetrical anticline from which the crest has been eroded. Restoration of the eroded part is shown by dotted lines. FIG. 2. Photograph of a symmetrical anticline in the bed of Walnut Creek, 5 miles south of Erie, Pa. the limb at the right is much greater than in the one at the left. A fold of this type deforms the west bank of the Chagrin River, a mile west of Chagrin Falls, Ohio. Figure 4 is a photograph of this fold. The steep dip downstream to the northeast is 29, and the gentler dip upstream is 11. The fold is about 160 feet wide, and deforms the rocks of a terrace 56 feet high. The rocks consist of alternating sandstones 6 STUDIES IN MINOR FOLDS and shales in the Bedford formation. 1 (See Table III.) The heavy sandstone bed, which stands out clearly in the picture, is 17 inches in thickness. This bed has been slightly faulted at the crest of the fold, the horizontal displacement being 2 feet and 10 inches. The dis- tinctness of the brecciated zone above this stratum suggests that the FIG. 3. Diagram of an unsymmetrical anticline FIG. 4. Photograph of an unsymmetrical anticline near Chagrin Falls, Ohio remaining strata above were affected by this fracture. While the steep side of the bank is weathered and partially covered with vegetation, the fact that a path was started from the top of the terrace along a natural depression in the line of the axis seems to be another indication that the fault affects all of the uppermost strata. Here, then, a fold below 1 C. H. Prosser, Geol. Surv. Ohio Bull. 15, 4th Ser. (1912), pp. 197, 198. STUDIES IN MINOR FOLDS 7 passes into a fault above. 1 This same relation of fold and fault was found 25 miles to the northeast along Paine Creek. This anticline is only one of a large number with marked asymmetry that exist in the area. Two distinct examples of this type occur within half a mile of FIG. 5. Diagram of an overturned fold Fig. 6. An overturned fold on the northwest shore of Lake Ontario near Bur- lington, Ontario. each other near Girard, Pennsylvania, and in them the highly inclined limbs are toward one another. Overturned anticline. When the stress or resistance differs greatly on the opposite sides of a fold, the strata on one side are raised and the 1 Chamberlin and Salisbury, Geology, I (1905), 516, Fig. 121; and Bailey Willis, Thirteenth Ann. Kept. U.S.G.S., Part II (1893), Plates 79, 93, 94. 8 STUDIES IN MINOR FOLDS top of the fold is thrust forward and bent over until the strata on both sides of the axis dip in the same general direction. By this process the overturned anticline is formed. Figure 5 is a diagram of an overturned fold. The axis is bent to the right to such an extent that the younger beds beneath the axis have their dips reversed toward the left, or in the FIG. 7a. Diagram of a recumbent anticline (after Van Hise) FIG. 76. Diagram of an overturned fold with recumbent top FIG. 8. An overturned fold with recumbent top, near North East, Pa. same general direction as those above the axis. Figure 6 shows an overturned anticline in the sandy shales on the northwest shore of Lake Ontario, 8 miles northeast of Burlington, Ontario. The fold is in the uppermost shales in a 1 4-foot terrace, and the strata in the over- turn are thrust up into the terrace material in a way to indicate the recency of the fold. This is one of several small folds above the end of a thrust fault whose plane has the low angle of 5 to 14. Two other STUDIES IN MINOR FOLDS 9 overturned folds were found in northern Ohio, one 3 miles south of Willoughby, along the Chagrin River, and one 6 miles southwest of Conneaut, along Conneaut Creek. On the south side of Lake Ontario, at Thirty Mile Point, there is an anticline with overturned axis. The overturned top of the fold incloses glacial drift beneath it. This fold is shown in Plate 19 in the back of the Niagara Folio. Recumbent fold. When an anticline is so far overturned tjiat the inverted strata approach a horizontal position, 1 it is called a recumbent fold. A diagram of such a fold is shown in Figure ja. This is taken from Van Hise, who in his article on "North American Pre-Cambrian Geology" gives an excellent discussion of the various types of folds. 2 No fold closely approaching the form of the one in Figure 7 a was found in the area studied, but Figure 7 b is a diagram of one one-fourth mile south of North East, Pennsylvania, on the east side of Sixteen Mile Gulf. The notebooks in the photograph of the same fold (Fig. 8), one on the axis and one down in the right foreground, are on the same stratum. By referring to Figure jb, it will be seen that the strata have been broken at the base of the right limb and overthrust to the right. In this figure the bed beneath e has its continuation at/g. This is the only example of a fold found in this area approaching the recum- bent type at all closely. Closed anticlinal fold. A closed anticline is one in which the limbs are pressed closely together. With reference to position of axial plane a closed fold may be upright, overturned, or recumbent. A diagram of an upright closed anticline is shown in Figure 9, and a photograph of one in Figure 10. This is the central part of a much larger fold, several of which are associated here on the flank of a very much larger anti- cline. For several feet from the crest the strata are parallel, as they are continuously in the carinate fold. A short distance from the top, however, the strata begin to diverge. This fold is in the midst of a folded area, in the thin beds of the Simpson formation in the Arbuckle Mountains, one-fourth mile below Crusher, Oklahoma. No folds as close as this were found in the Great Lakes region, though several approach the closed condition. Open anticline. An open anticline is one in which the strata spread widely from the axial plane. Open folds may have the axial plane vertical or inclined. In Figure n a diagram is given of a gentle, open fold. The strata swing up in broad, open curves, and the dip is slight 1 Bailey Willis, op. cit., p. 221. 2 C. R. Van Hise, Sixteenth Ann. Kept. U.S.G.S., Part I (1895), pp. 581-843. 10 STUDIES IN MINOR FOLDS from the axis on either side. The photograph of such an anticline is shown in Figure 12. It is in the east bank of Big Creek, 3 miles south- FIG. 9. Diagram of a closed anticline FIG. 10. Closed anticline in Simpson formation near Crusher in the Arbuckle Mountains, Okla. east of Painesville, Ohio. The fold is 60 feet wide, has a rise of 4^ feet at the crest, and the dip is 12 on either side. The strike of the beds, STUDIES IN MINOR FOLDS II of N:35W., and the direction of the axis, are well shown by the fall of the creek over the southwest limb. A sandy bed 15 inches thick in the midst of the shales is the cause of the fall, and this resistant bed stands out near the base of the bank, showing clearly the form of the fold. A large number of folds of this type were found, some larger, some smaller. FIG. ii. Diagram of a gentle open fold FIG. 12. Photograph of a gentle open fold along Big Creek, near Painesville, Ohio. Most of the large, open ones seem to belong to the earlier periods of folding, for in connection with the large open folds no evidence was found to indicate that any of them were as recent as the glacial period. Dome. A dome is an anticline in which the strata dip in all direc- tions from the center, or one with quaquaversal dip. A dome may be either circular or elongate. No domes were studied in the eastern area, but one was found in the Arbuckle Mountains of Oklahoma, occurring 12 STUDIES IN MINOR FOLDS in the Arbuckle limestone at the head of Falls Creek (Figs. 13 and 14). Numerous larger domes occur in the Henry Mountains of Utah, 1 in the Piedmont region of Maryland, 2 and in eastern Wyoming 3 the Black FIG. 13. An eroded dome in the Arbuckte limestone near the head of Falls Creek, Arbuckle Mountains, Okla. FIG. 14. A view of the dome in Figure 13 taken across the front foreground in the bed of the creek. The deep hole at the left is in the crest. 1 G. K. Gilbert, "Henry Mountains," U.S.G.S. (1877), Plates II and IV. 2 E. B. Mathews, Johns Hopkins University Circular, New Series, No. 7 (1907), pp. 27-34. * Sundance Folio, No. 127 (1905), "Structure Section Sheet." STUDIES IN MINOR FOLDS 13 Hills themselves being a large dome. 1 A dome of the elongate type, truncated by erosion, occurs in the Niagara limestone at Stony Island. This dome, in the southern part of Chicago, was an island at the late stage of Lake Chicago (the predecessor of Lake Michigan), when the outlet was to the southwest through the Des Plaines and Illinois rivers. cL FIG. 15. Diagram of a syncline between two anticlines FIG. 1 6. Syncline between two anticlines near Andover, Ohio Syncline. The syncline is the reverse of an anticline. According to Gilbert the strata in it dip in two directions toward the axis. 2 A single stratum dipping from both sides toward the axial plane forms a basin. And as the top of the anticline is called the crest, so the bottom of the syncline is called the trough. As in anticlines, so in synclines the axial planes may be vertical or inclined, and the fold may be open 1 N. H. Darton, Prof. Paper No. 65 (1909), p. 62. 2 G. K. Gilbert, Amer. Jour. Sci., 3d Ser., XII (1876), 21. 14 STUDIES IN MINOR FOLDS or closed. Figure 15 shows a diagram of a syncline between two anti- clines. The axes of the anticlines are marked by the lines ab and cd. The axial plane of the syncline lies about halfway between. There are three small anticlines with two intervening synclines on the north side of a small stream at the southern edge of the Andover quadrangle, Ohio. They are upstream a few rods east of the Lake Shore and Michi- gan Southern Railway. A photograph of one of these synclines and two of the anticlines is shown in Figure 16. While several synclines were found, there were very few as compared with the number of anticlines. Isoclinal or carinate folds. When a series of folds is so closely compressed that the strata in the limbs are all parallel, they are called isoclinal, and carinate is the name given to a single fold of this type, No isoclinal folds were found. Anticlinoria and synclinoria. When the large anticlines and syn- clines have within them a series of smaller folds, they are called anti- clinoria and synclinoria. Compound folds of this type are found in the Arbuckle Mountains, 1 but the larger ones are far too great in extent to get within the compass of a photograph. However, both in these mountains and in the Wichitas, smaller anticlinoria and synclinoria commonly have plunging axes, so the structure is shown by the edges of the strata that protrude in the plateaus. (See Fig. 21.) In the eastern area near the Great Lakes, anticlines and synclines are associ- ated in a number of places, but not in the form typical of the anti- clinoria and synclinoria. A part of a series of anticlines and synclines is seen in Figure 16. The fold shown in Figure 8 is adjacent to a larger anticline which has another sharp fold at the opposite end of it. Other examples of the association of anticlines and synclines are found along Elk Creek near Girard, Pennsylvania. In these instances, however, no larger fold seems to dominate them to give them the form of anti- clinoria or synclinoria. Monoclinal fold. "A monoclinal fold is a double flexure, connect- ing strata at one level with the same strata at another level." 2 It has one less flexure than the anticline. Figure 17 shows a diagram of a monoclinal fold in which the double flexure, marked by the lines cd and ef, joins the strata above with the same strata at the right below. A fold of this type is shown in Figure 18. This fold is in the flaggy Che- mung sandstones and shales along Sixteen Mile Creek, three-fourths of a mile northwest of North East, Pennsylvania. It deforms the fourteen- 1 C. A. Reeds, Okla. Geol. Surv. Bull. 3 (1910), pp. 51-53. 2 G. K. Gilbert, Amer. Jour. Sci., 3d Ser., XII (1876), 21. STUDIES IN MINOR FOLDS 15 foot terrace, and the loose shales at the top are uneroded. The mono- clinal fold so common in the Colorado plateaus 1 is uncommon in this area, only a few others having been found. The term monoclinal has been used with two other distinct mean- ings. The most common use is to express the structure of the rocks e FIG. 17. Diagram of a monoclinal fold FIG. 1 8. A monoclinal fold near North East, Pa. in an area in which the formations of a series are inclined in a single direction, irrespective of the manner in which the dip was acquired. Thus the structure on one side of an eroded dome is called a monocline. 2 The structure of the eastward dipping rocks of the Connecticut Valley 1 G. K. Gilbert, ibid., p. 21. 2 Belle F our che Folio, South Dakota (1909), p. 5. 16 STUDIES IN MINOR FOLDS is called monoclinal. 1 The ridges formed by the resistant formations on one side of an eroded anticline are called monoclinal ridges. 2 The third sense in which the term monoclinal has been used is to describe a series of folds in wh.ich the axes all are parallel. Van Hise has called a series of folds with parallel axial planes monoclinal folds. 3 "Homo- cline" has been suggested by R. A. Daly as a substitute for monocline. He says: For convenience the word homocline will be used as a general name for any block of bedded rocks all dipping in the same direction. The writer is inclined to follow the general, though not universal usage which defines mono- cline as a one-limbed flexure in strata, which are usually flat-lying except in the flexure itself. A homocline may be a monocline, an isocline, a tilted fault block, or one limb of an anticline or syncline. 4 This seems simply a multiplication of terms in suggesting another one for the most common usage for monocline. Monoclines readily could be distinguished as to origin by one of the prefixes tilt-, syn-, anti-, block-, or fault-. MINOR FOLDS IN ASSOCIATION WITH MAJOR FOLDS General relations and direction of axes. In areas of folded rocks, smaller folds are commonly associated with the larger folds, and not infrequently folds of several orders are found together. Bascom has recognized folds of four orders associated in the Piedmont district of Pennsylvania. 5 The minor folds may occur on or among the major ones, or they may occur adjacent to the major ones, on either or both sides. With reference to the trend of axes, in some instances the axes of the minor folds are approximately parallel with those of the major ones. Mathews and Miller describe an area in north central Maryland in which the minor sharp folds are parallel with the major open ones. 6 This parallelism seems to indicate that the minor folds are related definitely to the major folds. Second, the axes of the minor folds may be transverse to those of the major folds. Those mentioned above, described by Bascom, are of this type: Here the major and minor folds seem unrelated. And third, the direction of axes of minor folds 1 Chamberlin and Salisbury, Earth History, III (1906), n. 2 J. W. Powell, Amer. Jour. Sci., ad Ser., XII (1876), 416. 3C. R. Van Hise, Ann. Kept. U.S.G.S., XVI, Part I (1895), 621; Fig. 134, 658; and Fig. 148, 801. 4 R. A. Daly, Canada Dept. Mines Geol. Surv., Mem. 68 (1915), p. 53, note. s F. Bascom, Bull. Geol. Soc. Amer., XVI (1905), 306-8. 6 E. B. Mathews and W. J. Miller, Bull. Geol. Soc. Amer., XVI (1905), 362. STUDIES IN MINOR FOLDS 17 may be extremely variable, making their relation to the major folds uncertain. In describing the structure of southeastern Alaska, Brooks says the main trend of the major structures is northwest and southeast, but the axes of the minor folds are extremely variable in direction. 1 Minor folds in the Wichita and Arbuckle mountains of Oklahoma. To show some of the relations of minor to major folds, a few illustra- tions will be given from the Arbuckle and Wichita mountains of Okla- homa. These mountains are in two separate groups in the southern part of the state, the Wichitas being about 60 miles northwest of the Arbuckles. (See Plate II.) Each group is about 60 miles long and about twice as long as wide, the longer axes extending in a general east- west direction. The Arbuckles are the lower, the highest part, at the northwest, rising about 400 feet above the surrounding plains to a total of about 1,300 feet. The highest peaks in the Wichitas rise about 1,500 feet above the plain, to a total height of a little less than 2,500 feet. 2 Igneous rocks. Both .the Arbuckle and Wichita mountains consist of central masses of igneous rocks surrounded by great thickness of sedimentary formations. In the Arbuckle Mountains there are three areas of igneous rocks. A large area in the southeastern part, of about 148 square miles, con- sists chiefly of granite with minor amounts of quartz-monzonite and dikes of diabase, aplite, and pegmatite. The other two areas are com- paratively small ones, together being about 7 square miles in extent, in the northwestern part of the mountains. 3 These areas consist of por- phyry with some basalt and diabase dikes. The predominance of acidic rocks in the Arbuckle Mountains is marked, as the basic type is limited largely to dikes. In the Wichita Mountains the igneous rocks form a large elongate central mass around which are numerous scattered areas, particularly to the southwest and northwest. While granites and porphyries pre- dominate, there are, besides the dikes of basalt and diabase, very large areas of gabbro, so there is a very much larger amount of basic rock in the Wichitas than in the Arbuckles. In both groups of mountains the igneous rocks are pre-Cambrian in age. Against these massive igneous rocks the sedimentary formations have been folded. 1 F. H. Brooks, U.S.G.S. Prof. Paper 31 (1904), p. 29. 2 J. A. Taff, U.S.G.S. Prof. Paper 31 (1904), p. 54. 3 C. A. Reeds, Okla. Geol. Surv. Bull. 3 (1910), pp. 31-32. i8 STUDIES IN MINOR FOLDS TABLE I PALEOZOIC FORMATIONS OF THE ARBUCKLE MOUNTAINS, OKLAHOMA System Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Upper Cambrian Middle Cambrian (Modified from Wallis) 1 Formations Red beds Franks conglomerate f Caney shale \ Sycamore limestone Woodford chert (Hiatus) Bois d'Arc limestone Haragan marl (Hiatus) Henryhouse shale (Hiatus) Chimneyhill limestone Sylvan shale [ Viola limestone < Simpson formation [ Arbuckle limestone) Arbuckle limestone/ Reagan sandstone Thickness 0-500' (Max) 1600' 0-200' (Max.) 650' 0-90' 0-166' 0-223' c-53' 6o'-30o' 1 200 '-2000' 40oo'-6ooo' 0-500' Sedimentary rocks. In the Arbuckle Mountains, surrounding the pre-Cambrian igneous rocks, and resting unconformably upon them, is a series of sedimentary beds, 10,000 or more feet in thickness. The maximum for all the formations totals over 12,000 feet. These sedi- ments belong to the Paleozoic. To the north, west, and southwest lie the Permian Red Beds. At the southeast the Cretaceous overlaps and extends upon the granite, covering all older formations. A table showing the formations extending from the Middle Cambrian through Permian is given in Table I. Sedimentation was almost continuous from Middle Cambrian to near the close of the Mississippian, there being but three small breaks before the marked one between the Mississippian and Pennsylvanian. Toward the end of the Mississip- pian the area was uplifted, and the rocks were folded and faulted. Then these folded rocks were eroded almost to a peneplain before the Franks conglomerate of Pennsylvanian age was deposited. A second period of folding and erosion followed, making the Permian Red Beds unconformable upon the edges of all the older rocks. While all the 1 B. F. Wallis, Okla. Geol.Surv. Bull. 23 (1915), p. 32. STUDIES IN MINOR FOLDS 19 formations from Middle Cambrian to Upper Mississippian were deformed together, and all are involved in the larger structures, the minor folds occur within the limits of certain formations. As illustrations will be used only from the Arbuckle and Simpson, only these two formations will be described. The oldest sedimentary formation in the region, the coarse Reagan sandstone, is succeeded conformably by the Arbuckle limestone. This is the most competent formation in the region. It is heavy bedded, and has the great thickness of 4,000 to 6,000 feet. Most of the beds are a foot or more in thickness, and some are over 10 feet. In general the texture varies from fine even granular to compact, but some parts are coarsely crystalline, and some rather shaly members occur near the top. The Simpson formation reaches a thickness of 1,200 to 2,000 feet. The lower part consists of sandstone with shales, then limestone and more sandstone, and thinly bedded shaly limestones above. So in contrast with the Arbuckle limestone, the Simpson is relatively weak and incompetent. In the Wichita Mountains the sedimentary forma- tions have much the same characteristics as in the Arbuckles in so far as they are exposed, but the Red Beds still cover all but the Reagan, Arbuckle, and a little of the Viola. Structure of rocks. As noted before, all the Paleozoic formations except the Pennsylvanian and Permian were deformed together, the deformation including intense folding and faulting. The general trend of the larger folds is northwest-southeast, and they are several miles in width. Upon them are the smaller folds of one or more orders. On one of the large folds, C. A. Reeds 1 has recognized ten smaller longi- tudinal ones, and several still smaller transverse ones of a third order. In both groups of mountains the Arbuckle limestone exhibits these minor folds of several orders on the major ones. These major folds, many of which have plunging axes, have been truncated by erosion, and the edges of the strata in these truncated folds show clearly on the plateaus. Types of folds. As indicated above, some types of folds are found in the folded mountain areas that do not occur in the eastern lake region. The two types already illustrated the dome, and the closed fold will be treated more in detail, and some plunging folds illustrated and described. The dome shown in Figure 13 occurs in the channel near the head of Falls Creek, where a good section has been exposed by the stream 1 C. A. Reeds, Okla. Geol. Surv. Bull. 3 (1910), pp. 51-53. 20 STUDIES IN MINOR FOLDS cutting across it. It is a very close and conical fold to be formed in a heavy-bedded formation, and shows well how abruptly parts of this thick-bedded limestone have been folded. The bed at the left is 4 feet 3 inches thick. Another picture was taken across the eroded north end of the dome across the right foreground in Figure 13. This view is given in Figure 14, in which the abrupt curve in these heavy beds is shown. The stream has eroded a deep hole at the left from the crest of the fold. This is the most abrupt one of a series of small folds found on the large Arbuckle anticline. Another type of fold in a different relation occurs one-fourth mile below Crusher, on the southeast side of the Washita River. Here the FIG. 19. Diagram of a part of southwest limb of Arbuckle anticline, showing folds in the thin-bedded Simpson formation. The heavy beds at the left represent the Arbuckle limestone. minor folds are on the southwest limb of the same large Arbuckle anti- cline. The center of the large fold and the inner part of the southwest limb are occupied by the heavy beds of the Arbuckle limestone, and succeeding these toward the southwest are the thin beds of the Simpson. These relations are shown in Figure 19. The sharp folds shown in the diagram are in the thin beds in the Simpson. The beds to the right of the fold are the sandstones of the Simpson, while the thick beds at the left represent the Arbuckle limestone. This illustrates the difference in the response to the compressive stresses in the two formations. In the Arbuckle the readjustment has taken place between the heavy beds, while in the Simpson it has taken place across the thin beds, throwing these thin beds into acute folds. Figure 10 shows the central part of one of these close folds. These folds are in the position on the large STUDIES IN MINOR FOLDS 21 anticline in which there is the maximum of readjustment, for it is least at the crest of an anticline and the trough of a syncline, and most at intermediate points on the limb. 1 These minor folds not only show the difference in the response of the two formations, but they are a measure, in part at least, of the readjustments that have taken place between the beds in the Arbuckle limestone. Leith has stated succinctly this relation the folds in the weaker formations bear to the movements between the more resistant ones, as follows: Rocks within our field of observation are of varied competence. It follows then that in any folded area the structures of the weaker rocks are controlled by the folding of the stronger beds, and these tend to assume the "parallel" type of fold, in which the readjustment is between the beds rather than in them. This readjustment or slipping is concentrated in the interven- ing weaker layers. The structures of the weaker layers indicate the direction of the readjustment, and thus something of the structure of the competent beds. 2 More common than the dome and the closed fold are those of-a more open type which generally have plunging axes. Two broad open anti- clines with an intermediate synclinal trough are shown in Figure 20. They occur on the north side of the Wichita Mountains, where the general dip is northward toward the left. These are minor folds on a series of much larger ones succeeding one another across the broad exposure of the Arbuckle limestone. At this locality, this formation has exceptionally heavy beds, some of them being over 10 feet in thickness. A part of a more sharply plunging fold in the Arbuckle Mountains is shown in Figure 21, in which the beds are seen to curve around from the foreground toward the right. This plunging anticline is only one of a large number exposed over hundreds of acres of this plateau. The trees in the center of Figure 2 1 are on the sides of the channel of Falls Creek, near its head. A few miles to the north, the Arbuckle limestone is mineralized, and some zinc has been taken from it. When it is realized that minor folds may be the key to the solution of major complex structures, they are invested with a new significance. Summary of minor folds in association with major folds. From this brief consideration of minor folds associated with major ones, it is concluded that they are worthy of careful attention and study, as they may be an important part of a major structure. They are particularly significant when occurring in the weaker strata of a series, as they then give evidence of the magnitude and nature of the differential 1 C. R. Van Hise, Jour. GeoL, IV (1896), 208. 2 C. K. Leith, Structural Geology (1913), p. 114. 22 STUDIES IN MINOR FOLDS movements in the stronger adjacent rocks, and they may give a clue to the solution of the complex major structures. FIG. 20. A gentle syncline between two anticlines in very heavy beds of Arbuckle limestone on the north side of the Wichita Mountains. FIG. 21. Edge of strata in an anticline plunging to the left, near head of Falls Creek, Arbuckle Mountains. FOLDS IN THE MIDST OF HORIZONTAL OR GENTLY DIPPING STRATA Location and area. This study of minor folds began with a few small ones in the vicinity of Meadville, Pennsylvania. Part of these STUDIES IN MINOR FOLDS 23 folds had been described earlier. 1 From this locality the study was carried northward to Lake Erie, thence westward into Ohio and east- ward into New York. Most of the study was limited to a strip 15 to 30 miles wide, along the south side of Lake Erie, extending from Cleve- land, Ohio, to Dunkirk, New York, a distance of 140 miles. However, farther west, most of the larger streams between Cleveland and San- dusky were traversed for several miles from the lake, and points were visited in east central New York and on both sides of Lake Ontario in New York and Ontario. (See Plate I, general map.) This strip bordering the south shore of Lake Erie is at the north end of the broad structural basin lying between the Allegheny Plateaus to the southeast and the Cincinnati Arch on the west. There seems to have been an impression that in this area in which the rocks in general are flat r lying, no folds would be found. White says of this area: If there be any anticlinical and synclinal undulations at all in Erie and Crawford counties, they are so exceedingly flat that nothing short of an expen- sive system of measured borings, connected by instrumental surveys, would suffice to reveal their presence, measure their force and determine their direc- tion; which by the way should be from the northeast to southwest at some angle approximately parallel to the anticlinal rolls of Clarion, Butler and Beaver counties. 2 When some folds were found, an attempt was made to connect them with the Appalachian structure to the southeast, but it was con- cluded that they could not be related to the larger folds because their axes were variable, and because they were not parallel with those of the major structures to the southeast. 3 This study has brought out the fact that numerous small folds and faults exist in this region. TOPOGRAPHY OF THE AREA This area bordering the south side of Lake Erie is naturally divided into two provinces the Lake Plain and the Upland. The Lake Plain. The Lake Plain is a narrow strip varying from 4 to 6 miles in width. The elevation of this plain above the lake along its northern edge is variable. Northeast of Cleveland there is a 25-foot bluff, while north of Geneva, 12 miles west of Ashtabula, there is no cliff. For most of the 'distance eastward the cliff varies from 25 to 1 Smallwood and Hopkins, Bull. Syracuse Univ., 4th Ser., No. i (1903), pp. 18-24. 2 1. C. White, Second Geol. Surii. Pa., Kept. Q 4 (1881), p. 45. 3 Smallwood and Hopkins, op. cit., pp. 18^24; and I. C. White, op. tit., p. 45. 24 STUDIES IN MINOR FOLDS 60 feet. However, at points northwest of Girard and north of North East, Pennsylvania, it has a height of more than 125 feet. Along its southern margin this plain rises from 100 to 200 feet above the lake. The general slope of the plain toward the lake is 20 to 40 feet per mile, though some parts of it are very flat, as in the area west of Painesville, Ohio. The plain is trenched by several large streams crossing it, the most important of which are the Cuyahoga, Chagrin, Grand, and Ashta- bula rivers, and the Conneaut, Elk, Walnut, Mill, Sixteen Mile, Twenty Mile, Chautauqua, and Canadaway creeks. Most of these streams cross the plain through valleys with steep banks which frequently are 80 feet or more in height. Some of the streams have widened their valleys to a fourth- or a half-mile, and a few have locally widened them still more. Along the channel numerous terraces occur at various inter- vals above the flood plains. Between the major streams numerous small ones cross the plain. A few of these have deep channels, but most of them flow through slight depressions. The Upland The northern part of this Upland forms the divide between the streams tributary to Lake Erie and those flowing south. From Cleveland east to Girard the divide is about 25 miles south of the lake shore. Eastward from Girard it is less than 10 miles from the lake. At the inner edge of the Lake Plain there generally is an abrupt slope in the form of a lake cliff marking a former higher level of the lake. At the west the divide usually is flat, with elevations reaching above 1,300 feet (Lake Erie is 573' A.T.). Toward the east the divide rises to over 1,600 feet, or about 900 feet above the lake, though some of the hills in the Clymer quadrangle have elevations above i, 800 feet. The rise from the Lake Plain is less marked at the west than in the east. Five miles west of Westfield, New York, the rise is 500 feet in three-fourths of a mile. The Upland is very prematurely dissected, or in a youthful stage of erosion. The larger streams tributary to the lake have trenched their post-glacial valleys through glacial till deeply into the bedrock. The banks along many of the streams rise abruptly 80 or 100 feet above the valley floor, and in the gulf south of Westfield the banks rise 400 feet above the stream. As in the Lake Plain, so in the Upland, terraces with varying intervals occur in these valleys. The large streams form- ing a part of the southward drainage flow for the most part through broad, flat valleys, from which hills rise by several easy stages to heights of from 100 to 500 feet. Locally, where streams are impinging against the sides of old valleys, and in a few places where there is great inequality STUDIES IN MINOR FOLDS in hardness of rocks, as at Thompson's ledge, twelve miles southeast of Painesville, Ohio, steep slopes exist. Some of the small tributaries to these larger streams have cut deep valleys into hillsides, exposing thick sections of bedrock. TABLE II SHOWING PENNSYLVANIAN AND MISSISSIPPIAN FORMATIONS FOR PENNSYLVANIA, AND DEVONIAN, SILURIAN, AND ORDOVICIAN FOR NEW YORK (After Clarke and Schuchert, 1 and White 2 ) System Pennsylvanian Mississippian Devonian Formation f Sharon or Olean \ conglomerate Shenango beds Meadville beds Sharps ville sandstone Orangeville shale Cussewago beds Riceville shale Chemung beds Portage beds Genesee shale Tully limestone Hamilton beds Marcellus shale Onondaga limestone Schoharie grit Esopus grit Oriskany beds Kingston beds Becraft limestone New Scotland beds Coeymans limestone System Silurian Formation Manlius limestone Rondout waterlime Salina beds Guelph dolomite Lockport limestone Rochester shale Clinton beds Medina sandstone i Oneida conglomerate Richmond beds Lorraine beds Utica shale Ordovician < Trenton limestone Black River limestone Lowville limestone Chazy limestone Beekmantown limestone STRATIGRAPHY In the region along Lake Erie the exposed rocks extend in age from Upper Devonian to Lower Pennsylvanian. However, as other areas of 1 J. M. Clarke and C. Schuchert, Science, New Ser., X (1899), 876. 2 1. C. White, Second Geol. Surv. Pa., Rept. Q< (1881), pp. 55, 56. 26 STUDIES IN MINOR FOLDS older rocks were visited farther east and northeast, a few of the older formations will be characterized briefly. To indicate their relative position, the succession is shown in Table II. In this table the Penn- sylvanian and Mississippian are for northwestern Pennsylvania, and the Devonian, Silurian, and Ordovician for New York. ORDOVICIAN FORMATIONS Beekmantown limestone. This formation and the pre-Cambrian syenite on which it rests in all the exposure of the latter in the quad- rangle, were seen near the west edge of the Little Falls quadrangle at Middleville, 8 miles north of Herkimer, New York. About 200 feet of Beekmantown are exposed at this locality. 1 The Beekmantown is a gray, sandy dolomitic limestone. The thickness of the beds varies from 3 inches to 2 feet. Some very sandy beds 4 to 12 inches in thick- ness were observed in the ravine north of Middleville. Some low open folds occur in this formation similar to those in the following one. Lowville limestone. The Chazy formation being absent in this Little Falls area, there must be disconformity between the Beekmantown and the Lowville. 2 The Lowville is a medium to thin-bedded pure gray limestone, which near Middleville has a thickness of only about 20 feet. Gushing shows two good exposures of it in Plates 5 and 6, in Bulletin 77 of the New York State Museum series. In the latter of these two plates the low folds are well shown. Trenton limestone. The thin formation of limestone and shale above the Lowville the Black River was not exposed at the two areas visited in the Remsen quadrangle. The Trenton was seen at Graves- ville near the southern edge of the Remsen quadrangle, 20 miles north- west of Herkimer, and at Prospect, 4 miles northwest of Gravesville. " In general this formation may be said to be made up of thin bedded, dark bluish compact limestones with thin shaly partings." 3 The Tren- ton limestone is very fossiliferous. The formations in the Upper Ordovician will not be mentioned except to note that the next formation, the Queenston shale, may in part be equivalent to the Richmond. 4 Queenston shale. The Queenston shale is exposed at the lower end of the Niagara Gorge at Queenston, Ontario, whence it derives its 1 H. P. Gushing, New York State Mus. Bull 77, Geol. 6 (1905), p. 28. 2 H. P. Gushing, ibid., pp. 27-30. 3 W. J. Miller, New York State Mus. Butt. 126 (1909), p. 17. 4 E. M. Kindle and F. B. Taylor, Niagara Folio, No. 190 (1913), p. 6. STUDIES IN MINOR FOLDS 27 name. It is exposed eastward along the south shore of Ontario across the Niagara quadrangle, and in the valleys of the streams flowing into the lake. It is particularly well exposed in the gorge of Eighteen Mile Creek, two miles south of Olcott, New York. The same type of for- mation, with colors and physical characteristics the same, is exposed on the north shore of Lake Ontario to the east of Burlington, both along the shore and in valleys of streams tributary to the lake. The Canadian geological map shows that the Silurian extends around the west end of Lake Ontario and on the north side well up toward Toronto. 1 The Queenston formation consists chiefly of friable shale with some interca- lated thin sandstone beds. The predominating color is red, but beds of green and gray shales and sandstone occur, interspersed through the red shales. The total thickness, determined from deep wells, is 1,200 feet, only 300 feet of which are exposed. 2 Passing over the rest of the Silurian formations and the Lower and Middle Devonian, the Portage group in the Upper Devonian will next be considered. (See basal part in center of Table III.) TABLE III UPPER DEVONIAN AND LOWER MISSISSIPPIAN FORMATIONS FOR OHIO, PENN- SYLVANIA, AND NEW YORK System Lower Mississippian Ohio' Sunbury shale Berea grit Bedford shale Cleveland shale Pennsylvania* Orangeville shale Corry sandstone New York* Knapp formation Cussewago sandstone Riceville shale Oswayo formation Upper Devonian Chemung Chagrin formation Girard shale Huron shale Portage 6 Atlas of Canada, No. 5, "Geology, East Sheet." Cattaraugus Chemung Portage beds? 2 Kindle and Taylor, op. cit., p. 5. 3 C. S. Prosser, Geol. Surv. Ohio Butt. 7, 4th Ser. (1905), p. 4. * 1. C. White, Second Geol. Surv. Pa., Rept. Q< (1881), pp. 117-20. s Elkland-Tioga Folio (1903), p. 5; Warren Folio (1910), p. 3; C. Schuchert, Bull. Geol. Soc. Amer., XX (1908), 548; L. C. Glenn, N.Y. Stale Mus. Bull. 69 (1903), pp. 967-95. 6 J. M. Clarke, N.Y. State Mus. Bull. 69 (1903), p. 853. 7 D. D. Luther, N.Y. State Mus. Bull. 69 (1903), pp. 1000-1029. 28 STUDIES IN MINOR FOLDS UPPER DEVONIAN FORMATIONS Portage group. White gives the name of Portage to the oldest rocks exposed in northwestern Pennsylvania. Coming from beneath the lake 2 miles east of the Ohio state line, they rise until 475 feet are exposed at the New York state line. 1 These rocks consist of gray shales and flaggy sandstones. The layers of sandstone usually are 12 inches or less in thickness, occasionally 2 feet. White follows Hall's earlier identification of these rocks as Portage. 2 Clarke, however, says there is no Portage in northwestern Pennsylvania. 3 The Portage group has been restudied by Clarke and Luther 4 in its type section at Portage on the Genesee River and in its general expo- sures, and they have made a detailed geological map for the group in western New York, which shows the rocks of this group extending from Seneca Lake in a strip with irregular borders to the shore of Lake Erie. According to this map the most westward exposure of the Portage extends a little west of Westfield, New York. As there is a general dip of the formations here toward the southwest, only a marked anti- cline or a fault would expose the Portage again in Pennsylvania. The Portage group, as recently described, consists of nine formations shown in the following table, in which the thickness of each formation and the total thickness are given. 5 TABLE IV PORTAGE GROUP (After Clarke and Luther) Formations Thickness 1. Passage shales 3' 2. Middlesex black shale 32' 3. Cashaqua shale 165' 4. Rhinestreet black shale 53' 5. Hatch shale 203' 6. Grimes sandstone 25' 7. Gardeau shales and flags 372' 8. Portage sandstones ~. 187' 9. Wiscony shale 167' Total , 1207' 1 1. C. White, Second GeoL Surv. Pa., Rept. Q* (1881), pp. 119-20. 2 J. Hall, Nat. Hist. N.Y. 4 th Dist., Part IV (1843), P- 238. 3 J. M. Clarke, N.Y. State Mus. Bull. 6g, Paleon. 9 (1902), p. 853. J. M. Clarke, ibid., pp. 1000-1029, and Geol. Map; also J. M. Clarke and D. D. Luther, N.Y. State Mus. Bull. 118, Paleon. 18 (1908), pp. 1-69. 5 Clarke and Luther, ibid., p. 1010. STUDIES IN MINOR FOLDS 29 This total thickness is a little above Hall's earlier estimate of over 1,000 feet. 1 The foregoing table shows the general character of the Portage in New York. If the northwest dip continues constant, so that none of the Portage group is exposed along the lake in Erie County, Pennsylvania, the rocks there formerly called " Portage" must be younger. Dr. Clarke has reached this conclusion, for he says: "The 'Portage' and 'Girard Shales' of Erie County, Pennsylvania, are later than Portage time." 2 Also, Chemung brachiopods have been collected near the lake north of North East, Pennsylvania, in what was con- sidered the lower part of the "Portage" for that locality. 3 Huron shale. The position of the Huron shale in northeastern Ohio is in question. The name was given to the black bituminous shales exposed along the Huron River. Prosser took the position that the Huron west of Cleveland is at least in part synchronous with the Chagrin to the east. 4 If the two are in contact at all, the darker lower shales may then be called Huron. Girard shales. The Girard shales appear above the lake a short distance east of the Ohio state line, and the basal beds rise to 475 feet above it at the New York state line. They are gray and grayish-blue shales containing a few thin beds of sandstone, and have a thickness in Pennsylvania of 225 feet. 5 They are very friable and easily eroded. In some localities they contain large calcareous concretionary lenses, while in others, cone-in-cone is common. The relation of the Girard shales to the Chagrin is seen in the following statement: "The Girard shales lithologically are very similar to the lower part of the Chagrin formation as seen in northeastern Ohio, of which they are the eastern continuation. 6 They now are thought to be Chemung in age. 7 They contain few fossils. Chagrin formation. The Chagrin, or Erie, shales are soft bluish gray, containing a few thin sandstone beds and locally calcareous beds. The upper part of the Chagrin is considered Chemung. 8 As just noted, the lower part is equivalent to the Girard shales. The Chagrin 1 J. Hall, op. cit., p. 238. J. M. Clarke, Bull. Geol. Soc. Amer., XIV (1902), 536. 3 D. D. Luther, N.Y. State Mus. Bull. 6p, Paleon. 9 (1903), p. 1028. 4 C. S. Prosser, Geol. Surv. Ohio Bull. 75, 4th Ser. (1912), pp. 515, 519. s I. C. White, Second Geol. Surv. Pa., Rept. Q" (1881), pp. 118, 119. 6 C. S. Prosser, Geol. Surv. Ohio Bull. 15, 4th Ser. (1912), p. 451. 7 C. S. Prosser, ibid., p. 451. 8 C. S. Prosser, op. cit., pp. 462-64. 30 STUDIES IN MINOR FOLDS formation is widely distributed in northeastern Ohio, and extensive expo- sures of it may be seen in nearly all the valleys of the rivers and larger creeks. Chemung formation. The Chemung of northwestern Pennsylvania is described as composed of alternate groups of shale and sandstone, with a thickness of 325 feet. 1 If the rocks called "Girard shales" and " Portage" are also Chemung, the total thickness would be 1,025 feet. As noted above, the Chemung is continued into Ohio as the upper part of the Chagrin formation. Hall says of the Chemung that "this group consists of a highly fossiliferous series of shales and thin-bedded sand- stones, sometimes in well-defined and distinct courses, and an infinite variety resulting from the admixture of the two ingredients." 2 The colors are green, gray, and black. The formation becomes pebbly and conglomeratic toward the top. The Chemung has a thickness of 1,120 feet in the Warren quadrangle. 3 This quadrangle is in the northern part of Warren County, Pennsylvania, which is east of Erie County in the same state, and south of Chautauqua, the most western county of New York. Cleveland shale. In northeastern Ohio the uppermost formation recognized in the Devonian is the Cleveland shale. 4 It is a carbo- naceous shale of brownish-black color and homogeneous texture. In thickness it varies from 30 to 200 feet. 5 East of Cleveland the forma- tion decreases in thickness and disappears in Trumbull and Ashtabula counties. 6 Edward Orton includes the Cleveland, Erie (Chagrin), and Huron shale all under the Ohio shale. 7 Cattaraugus formation. In southwestern New York and in north- western Pennsylvania, the Cattaraugus is the name given to a thick formation of Upper Devonian age next above the Chemung. 8 In the Gaines quadrangle in northern Pennsylvania this formation consists of red, gray, and green shales alternating with brown and green sand- stones. Northward, in southern New York, several conglomerate mem- 1 1. C. White, op. cit. (1881), p. 117. > J. Hall, Nat. Hist. N.Y. 4 th Dist., Part IV (1843), P- 252. 3 Warren Folio, No. 172 (1910), p. 3. * C. S. Prosser, Geol. Surv. Ohio Bull.. 15, 4th Ser. (1912), pp. 16-21. s E. Orton, Geol. Surv. Ohio, Econ. Geol., VI (1888), 26. 6 C. S. Prosser, Geol. Surv. Ohio Bull. 15, 4th Ser. (1912) pp. 509-14. 7 E. Orton, Geol. Surv. Ohio, Econ. Geol., VI (1888), 23. 8 L. C. Glenn, N.Y. State Mus. Bull. 69, Paleon. 9 (1903), pp. 971-78; and Gaines Folio, No. 92 (1903), columnar section at back. STUDIES IN MINOR FOLDS 31 bers are recognized in the midst of the formation. In the Gaines area it is about 500 feet in thickness. DEVONO-CARBONIFEROUS FORMATIONS Under this head are classed several transitional formations which have not been definitely assigned either to the Devonian or Mississippian. Two will be noted here, the Riceville shale and the Oswayo formation. Riceville shale.- In Crawford County, Pennsylvania, the Riceville shale consists of 80 feet of grayish-blue shales and shaly sandstone. Correlation with parts of Cleveland and Bedford shales of Ohio have been suggested. 1 The Riceville contains many fossils common in the Chemung. Oswayo formation. In northern Pennsylvania a series of shales and sandstones about 1,000 feet thick has been assigned to this formation, and it has been classed as Devono-Carboniferous in age. 2 In south- western New York it is classed with the Carboniferous formations by Glenn. 3 MISSISSIPPIAN FORMATION Bedford shale. Of the Mississippian formations given in Tables II and III, folds were found only in the Bedford shale, so only that one will be described. It is placed at the base of the Mississippian forma- tions of northeastern Ohio, and succeeds the Cleveland shale, being suc- ceeded by the Berea grit, that being correlated with the Cussewago and Corry formations of Pennsylvania. 4 The Bedford formation is extremely variable in thickness, for its top surface is very irregular, marking an unconformity between it and the Berea. It consists chiefly of bluish-gray and chocolate-colored shales with a varying amount of sandstone interspersed. In its type locality it has a thickness of 88 feet. 5 PENNSYLVANIAN FORMATION Sharon or Olean conglomerate. The youngest formation in the region under consideration is the Sharon or Olean conglomerate. It has a marked unconformity at the base, and is found only on the tops of the highest hills back a considerable distance from the lake, in Ohio, Penn- sylvania, and New York. It is a coarse conglomerate with white 1 1. C. White, Second Geol. Surv. Pa., Rept. Q< (1881), p. 97- 3 Gaines Folio, No. 92 (1903), p. 2. 3 L. C. Glenn, op. tit., p. 978. < C. S. Prosser, Geol. Surv. Ohio Bull. 15, 4th Ser. (1912), pp. 352, 511. s C. S. Prosser, ibid., p. 87. 32 STUDIES IN MINOR FOLDS quartz pebbles generally about one-half to three-fourths of an inch in diameter. Large parts of it commonly are strongly cross-bedded. At Thompson's Ledge in northern Ohio, it has a thickness of 80 feet; 1 northeast of Meadville, Pennsylvania, 40 feet; and in the Gaines quad- rangle, in north-central Pennsylvania, 60 to 100 feet. 2 No indurated rocks younger than those of early Pennsylvanian age have been found in this area. QUATERNARY DEPOSITS Pleistocene. The deposits of Quaternary age consist chiefly of glacial till, sand, and gravel. The glacial drift is very irregularly dis- tributed. In general it is thinnest on the hilltops and thickest in the valleys, varying from nothing on the former to nearly 500 feet in the latter. The deposits of the ground moraine are very irregular in thick- ness, but there is still greater irregularity in the broad complex terminal moraines. Locally, extensive deposits of gravel and sand have resulted from glacio-fluvial work in the form of kames and kame terraces. Post-glacial. Post-glacial deposits have been formed in lakes and by streams. In small lakes some peat and much marl have been deposited, while streams have made deposits in the form of alluvial fans, flood plains, and deltas. GEOLOGIC HISTORY As noted above, the oldest rocks exposed in the specific area under consideration are of Upper Devonian age. However, the deep well at Presque Isle, Erie, Pennsylvania, at a depth of 4,450 feet is thought to have penetrated 170 feet of the Trenton limestone, and the general formations of the New York section intervening between the Trenton and the Chemung are represented. 3 Marine conditions seem to have been dominant in the area, from early Paleozoic until toward the close of the Mississippian. In the earlier periods limestone predominated; toward the close, clastic formations. All older formations are con- cealed beneath Upper Devonian. The formations of this epoch indicate shallow marine deposition with fine silt as the chief sediment, though alternating rather frequently with sand. There seems to have been no break in sedimentation at the close of the period, and no good line of demarcation has been drawn between Devonian and Mississippian rocks. Marine deposition continued 1 C. S. Prosser, Geol. Surv. Ohio Bull. 15, 4th Ser. (1912), pp. 286-88. 2 Gaines Folio, No. 92 (1903), columnar section in back. 3 C. S. Prosser, Geol. Surv. Ohio Bull. 75, 4th Ser. (1912), pp. 412-22. STUDIES IN MINOR FOLDS 33 through a large part of the Mississippian period, with clastic materials greatly predominating. Toward the close of the latter period the area was elevated, and suffered erosion before the Pennyslvanian rocks were deposited. After this erosion interval the region was depressed, and a coarse conglomerate with white quartz pebbles spread widely over the area. Above this conglomerate shales and coal were deposited, but they have been completely removed by erosion from most of the area. If Permian or younger rocks ever were deposited, all evidences of their presence have been carried away. It is thought then that this area has been above the sea ever since a time near the close of the Paleozoic. PHYSIOGRAPHIC HISTORY Since the Paleozoic, in the vast periods of erosion which followed, the history of this area doubtless closely paralleled that of southwestern Pennsylvania. 1 In the latter region erosion is thought to have reduced the land to a peneplain before the close of the Cretaceous, and this first great base-level is called the Schooley Peneplain. Succeeding uplift initiated a second cycle of erosion which nearly obliterated the former plain, developing the Harrisburg as the second peneplain in early Terti- ary time. After subsequent uplift and erosion a third less complete peneplain the Worthington was developed toward the close of Terti- ary time. A series of straths and terraces below the latest pene- plain is taken to indicate still later periodic elevations of the region. The deformation of the Harrisburg or late Tertiary peneplain has been worked out in considerable detail by M. R. Campbell. He finds it has a domelike uplift, so that it is 1,600 feet higher in northern Pennsyl- vania and southern New York than in southeastern Pennsylvania. (See Plate III.) 2 Doubtless as many peneplains were developed in north- western Pennsylvania as in the southwestern part of the state, but some of the evidences of them have been obliterated by the heavy deposits of glacial drift in the northern region. The tops of the higher hills easily are recognized as remnants of a peneplain. Below these, broad flat areas of great extent seem to rep- resent a second less complete plain. Below this broad plain, terraces of varying width can be recognized, though they are materially modified by glacial deposits. During the last period of erosion preceding glacia- tion, the valleys were cut from 800 to 1,000 feet below the hilltops. Accordingly, when the continental glaciers invaded this area, it was one 1 R. W. Stone, Kept. Top. and Geol. Surv. Com. Pa. (1906-8), pp. 120-22. 2 M. R. Campbell, Bull. Geol. Soc. Amer., XIV (1903), 277-96. 34 STUDIES IN MINOR FOLDS of considerable relief. It was covered by that part of the glacier called the Grand River Glacial Lobe. 1 Glaciers had a marked effect on the topography of the area. While glacial erosion seems to have been relatively unimportant, glacial deposition and drainage changes were of great importance. Evidences of only two glacial epochs in this area have been found. 2 Glacial erosion. At the quarry i mile northeast of Meadville, Penn- sylvania, planation, grooving, and striation resulted from erosion, but the glacier did not erode the rocks deeply, for evidences of former weathered surfaces exist below the plane cut by the ice. Doubtless the tops of the hills were not lowered materially by glacial erosion, but were only smoothed and rounded in contour somewhat by its action. Glacial deposition. Glacial deposition did affect the topography very materially. Terminal moraines with accessory kames, kame terraces, outwash plains, and ground moraines with drumlins were formed. Some of the old valleys were filled in with much drift, 200 to 300 feet of drift being common, and nearly 500 feet being reached near Meadville, Pennsylvania. 3 In the moraines the drift was deposited in rounded hills both in valleys and on uplands alike. Drainage changes. The effect of glaciation on the drainage systems of this area was very marked. 4 Several large northward-flowing sys- tems were practically obliterated, and their drainage areas added to systems of the southward-flowing streams. There was then a marked shifting of the divide northward. Owing to conditions near the close of the glacial period, several of the northward flowing streams reach the lake by very peculiar courses. As the ice receded from the area toward the northeast, drainage was established westward along its southern margin, after it receded north of the divide. The southern margin remained constant at a number of places for a sufficient length of time for channels to be developed parallel with it. As a result several streams run parallel with the lake shore for a considerable part of their course. Another condition favoring this parallelism is the trend of the morainic deposits, which are about parallel with the lake shore. Because of these deposits some of the streams were deflected westward on the south side of them. Examples of this type of course are Twenty Mile, Walnut, Elk, and Conneaut creeks, and Ashtabula and Grand rivers. Thus the headwaters of the stream in Gage Gulf are but 3 miles from 1 F. Leverett, U.S.G.S. Mono. 41 (1902), Plate 15. 2 F. Leverett, ibid., pp. 272-74, and Plate 15. * F. Leverett, ibid., p. 458. 4 F. Leverett, ibid., pp. 128-44. STUDIES IN MINOR FOLDS 35 the lake, but it flows in a westerly direction for 9 miles, then 3 miles to the lake. From the abrupt turn at Conneaut Creek north of Albion it is but 6 miles to the lake, but from that point it goes 20 miles west- ward and then over 9 miles northeastward to the lake. The Grand River also runs parallel with the lake for over 20 miles within 8 miles of the shore. Glacio-Lacustrine substage. As the ice receded from the lake basin it was occupied by water standing at very much higher levels for a series of stages. To these various stages the names of Maumee, Whittle- sey, Warren, and Dana were given. At the Whittlesey stage the water was 211 feet above the present level of Lake Erie at the New York state line. 1 Besides these higher lake stages there were numerous small ice- front lakes in northwestern New York and Pennsylvania. 2 Post-glacial changes. Since the glaciers receded, the streams of this area have been active. While a few of them seem to have accom- plished little, most of them have cut their channels through the drift and into the bedrock beneath. Many of these channels are 80 to 100 feet in depth, while the one south of Westfield, New York, is over 400 feet in depth. In this down-cutting process, numerous terraces have been left marking the old high-stream levels. As noted above, stream deposits have been made in the form of alluvial fans, alluvial plains, and deltas. Similarly, Lake Erie has been actively at work undercutting the cliffs along its southern margin, causing the shore to recede southward. Sediments derived by shore erosion and by transportation of streams have been deposited along the beach in bars or hooks, or carried to the deeper parts of the lake basin. STRUCTURE OF THE ROCKS General structure. The rocks of the area in general appear to be flat-lying, although the distribution of formations on the geologic map shows a belted arrangement characteristic in regions with monoclinal dip. The oldest rocks are exposed along the lake, and successively younger ones toward the south. The dip of the rocks in northwestern Pennsylvania has been figured to be about 20 feet per mile southward and 10 feet per mile westward. 3 The amount of dip, however, is not constant, but increases toward the northeast. X H. L. Fairchild, N.Y. State Mus. Bull. 106 (1907), pp. 42-44. 2 H. L. Fairchild, ibid., pp. 33-41. 3 J. P. Lesley, Second Geol. Sun. Pa., Rept. Q 4 (1881), pp. 45-49. 36 STUDIES IN MINOR FOLDS Local structures. In this general area of very gently dipping rocks, numerous irregularities occur in the form of small folds and faults. The faults are all thrust faults, but among the folds, besides the specific types before mentioned, several general types will be considered. GENERAL TYPES OF FOLDS Intra-formational folds. While some of the folds in the region south of Lake Erie are limited to only a few feet in vertical extent, it is sig- nificant that they do not commonly show close, overturned, and recum- bent types, which are very common in minor intra-formational folds. A close fold of this type, occurring in the Arbuckle Mountains of Oklahoma, is shown in Figure 10. This fold is in the weak calcareous beds of the Simpson which lie between the heavy Simpson sandstones and the Arbuckle limestone. To the east of the Lake Erie region at Trenton Falls, New York, intra-formational folds occur in the Trenton limestone at two horizons, the rest of the strata being parallel and not folded. These folds recently have been illustrated and described by W. J. Miller. 1 He says: Within the folded zones the layers are, in rare instances, scarcely dis- turbed; sometimes they are only gently folded; most commonly they are highly twisted or contorted; while occasionally some of the layers are broken, and pushed or faulted over others. Earlier, L. Vanuxem illustrated and described these folds as follows: For thirty or more feet in length, and from three to five feet in thickness, the rock exhibits extraordinary contortions for one whose layers are so regu- larly disposed, forming almost semicircular curvatures, and not unlike the writhings of a huge serpent. When the contortions are observed, they show a crystallized white limestone, enveloped in the usual calcareous shaly materials, proving that the disturbance was caused by the crystallization of the white limestone forming a layer. 2 Later, the same folds were described and illustrated by T. G. White. 3 Folds of this type in western Pennsylvania have been descrbed by R. R. Hice as follows: At the site of Dam No. 5 on the Ohio River, on the eastern edge of the Beaver quadrangle between the towns of Rochester and Freedom, a recent railroad cut exposes, in a distance of 600 feet, a series of foldings involving the strata between the base of the Lower Kittanning clay and the horizon of ' W. J. Miller, Jour. Geol., XVI (1908), 428-33. 2 L. Vanuxem, Geol. of $d Dist. N.Y. (1842), p. 53. 3 T. G. White, Trans. N.Y. Acad. Sci. (1895), pp. 71-96, Plate 3 A. STUDIES IN MINOR FOLDS 37 the Middle (Upper) Kittanning coal (about 35 feet) in no way involving the underlying strata or those above the horizon of the Middle Kittanning. 1 W. G. McGee has described some similar small folds and faults in the Cedar Valley (Devonian) limestone of northeastern Iowa. 2 While the folds are within the single formation so far as observed, they are more gentle than intra-formational folds commonly are, and in the illustration given, the deformation is near the surface. The folding is associated with brecciation in the limestone, and as brecciation was coeval with deposition, the deformation and brecciation were thought to have been contemporaneous. The same type of folds occurs in the Quaternary clays along the Black River canal feeder about 3 miles from Boonville, New York, where a series of closely folded layers in a folded zone occurs between unfolded parallel strata on either side. 3 I. C. Russell also describes and illustrates intercalary folds in Quaternary deposits in the Lake Mono region of California, and concludes that they were formed in some manner at the time of deposition. 4 E. M. Kindle explains folded sands and clays between non-folded beds in Nova Scotia and southern Ontario by movement of soft muds beneath, due to irregular weighting. 5 The only folds of this type found in the Lake Erie region are extremely small and unimportant (Fig. 22). On the northeast side of Twenty Mile Gulf, one-fourth mile west of the Pennsylvania-New York state line, a small fold in the vertical bank is inclosed between horizontal strata above and below. The fold is up about 25 feet in an 8o-foot bank. Only about 5 feet of strata are involved. The thickening of the strata above the fold either side of the crest indicates that a few of the beds were slightly deformed soon after their deposition, before the super- jacent stratum was deposited, as the latter fills in the depression either side of the crest of the fold. The second and third beds above the fold become uniform in thickness and horizontal in position. Parallel folds. Numerous small folds with axes parallel with the trend of valleys occur in the floor of many of the post-glacial valleys of the area. The folds generally are small, involve only a few feet of strata, and usually are limited entirely to the valley floor, affecting the 1 R. R. Hice, Bull. GeoL Soc. Amer., XXII (1910), 716-17, Abs. 2 W. G. McGee, Eleventh Ann. Kept. U.S.G.S., Part i (1889-90), pp. 337~38. 3 L. Vanuxem, GeoL jd Dist. N.Y. (1842), pp. 213-15. or to get a shortening of one foot there would be 2,000 feet of depression. But possibly these defor- mations relieved stresses for a mile either side of the folds, so we should consider the distance for the shortening for 4 miles instead of two. Even for 4 miles there would need to be a depression of 1,000 feet to cause i foot of thrust. Along Elk Creek, south of Girard, over n feet of shortening has occurred within a mile. The 7 faults farther east in 1 H. F. Bain, U.S. Geol. Surv. Bull. 294 (1906), p. 44. 2 T. C. Chamberlin and R. D. Salisbury, Geology, I (1905), 580. STUDIES IN MINOR FOLDS 59 central New York, as noted above, have a total displacement of 75 feet, and these occur within an arc of 10 miles. The displacements in the eastern area should not be added, for they occur in four different locali- ties, Marcellus, East Onondaga, Jamesville, and Manlius, almost in an east-and-west line. As the shortening is a north-and-south one, they probably represent a continued deformation. But a single displace- ment at East Onondaga is 42 feet, one at Jamesville 15 feet, and four others 12 feet. If depression here has taken place over an arc 10 miles across, the ratio would be i : T J- Q-, or there would be a settling of 400 feet to i foot of thrust. So to get 42 feet of thrust a depression of 16,800 feet would be required. Also, the shortening noted above does not include several folds that occur in the general region with the faults. Thus from a quantitative standpoint the shortening of the arc of a segment due to settling from compacting of subjacent rocks, is far too small to cause thrusts of even the small magnitude of those in the Lake Erie region or in central New York. Vertical pressure of valley walls. W. O. Crosby 1 is credited with having suggested the weight of the overlying strata as an explanation for the marked small folds at Trenton Falls, New York, but W. J. Miller concludes that the overturned folds cannot be accounted for in this way. Smallwood and Hopkins, in their discussion of the origin of the folds near Meadville, consider the possibility of the weight of the valley walls being sufficient to deform the rocks in the bottoms of the valleys, but conclude that they are not sufficiently high to develop an adequate force. 2 This conclusion seems well founded, for the walls of the deepest sharp post-glacial valleys in the vicinity of Meadville reach scarcely 100 feet in height. Sandy shales and some sandstones, as well as the argillaceous shales, are involved in the folds there. The deepest valley on the southern border of Lake Erie, near Westfield, New York, is 400 feet deep. Figuring 165 pounds per cubic foot for the shales and sandstones in the walls of these valleys, there would be only about 460 pounds pressure per square inch on the rocks at the bottom of the valleys beneath the walls 400 feet high, and 115 pounds on those with walls of 100 feet. The crushing strength of the weaker sandstones can hardly be lower than 1,500 pounds per square inch. 3 This is far above either *W. O. Crosby, Jour. Geol., XVI (1908), 430; and Man. N.Y. Acad. Sci., XV (1895-96), 90. 2 Smallwood and Hopkins, Bull. Syracuse Univ., 4th Ser., No. i (1903), pp. 18-24. s L. V. Pirsson, Rocks and Rock Minerals (1915), p. 324. 60 STUDIES IN MINOR FOLDS figure for the pressure due to the weight of rocks in the walls of the valleys. It is significant, too, that most of the smaller folds of the area, occurring in the floors of valleys parallel to the walls, are found in those with relatively low walls of about 100 feet or less. G. F. Becker has given an excellent summary of the rupturing effect of stress due to gravity as follows: When gravity acts on a mass, homogeneous strain is, strictly speaking, impossible, excepting within infinitesimal limits of space, each level surface being subjected to greater pressure than the next above it. On the other hand, the forces involved in the deformation and fracture of rocks are very great, except in some extreme instances, such as that of moist clay. For ordinary firm rocks the ultimate strength is such that a column of from one to several thousand feet in height would be needful to produce at its base a pressure sufficient to produce rupture. Consequently in masses of such material from a few score of feet to a few hundred feet in thickness, gravity plays but a small part compared with the rupturing stress. 1 His conclusions are in conformity with the figures above, showing that the weight of these valley walls is far below that necessary to rupture solid rocks. Folds due to relief from compression. Folds occurring in the quarries or valleys frequently are attributed to relief of pressure by the removal of rock from above. Such an explanation has been given for a small anticline in the Queenston shale at the head of Hopkins Creek estuary, south of Lake Ontario, of which it is said: "It probably is a secondary result of erosion, the removal of the overlying rocks permitting relief from compression." 2 But relief from vertical compression alone could have nothing to do with the formation of folds. If marked stresses already exist in the rocks, and the superficial strata by their strength and weight prevent folding, then their removal might permit the stresses resident in the subjacent weaker rocks to become effective by throwing those weaker rocks into folds. Folds due to weight of delta. In several places in the western part of the United States, folds occur at the base of large deltas. I. C. Russell has described and figured folds which occur at the base of the Provo delta, along the Logan River, Utah. He suggests as an explanation of the folds, that the weight of 300 feet of delta material pressed out some of the soft, freshly deposited sediments and arched them up into a series of folds, developing the folds of the series successively as it was built 1 G. F. Becker, Bull. Geol. Soc. Amer., IV (1893), 49-50. 3 Niagara Folio, No. 190 (1913), p. 15. STUDIES IN MINOR FOLDS 61 forward. He computes the difference between weight of water and weight of delta as 75 pounds per square inch. 1 Russell thinks the folds in the lower part of deltas in Lake Lahontan, Utah, and in the Mono Valley, California, also are due to the weight of deltas. 2 The deformation of soft, unindurated sediments by the weight of a thick delta deposit seems very possible, but a similar weight would have little effect on indurated shales and sandstones such as are folded and faulted in the Lake Erie region. Besides, no deltas of the type and mag- nitude described by Russell are found in this area. Deformation due to landslides. Numerous small anticlines occur in the bottom of post-glacial valleys in the vicinity of Meadville and in the tributaries of Walnut Creek, south of Erie. The valleys are narrow at the bottom and have steep sides. Generally the folds are about parallel with the trend of the valley. Some of these folds are asso- ciated with landslides, and W. M. Smallwood and T. C. Hopkins at- tribute the formation of the folds largely, if not entirely, to this cause. 3 However, many of the folds are in no way related to landslides, and numerous large landslides have caused no deformation of the strata. As the height of the valley walls is generally less than 100 feet, the force from the slow settling of a comparatively small landslide down a 45 slope, is thought to be entirely inadequate to deform 8 to 20 feet of well-indurated shales and sandstones commonly involved in the folds in the valley floors. R. T. Chamberlin 4 has noted much larger folds of this type in materials of the slides along Lakes Zug and Zurich in Switzer- land, where the rocks beneath the slide were undisturbed. A few of these folds, particularly some in the sharp southern tribu- taries of Walnut Creek, before referred to, not only are related to land- slides, but clearly are a part of them. In at least two instances the landslides were forced across the narrow floor of the valleys, and the strata in the lower part of the slide were buckled into anticlinal form by the weight of the upper part of the slide. It would, however, take far less force to buckle up in the end of a slide the strata which have been loosened by the movement, than would be required to deform the undisturbed bedrock of the valley floor. Should there be a prominent joint in the rock a short distance back in the valley wall about parallel with the trend of the valley, cutting 1 Credited to I. C. Russell by G. K. Gilbert, U.S.G.S. Mono., I (1890), 162. 2 1. C. Russell, Eighth Ann. Kept. U.S.G.S., Part i (1886-87), p. 310. 3 Smallwood and Hopkins, Bull. Syracuse Univ., 4th Ser., No. i (1903), pp. 18-24. 4 Unpublished correspondence. 62 STUDIES IN MINOR FOLDS beneath the edge of the valley floor and sloping steeply in the wall, it might determine the plane of a break in the wall. Should a large mass of rock with broad top rest on a relatively small base, the pressure on that base would be greatly increased, and might cause the surface layers of a weak rock to buckle in the bottom of the valley in front of it. It may be well to inquire, in this connection, whether it is possible for folding in a valley floor to initiate landslides. If, because of lateral stresses, the rocks fold up in the valley floor, there might be a slight giv- ing under the edge of the valley in a way to loosen the strata above and cause a landslide. The illustration of a fold at the base of a cliff antedating a slide is given by F. R. Van Horn. 1 It is in the quarry of the Cleveland Brick and Clay Company, along Mill Creek, at Cleveland. Back a distance from the edge at the top of the quarry wall, which is 112 feet high, a fissure opened August 17, 1908. The following day a small anticline 200 feet long, started to buckle up at the base of the bank, in the shales weakened by the blasting. The day following the initiation of the buckling, the mass of rock, estimated to weigh 87,732 tons, started to settle. It was stated definitely by the quarryman, who watched the phenomena, that the buckling preceded any settling of the mass. After the settling began, it continued at the slow rate of 6 feet in two weeks, and of 20 feet in a little over four months. After the excavation of most of the shale of the slide, it appeared that a joint crack had reached up 80 feet of the 112 in the quarry wall, this crack having determined the location of the fissure at the top of the bank. If this joint crack extended a short distance below the floor of the quarry, the identical conditions exist which are postulated with reference to the valley wall in a landslide described above. In this instance the slide and the small fold parallel with its front seem definitely related. The amount of weakening by the shattering of the shales in the base of the high bank and on the quarry floor is not known, but it may have been very considerable. Another consideration to be noted in connection with folds related to landslides, both in this quarry and in the valleys in the area studied, is that lateral stresses of importance may already exist in the rocks. If such stresses do exist, and to them are added the relatively smaller ones initiated by the landslides, the combined stresses are then suffi- cient to overcome the strength of the rocks and bow them up. In this case, while definite stresses are added by action of the landslide, their minor force may be looked upon as a trigger which has added its slight 1 F. R. Van Horn, Bull. Geol. Soc. Amer., XX (1908), 625-32. STUDIES IN MINOR FOLDS 63 power to the larger forces it sets in motion. Reasons for thinking that such lateral stresses exist in the rocks of the area studied will be given later. Pressure of natural gas. In this area gas occurs under low pressure at an average depth of 600 feet. At one locality it is constantly escap- ing from the crest of the anticline shown in Figure 38. I. C. White 1 has attributed the " minute cracked anticlines" of the area to the pres- sure of natural gas. However, the wells produce little gas, and that little under low pressure. Furthermore, the folds occur both within and without the areas productive of gas. While the pressure of natural gas clearly produces stresses, it is thought that they are far too weak, with the low pressures common here, to deform the rocks. Besides, the release of strains due to gas pressure doubtless would come in the forma- tion of domes and symmetrical folds rather than in the markedly unsym- metrical ones common in the area under consideration. Differential movements. W. J. Miller 2 has suggested that differ- ential movement in the mass of the Trenton limestone, in the large fault at Prospect Village near Trenton Falls, New York, has caused the folds in two horizons which lie between unfolded strata. The suggestion is excellent, and it seems probable that as the end of the upthrown part of the fault has been greatly eroded, these small folds are the best existing index of the amount and character of the lateral movement in the fault. In several instances in the area studied only a few feet of the superficial rocks seem to be involved in the folds. In these few cases the beds underneath are undisturbed, while the more superficial ones are deformed, thus seeming to indicate either more acute stresses or greater weakness in the upper than in the lower rocks. But these very superficial folds seem to be in no way related to a larger fault. In many of the folds, however, the deformation appears to involve the rocks to a considerable depth as well as the superficial ones, but none are related to a large fault. Glaciation. Many folds of varying types and magnitude have been attributed to the action of glaciers. Glaciation has been so much abused in having unreasonable destructive and deformative acts attributed to it, that the prevailing tendency now is to scrutinize with care the charge "due to glaciation." The real deformation of rock which may rightly be attributed to glacial action is now thought to be very moderate. The work attributed to glaciation by J. A. Udden 3 and F. W. Sardeson 4 1 1. C. White, Second Geol. Surv. Pa., Kept. Q< (1881), p. 120. 2 W. J. Miller, Jour. Geol., XVI (1908), 428-33. 3 J. A. Udden, III. Geol. Surv. Bull. 8 (1908), pp. 255-67. 4 F. W. Sardeson, Jour. Geol., XIII (1903), 351-57. 64 STUDIES IN MINOR FOLDS seems excessive. Udden describes and illustrates many small folds and faults in the walls of valleys and in coal mines, some of the faults having a displacement of 30 to 100 feet. He thinks the folds and faults near Peoria, Illinois, are disturbances in the upper part of the soft bed rock, caused by the pres- sure and motion of a continental ice sheet in the Pleistocene period; that they are planes marking the outline of immense blocks of large tracts of the upper- most coal measure strata covering probably hundreds of acres of land which have been dislodged from their position, displaced, fractured, rotated hori- zontally and at times vertically, and partly ground to till. 1 Sardeson speaks of glaciers "plowing up bed rock," 2 and transmitting stresses through beds of gravel and till 16 to 20 feet in thickness, over which they are moving with such efficiency as to fold and overthrust the Galena limestone beneath the gravel and drift. E. M. Kindle and F. B. Taylor 3 have described and illustrated an anticline at Thirty Mile Point, along the southern shore of Lake Ontario just east of the Niagara quadrangle, and they attribute the origin to the pressure of glacial ice, in spite of the fact that it is overturned into the glacial till and includes till definitely beneath the overturned top of the fold. In G. K. Gilbert's 4 earlier description of this fold he advances two hypotheses for its origin, but does not conclude definitely as to either one. If due to glaciation his idea is that the waning ice stopped just north of this, then moved forward again enough to form the fold and overturn it into the drift. The shore has undoubtedly been eroded back a long distance in the sandy shale at this locality, possibly a mile or more. If the glacier moved forward several times over the area, it seems strange that it should wait until a last slight advance to deform the rocks. J. Le Conte attributes folds in several horizons in clays along Rush Creek, in the Mono Valley, California, to glaciers or icebergs, 5 but I. C. Russell determined that the glaciers did not come within 3 miles of the area described, and that the folds were so widespread, and many of them of such a nature, that they could not have been formed by icebergs. 6 1 J. A. Udden, op. cit., p. 265. 2 F. W. Sardeson, op. cit., p. 356. 3 Kindle and Taylor, Niagara Folio (1913), p. 15, and Plate 19. * G. K. Gilbert, Bull. Geol. Soc. Amer., X (1898), 131. 5 J. Le Conte, Amer. Jour. Sci., 3d Ser., XVIII (1897), 40- 6 1. C. Russell, Eighth Ann. Kept. U.S.G.S., Part i (1889), p. 309. STUDIES IN MINOR FOLDS 65 There are but two localities in the area studied in which folds of such a character were found that their origin could be attributed to glacial pressure. One was in an open-pit coal mine 5 miles south of Conneautville, Pennsylvania, and the other in shales along Canadaway Creek ij miles west of Dunkirk, New York. In both instances the topographic situation, presenting a steep slope toward the north, the general direction from which the ice came, favored the transmission of stress into the coal in one case and the shale in the other, as the ice advanced against these transverse barriers. In both instances the folds are small, only a few feet of strata being involved, and the rocks much broken, indicating that the folding took place without much pressure from above. The two small folds in the coal are shown in Figure 35, and being indistinct they are outlined by the dotted line. The hammer is i if inches long. About 6 feet of broken and crushed coal lie above the folds, and a few feet to the right of them, toward the north, the glacial till is intermingled with the broken coal. The coal at the top of the hill is an outlier of a more extensive formation to the south. Appar- ently the ice, advancing against the steep northern slope of the hill, shattered and slightly deformed the northern end of the coal vein before it was deflected above the top of the hill. The folds in the Dunkirk shale of the Portage group, near Dunkirk, are shown in Figure 36. Three small anticlines occur in the series, and each is about 10 feet wide. The sandstones beneath the shales are undisturbed. The thickness of the shale is 5 feet, and it is overlain by 3 feet of glacial drift and soil. A short distance to the north of the folds the shale presents an abrupt slope to the north. Because of the topographic situation with reference to the advancing ice front these very small folds in coal and shale seem readily attributable to the pressure of glacial ice. The fact that larger folds due to glacial pressure were not found in this area does not constitute proof that they do not occur elsewhere. It is thought, however, that this extremely minor type of folding is all that can be expected from the glacial ice pressure, because of its softness, and plastic adjustment under com- pression. A detailed study of the major and minor deflections of the ice due to various types of barriers would be instructive in this connection, but it cannot be followed here. Brief attention is called to the plastic adjust- ment of the ice under pressure as indicated by the manner in which it fitted into the irregularities of the rock surface. An illustration of this adjustment is seen in the south quarry at Stony Island, in Chicago, 66 STUDIES IN MINOR FOLDS where the underside of a small projecting ledge of the Niagara limestone has been polished by the wear of the ice and its tools. In another FIG. 35. Two small folds in the north edge of an open coal mine. The hammer is ii f inches long. Five miles south of Conneaut Lake, Pa. FIG. 36. Small folds in 5 feet of Dunkirk shales i^ miles west of Dunkirk, N.Y. The thick homogeneous beds are outlined by the dotted lines. locality, on Kelly's Island, in the western part of Lake Erie, the ice was forced with great pressure through the sinuosities of a tortuous channel without breaking off the interlocking spurs of the rock. 1 T T. C. Chamberlin and R. D. Salisbury, Geology, III (1906), 349, Fig. 485. STUDIES IN MINOR FOLDS 67 Drag of icebergs. In a few instances folds in unconsolidated clays and sands have been attributed to the drag of icebergs over them. J. Le Conte 1 has suggested that explanation for the folds in the clays and sands in the beds deposited in Lake Mono. A like cause has been credited by R. D. Salisbury and W. W. Atwood 2 with the deformation of layers of sand and silt in the glacial lake deposit in the Baraboo region of Wisconsin. I. C. Russell 3 questions the possibility of any type of ice origin for the folds in the former case, but it seems quite possible that local, narrow, irregular areas of unconsolidated sediments might be so deformed. If the grounding and drag of icebergs have caused the defor- mation of lake beds, the general direction of the current from the ice front to the outlet of the lake would indicate the course of the icebergs, and the axes of the folds would be about normal to the direction of their course. Tangential compression. The closed folds and thrust faults of the Taconic, Appalachian, Ouachita, Arbuckle, and Wichita mountains give evidence that stresses of great potency of a tangential nature have been developed at various periods of the earth's history. It is not the pur- pose here to seek the ultimate origin of these lateral stresses, but simply to suggest the possibility of their presence as a cause for the deformation of the region under consideration. And it is thought that most of the folds and faults are due to widespread lateral stresses, which have been directed in such a way as to cause these minor movements. Summary of origin of folds and faults. Of the various causes to which the formation of folds and faults have been attributed, a number of them seem to have no application to the deformation of rocks in the area under consideration; namely: igneous activity, expansion due to heat from adjacent igneous rocks, increase in temperature at the close of the glacial period, crystallization, solution of underlying formations, alteration of iron sulphides, weight of delta, gas pressure, differential movement of beds in a large fault, and drag of icebergs. Three others pressure of valley walls, release of pressure by erosion, and landslides have been shown wholly inadequate to develop stresses sufficient to deform any considerable thickness of strata in place. If they have had any effect, it has been by the addition of their relatively slight force to the much greater stresses already in the rocks. Expansion due to heat, freezing of water, and hydration of minerals, may have helped to form some of the smaller folds, but it is believed 1 J. Le Conte, Amer. Jour. Sci.-, 3d Ser., XVIII (1879), 4-Q- 2 Salisbury and Atwood, Jour. Geol., V (1897), 143. 3 1. C. Russell, op. cit., p. 309. 68 STUDIES IN MINOR FOLDS that these agents, in the main, have been secondary, increasing dips started by other forces rather than initiating them. In two instances very small folds involving only 5 feet or less of strata, are thought to be due to the pressure of glacial ice, which because of their topographic form and situation, presented very abrupt barriers to the front of the ice. All of the larger and intermediate folds, and many of the smaller ones, are thought to be due to widespread tangential stresses in the rocks, the origin of which will be considered later. AGE OF THE FOLDS AND FAULTS The age of the folds and faults will be considered under three heads pre-glacial, glacial, and post-glacial. An attempt also will be made to determine still more explicitly the age of some of the post-glacial deformations by their relation to a series of terraces. Pre-glacial. A number of the larger open gentle folds are considered pre-glacial, the low fold shown in Figure 1 2 being one of the smaller ones belonging to this group. A number of others were found, but no good pictures were secured. Still more marked folds of this age and type occur farther west in Ohio, outside this area. If an attempt were made to fix the age more closely we can say they are post-Lower Mississip- pian and pre-Pleistocene. Both Upper Devonian and Lower Missis- sippian rocks were involved in the deformations, and there was no break in the sedimentation between the Devonian and the Mississippian periods. Possibly some of the folds were formed toward the close of the Missis- sippian or at the beginning of the Pennsylvanian period, as there is a general unconformity between the rocks of these two systems. Or they may be referable to the Permian, a series of small, gentle folds being formed here when the larger and more intense ones were developed in the Appalachians to the southeast, though they seem rather far removed to be genetically related to the latter. Or there is a possibil- ity they may have been formed still later, between the Permian and Pleistocene. Glacial. As has been noted above in connection with the origin of folds, a few small ones are thought to be due to the pressure of glacial ice, and accordingly are of Pleistocene age. They are considered to belong to the early part of this period, being formed when the advancing ice first was opposed by rather abrupt barriers. Soon after the first impact of the front of the glacier with the barriers, the ice doubtless was deflected above them. It then began to erode the crest of the escarpments and to deposit till at its base, thus developing an easy STUDIES IN MINOR FOLDS 69 gradient over which to move. The folds of this age that were observed are in the surface coal mine (Fig. 35) and in the shales near Dunkirk (Fig. 36). It is possible also that some of the folds and faults, though not attributable to glacial forces, may have been developed during the Pleistocene period, but distinctive evidence marking them of glacial age is wanting. The sharply overturned anticline at Thirty Mile Point, on the south shore of Lake Ontario, has been called glacial in origin, and if that assumption is correct, it is glacial in age. If so, it was formed and overturned after deposition of the till which it incloses beneath the overturned crest. Post-glacial. The deformation of glaciated surfaces and of glacial deposits, and of uneroded tops of protruding folds and faults, is recog- nized as evidence of post-glacial origin. G. K. Gilbert was among the first to recognize the post-glacial age of folds near Caledonia and in Chautauqua and Jefferson counties, New York. 1 G. F. Mathews also recognized numerous post-glacial faults in the slates at St. John, New Brunswick. 2 The faults are very small, having a displacement of from less than an inch to 5 inches, but the glacial surfaces are distinctly deformed by these faults. Much more recently A. C. Lawson described numerous similar post-glacial faults of the same slight magnitude, 5 miles west of Banning, Ontario. 3 J. B. Woodworth has studied a series of small post-glacial faults in eastern New York, and has com- pared them with faults of like age in New England, Quebec, and New Brunswick. 4 Of the faults that occur in the five areas described in eastern New York, all are thrust faults in highly folded strata extending from Lower Cambrian to Lower Silurian. He has tabulated the move- ments in several of the series of small faults, and has taken the average displacement per yard for two adjacent areas, and gets as the result i . 9 inches in a yard, or 336 . 7 feet per mile. 5 No such minutely deformed glaciated surfaces were found in the area studied. Possibly this is because of the difference in the general attitude of the rocks. In the Lake Erie region they are in general flat-lying, while in eastern New York they are highly folded. 1 G. K. Gilbert, Amer. Geologist, VIII (1891), 320-31; and Proc. Amer. Assoc. Adv. Sci., XXXV (1887), 227. 2 G. F. Mathews, Amer. Jour. Sci., 3d Ser., XL VIII (1849), 5 OI -3- 3 A. C. Lawson, Bull. Seis. Soc. Amer., I (1911), 159-66. 4 J. B. Woodworth, N.Y. State Mus. Bull. 107, Geol. 12 (1907), pp. 5-28. s J. B. Woodworth, ibid., p. 19. 70 STUDIES IN MINOR FOLDS Faults and folds with uneroded tops are in reality a phase of defor- mation of glaciated surfaces, because the loose, easily eroded tops rise above the general level of the glaciated surface and deform the basal part of the super jacent glacial deposits. The crest of a fold pro- truding up into the glacial till and including till beneath its overturned top, occurs along the southern shore of Lake Ontario, at Thirty Mile Point. Illustrations of this fold have been made by G. K. Gilbert, 1 and E. M. Kindle and F. B. Taylor. 2 This fold has a sharp crest of much shattered sandstones and shales extending up into the glacial clays in a way which indicates that the fold was formed after the drift had been deposited and the ice had receded. Similarly sharp crested small anticlines, though not overturned, have been illustrated by F. R. Van Horn. 3 These folds in the Chagrin shales have their sharp crests protruding into the superficial glacial till in a manner to mark them clearly as post- glacial. Had they been pre-glacial, the tops of soft, broken shale would have been eroded. A fold of this type with the top protruding into the drift above it occurs along the lake shore 3 miles east of Erie (Fig. 27). By their relation to terraces and terrace deposits, the age of folds and faults can be determined still more definitely, so we can say they are not only post-glacial but post- terrace in age. Evidently if a fold or fault deforms the terrace, or if the loose shales at the top are uneroded in the terrace, they are younger than the terrace. E. M. Kindle and F. B. Taylor have recognized small folds in the Queenston shale south of Lake Ontario, that deform low terraces and so are known to be more recent than those terraces. 4 In the region south of Lake Erie, terraces are deformed by folds in this same manner. The anticline shown in Figure 38 deforms a 43 -foot terrace. The gentle limb at the left is 200 feet wide and the steep one at the right 12 feet wide. Above the crest of the anticline a distinct ridge extends across the surface of the terrace in the direction of the axis. A view of this ridge above the fold is shown in Figure 37. Other folds, above which distinct low ridges occur in the terrace, in line with the axis of the fold, are shown in Fig- ure 18, where the terrace is 14 feet high, and in Figure 26, where the terrace is about the same height. 1 G. K. Gilbert, Bull Geol. Soc. Amer., X (1898), 133, Fig. 2. 2 E. M. Kindle and F. B. Taylor, Niagara Folio, No. 190 (1913), Plate 19. 3F. R. Van Horn, Bull. Geol. Soc. Amer., XXI (1009), 771-73, Figs, i, 2, and Plate 54. 4 E. M. Kindle and F. B. Taylor, op. cit., p. 15. STUDIES IN MINOR FOLDS 71 A large number of folds were found with their uneroded crests extending up definitely into the terrace material above. Protrusion by a fold in which the topmost beds are resistant, might be expected, FIG. 37. A low ridge deforming the terrace above the fold in Figure 38 FIG. 38. An unsymmetrical fold deforming a 43-foot terrace, the top of which is shown in Figure 37. Along Elk Creek i mile southeast of Girard, Pa. if the folds are older than the terrace; but when the top of the fold con- sists of thin-bedded shales and sandstones, the uneroded crest gives evidence that the fold has been developed since the formation of the 7 2 STUDIES IN MINOR FOLDS terrace, or since the stream abandoned the old higher channel. Two illustrations of anticlines from which streams have eroded the crests FIG. 39. Anticline in the Trenton limestone related to the great thrust fault at Prospect, N.Y. Having eroded the crest from the fold, West Canada Creek has shifted its channel to the right. FIG. 40. A fold from which the crest has been eroded and flood-plain material deposited on the eroded top. Along the Ashtabula River, near Kingsville, Ohio. are shown in Figures 39 and 40. The former is a fold in the Trenton limestone at the edge of the large fault at Prospect, New York. Here, STUDIES IN MINOR FOLDS 73 West Canada Creek, having eroded the crest of the fold, has cut its channel more deeply to the right, and has shifted in that direction. FIG. 41. Fold with crest uneroded in a 20-foot terrace. The book above the center of the picture covers the crest, and the terrace deposits extend down into the shale between the two books. Near North East, Pa. * boo * > - '" .* v ^i^^^^'^^^^^^ m FIG. 42. Small anticline with crest uneroded in a low terrace. Open notebook behind the roots, and camera case near end of small thrust, mark top of shale. Seven miles southeast of Painesville, Ohio. Another anticline with eroded crest is shown in Figure 40. Since the erosion of the crest the stream has shifted, and the materials of the 74 STUDIES IN MINOR FOLDS flood plain have been deposited on the truncated edges of the strata, indicating that the anticline is older than the flood plain. While a large number of folds having their crests uneroded occur, it seemed difficult to get pictures clearly illustrating this condition. The loose shales at the top of the folds frequently are very like the fine material above, so the contrast is not sufficiently marked to show well in a picture. Figures 41 and 42 show two folds from which the loose shales at the crest have not been eroded, thus marking them younger than the terraces in which they occur. Figure 41 shows the detail of the crest represented in Figure 8. It occurs along Sixteen Mile Creek, one-half mile south of North East, Pennsylvania, in a terrace 20 feet high. The anticline has been overturned upstream, and the crest (covered by the upper book) has been forced up into the midst of the coarse terrace material. The lower edge of the book at the right rests on the topmost part of the shale to the right of the anticline. The crest of this fold, overturned upstream, clearly is post-terrace. Fig- ure 42 shows another small anticline, from the crest of which the loose shale has not been eroded. The open notebook (back of the roots to the left of the crest), marks the juncture of shale with flood-plain material, while the camera case marks it down on the right limb of the fold. A small fault with a horizontal displacement of 2 feet and 3 inches occurs at the right. Figures 18, 26, and 38 also represent folds in terraces with their crests uneroded. As indicated above, the age of these folds is fixed clearly as younger than the terraces in which they occur, in each case where the non-resistant crest is uneroded. In the same manner that the uneroded top of a fold indicates that it is more recent than the terrace on which it occurs, so the uneroded top of a fault indicates its recency. Two faults illustrating the differ- ence between a pre-terrace fault and a post-terrace one are shown in Figures 43 and 44. Both are in the upper part of the Chagrin shales along Euclid Creek, about 9 miles east of Cleveland. Both are over- thrust upstream in a general southeasterly direction. The one in Figure 43 is in a terrace 10 feet high, 2\ miles south of Euclid, and the other is three-fourths of a mile farther upstream, in a terrace 8 feet high. In Figure 43 the thin sandstone bed above (a) on the downthrow side of the plane is the same as the one above (a") on the upthrow side. The overthrust of the bed from the right of (a) would carry it to some such point as (a'), but the vertical extension of the overthrust has been eroded to the gradient plane of the former stream channel upon which the upper material of the terrace has been deposited. Thus the eroded STUDIES IN MINOR FOLDS 75 top indicates clearly that the displacement occurred before the forma- tion of the terrace. In contrast with this condition, the fault shown in Figure 44 has the upthrow side still uneroded in a terrace 8 feet in height. FIG. 43. A small fault with eroded top in the Chagrin shales near Euclid, Ohio. The same bed occurs above (a) and (a"). FIG. 44. Overthrust fault with top uneroded, south of Euclid, Ohio The fault plane dips at a very low angle in this upper part. At the left, to the northwest, it dips more abruptly beneath the floor of the valley. The horizontal displacement is 5 feet, extending from (a) to 76 STUDIES IN MINOR FOLDS the camera case above at the right. The end of the upthrown side abuts against the flood-plain material at the right, and the soft shales have not been eroded from the top. This uneroded top proves that the displacement is younger than the terrace. NATURE AND ORIGIN OF STRESSES IN ROCKS IN AREA Where and how have the stresses originated that formed the folds and faults ? Are they merely local, or are they general for the region ? Are they cumulative, or residual? As the magnitude of the deforma- tions may have direct bearing upon these questions, that will be noted first. A few of the folds have a width of from 300 to 500 feet, but most of them are less than 100 feet. In regard to the thickness of the strata involved, some folds deform terraces 40 to 60 feet high, and extend an unknown distance below the bottom of the valleys. The size of the folds would seem to indicate that any single fold does not reach a very great depth. In some instances faults below grade into folds above, and these faults may reach down for a considerable distance. In the case of one fold along Elk Creek, south of Girard, Pennsylvania (Fig. 38), there is a constant escape of gas from the crest where the stream crosses it. The gas horizons of this region lie chiefly between 500 and 700 feet in depth. This seems to indicate that not alone the most superficial strata are affected by the deformation. A number of folds of the same magnitude occur in the area. Cumulative stresses. That the earth's crust has a very considerable rigidity which must be overcome before deformation can take place is a fact now generally recognized. That the great movements of the earth's crust are periodic has been emphasized by many eminent geologists. 1 This periodicity has been proved by the earth's history. R. T. Cham- berlin recently has summarized the diastrophic periods of the Paleozoic, noting their relative importance and value as criteria for separating the rock systems. 2 Between the periods of great deformation, inter-periods of relative quiescence have occurred, during which land masses remained in a stable condition, while erosion has proceeded sufficiently long to reduce areas to the base-leveled condition of a peneplain. Because of the rigidity of the earth, and of this periodicity of the great diastrophic movements, it is conceivable that there are long periods of stress accumu- X T. C. Chamberlin and R. D. Salisbury, Geology, I (1905), 588-89, and III, 192-93; T. C. Chamberlin, Jour. Gcol., XVII (1909), 689; C. Schuchert, Bull Geol. Soc. Amer., XX (1908), 500; Bailey Willis, Science, XXXI (1910), 246-48. 2 R. T. Chamberlin, Jour. Geol., XXII (1914), 315-45. STUDIES IN MINOR FOLDS 77 lation, in which the stresses become more and more acute until they are sufficiently strong to overcome the strength of the earth's crust and its rigid interior. When this mastering degree of intensity has been reached, readjustment by movement takes place in the various types of deforma- tion. These adjustments continue until all the stresses are more or less perfectly compensated. After each great deformative period there follows a period of quietude in which the accumulation of stresses again begins to develop toward a higher and higher intensity. If there are these long intervals of stress accumulation preceding each diastrophic movement, it is possible that although incompetent to deform the. great masses of the earth's exterior, they still may have sufficient force to overcome locally some of the weaker parts, particu- larly if in addition to the more general stresses the rocks of an area are subjected to additional local ones. That these cumulative stresses are likely to be widespread may be deduced from the large areas affected by compensating adjustments. It is conceived then that during a period of relative quietude for this area, stresses have accumulated which, possibly by the addition of local stresses, to be noted later, have reached an intensity sufficient to deform the rocks on a small scale. If the great periodic or inter-periodic stresses have been accumulating in this area, and have contributed, in part at least, to the deformation of the rocks, the small folds and thrust faults indicate that they are largely lateral, or tangential. Residual stresses. In connection with the climactic accumulation of stresses with the consequent strains and deformative movements, it was noted that the compensation of the force developed might be more or less complete. If the compensation were absolutely complete, the new era following the deformative one would start out with a clean record. Thus one series in a cycle would be: stresses developed slowly to great intensity, great diastrophic movements, stresses all relieved, and rocks perfectly at ease until the beginning of a new series of cumulative stresses is initiated. Another conception may be that compensation is not com- plete; that after all the force of the great stresses competent to cause deformation has been relieved, there still remain residual stresses, which, though real in character, are incompetent to carry the deforma- tion farther. Thus the new era succeeding the deformation period would have bequeathed to it some of the stresses which may have been initiated early in the preceding period of general quietude. Still another conception may be that compensation is variable throughout any large area affected by deformative forces, not alone 78 STUDIES IN MINOR FOLDS because of the variable rigidity and elasticity of the rocks, but more largely in the localization of the forces engendered by the stresses, and the alignment of those forces in one general direction. Illustrations of this localization and alignment are seen in the closely folding, over- turning, and thrust faulting of the central and southern parts of the Appalachians. If it be true that a great -thrust of part of the earth's exterior, and possibly a part of the interior, was directed against the southeastern margin of the Appalachian region, it is possible that compen- sation was most perfect along this margin, where the force was applied and where the close folds were overturned, broken, and overthrust one far upon another. Toward the northwest, compensation may have been less complete as the folds became more open. Still farther to the northwest, beyond where the gentle, open folds were formed, lateral stresses may have developed which, though considerable in intensity, were incompetent to deform the rocks. It is the conception, then, for the area studied, that tangential stresses very considerable in amount were initiated here at the time when the marked diastrophic movements formed the great Appalachian structural unit, and that these stresses have remained as a residuum since that time. It is thought that to these residual stresses later stresses were added, making their combined force sufficient to fold and rupture the rocks. J. Barrell, in his recent studies, has expressed the idea that strains within the earth's crust may be borne for very long periods of time. 1 RELATION TO LARGER MOVEMENTS Having noted that very considerable residual stresses probably have existed for an extremely long time, it is the purpose now to link the minor movements of this area with much larger ones; the up tilt- ing of the northern part of the Great Lakes area and the region to the northeast, and the uplift and deformation of the Harrisburg pene- plain in northern Pennsylvania, northeastern Ohio, and southern New York. Northeastward tilting. It has long been recognized that the region of the Great Lakes and the area to. the northeast of these lakes have been undergoing a tilting movement during and since the Pleistocene. In this movement the' rise has increased in magnitude from the Great Lakes region toward the northeast. From a large number of measure- ments of changes in the levels of the lakes on the north and south sides, 1 J. Barrel), Jour. GeoL, XXII (1914), 310. 2 G. K. Gilbert, Eighteenth Ann. Kept. U.S.G.S., XVIII (2) (1896-97), 636. STUDIES IN MINOR FOLDS 79 G. K. Gilbert 1 has deduced the result of a tilting .42 of a foot for 100 miles in one hundred years. The tilting during and since the Pleistocene also has been measured and computed, by determining the amount of warp- ing of the old water lines marked by beach ridges and cut bluffs which are now far above the present level of the lakes. A large number have worked on the problems of the uplift and tilting of this lake area, and H. L. Fairchild in a recent article has given the bibliography of this work. 2 The direction of the isobases for New York and northern Penn- sylvania, given by Fairchild "with an inclination from the latitude parallels of 20, 70 divergence from the meridians," 3 corresponds closely with the extensions of those given by J. W. Goldthwait for the Lake Michigan -Huron region; 4 also with those given earlier by G. K. Gilbert for the entire Great Lakes region. 5 Just where the hinge for this uplift should be placed with reference to Lake Erie is not definitely settled. Goldthwait apparently would place it at the extreme eastern end of the lake, 6 while Fairchild would place it west of the center. If then, the hinge line of this uplift passes through, or along, the edge of the area studied, this area is in a critical position with reference to the uplift. The position is critical because of tangential stresses which may have been related to the tilting in a zone parallel to the hinge. That the tilting and tangential movements have been synchronous is suggestive. The tilting begun during the Pleistocene continues to the present. Numerous folds and faults are post-glacial, some giving evi- dence of reaching almost to the present time. If tangential stresses are related to this uplift, folds resulting from them should have their axes trending in a general way parallel with the hinge line and the isobases. As noted above, the isobases trend about 80 west of north, when extended as straight lines from the Lake Michigan-Huron region, but if the isobases curve southward across Lake Erie, the direction would be north 60 or 70 west. Table V gives the direction of the axes for folds in the Lake Erie region, of which illustrations are shown, and is quite representative. While a very few have axes either about east and west or north and south, two-thirds have a northwest trend, the majority lying between N.40W. 'G. K. Gilbert, Eighteenth Ann. Kept. U.S.G.S., XVIII (2) (1896-97), 636. 2 H. L. Fairchild, Bull. Geol. Soc. Amer., XXVII (1916), 255-62. 3 H. L. Fairchild, ibid., 238, Plates 10 and 12. 4 J.W. Goldthwait, Canada Dept. Min. Geol. Surv. Mem. 10 (1910), Fig. 3, opp. p. 40. 5 G. K. Gilbert, op. cit., p. 640, Fig. 100. 6 J. W. Goldthwait, Bull. Geol. Soc. Amer., XXI (1910), Plate 5, opp. p. 233. 8o STUDIES IN MINOR FOLDS and N.8oW., and about one- third have a northeast trend. The trend of the folds may seem too variable to align them into one or even two series, but minor folds of any series are not necessarily closely parallel with one another. For comparison, the trend of a number of folds on the northwest border of the Appalachians was determined from the geological folios and arranged in Table VI. This table shows a large variation in the trend of the axes of the smaller folds of a series adjacent to the larger Appalachian structures. A comparison of Tables V and TABLE V SHOWING DIRECTION OF AXES OF FOLDS IN THE LAKE ERIE REGION Fig. Fig. Fig. Fig. Fig. Fig. 2. 27. 28. 12. 40. 4- Northwest Trend N.ioW. Fig. 14. N.5oW. N.2oW. Fig. 31. N.6oW. N. 3 oW. Fig. 33. N.6oW. N. 3 sW. Fig. 25. N.8oW. N.4oW. Fig. 34. N.8oW. N.45W. Fig. 42. N.8oW. East -West and North-South Trend Fig. 23. E.-W. Fig. 38. N.-S. Northeast Trend Fig. ii. N.ioE. Fig. 26. N.25E. Fig. 8. N. 35 E. Fig. 41. N. 35 E. Fig. 24. N.5oE. TABLE VI SHOWING TREND OF AXES OF FOLDS ALONG THE NORTHWESTERN EDGE OF THE APPALACHIANS IN PENNSYLVANIA N.isE. N. 35 E. N.8oE. N.i5E. N. 4 oE. N.8oE. N. 2 8E. N. 45 E. N.30E. N.6oE. N.42W. N. 35 E. N. 7 oE. N.8oW. VI shows about the same degree of variation in both, even though the directions in the latter represent the axes of folds along the border of the Appalachians. Doming of peneplain. M. R. Campbell 1 has adduced evidence on a physiographic basis to show that the Harrisburg peneplain has been deformed by an irregular domelike uplift, with the maximum of eleva- tion in McKean and Potter counties, Pennsylvania, and in southern New York. This peneplain rises from 500 feet near Harrisburg, to 2,200 feet in the northern part of the state. The area along the southern border of Lake Erie forms the northwestern part of this domelike uplift, 'M. R. Campbell, Bull. Geol. Soc. Amer., XIV (1902), 277-96. a M 2 7> 3> 4 2 ; sandstones, 14 Cherokee shales, 54 Chimney Hill limestone, 18 Cincinnati Arch, 23 Clarke, J. M., 27, 28, 29, 41; and Luther, 28; and Schuchert, 25 Cleveland, 3; Brick and Clay Company, 62; shale, 27, 30 Clinton beds, 25 Closed anticline, 10, 27, 28, 29, 41, 82 Clymer quadrangle, 24 Coeymans limestone, 25 Compensating adjustments, 77, 78 Cone-in-cone, 29 Conneaut Creek, 24, 34, 35; Ohio, 4 Conneautville, Pa., 65 Connecticut Valley, 15 Cook Point, 48 Coon, W. E., 4 Corry sandstone, 27, 31 Crawford County, Pa., 31 Crosby, W. O., 59 Crusher, Okla., 9, 20 Cumulative stresses, 76 Gushing, H. P., 26 Cussewago beds, 25, 27, 31 Cuyahoga River, 24 86 STUDIES IN MINOR FOLDS Daly, R. A., 16 Dana Lake, 35 Darton, N.H., 13 Deep well at Erie, Pa., 57 Deformation, by landslides, 61; by solu- tion, 54; of terrace, 70 Des Plaines River, 52 Devonian, 18, 25, 27, 28, 29, 30, 68 Devono-Carboniferous, 31 Dewitt, N.Y., 48 Diabase, 17 Differential movements, 63 Disconformity, 26 Displacement in faults, 44-47, 56, 69 Dome, n, 19, 20 Doming of peneplain, 33, 80, 81, 82 Drainage changes, 34 Dunkirk, N.Y., 3, 49, 65 East Onondaga, 59 Eighteen Mile Creek, 27 Elk Creek, 14, 24, 34,- 41, 45, 47, 58, 76 Erie, 4, 41, 42 Esopus grit, 25 Euclid, Ohio, and Creek, 74 Fairchild, H. L., 35, 69 Falls Creek, 12, 19 Faulting, 57 Faults, 41, 43-48 Ferrous sulphate soluble, 51 Folds, 1-82; carinate, 14; due to com- pacting, 57; gas pressure, 63; intra- formational, 36, 37; isoclinal, 14; monoclinal, 14; origin of, 48-68; parallel, 37, 38; transverse, 38, 39; types, 4-16, 19, 20 Folios with folds due to vulcanism, 48 Franks conglomerate, 18 Freedom, Pa., 36 Gabbro, 17 Gaines, folio, 30, 31, 32; quadrangle, 30 Galena dolomite, 58 Gardeau shales, 28 Gartz, Frank, 4 Gas, escape, 63; pressure low, 63 Gasconade formation, 54 General relations and direction of axes, 16 General, structure, 35; types of folds, 36 Genesee, River, 28; shale, 25 Geneva, Ohio, 23 Geologic history, 32 Gilbert, G. K., 4, 13, 14, 15, 49, 61, 64, 69, 70, 78, 79 Girard, Pa., 4, 7, 14, 43, 45 Girard shales, 27, 29, 41, 43, 44, 51 Glacial, age of folds, 68; deposition, 34; erosion, 34 Glaciation, 63, 82 Glacio-Lacustrine substage, 35 Glenn, L. C., 27, 30, 31 Goldthwait, J. W., 79 Grand River, 24, 34; Glacial Lobe, 34 Granite, 17 Gravity, effects of, 60 Grimes sandstone, 28 Gulf Coastal Plain, 54 Hall, J., 28, 29, 30, 50 Hamilton beds, 25 Haragan marl, 18 Harrisburg peneplain, deformed, 78, 80, 81, 82; age of, 33, 81 Hatch shale, 28 Henry Mountains, 1 2 Henry house shale, 18 Hice, R. R., 36, 37 Hinge of uplift, 79 Homocline, 16 Hopkins Creek Estuary, 60 Hopkins, T. C., 23, 50, 59, 61 Huron, River, 29; shale, 27, 29, 51 Ice expansion as cause of folds, 49/82 Igneous activity as cause of folds, 1 7 Intra-formational folds, 36, 53 Introduction, i Isobases of tilted area, 79 Isoclinal folds, 14 Ithaca, N.Y., 48, 55 Jamesville, N.Y., 59 Jefferson County, N.Y., 49, 69 Joplin folio, 54 INDEX Kelley's Island, 66 Kennedy, W., 54 Keweenawan dolomite, 48 Kindle, E. M., 37; and Taylor, F. B., 26, 27, 64, 70 Kingston beds, 25 Kingsville, 4 Knapp formation, 27 Lake, Erie, 3, 28, 35, 38,49, 69; Lahon- tan, 61; Mono, 37, 67; Ontario, 3, 27, 60, 64; Plain, 23; Zug, 61; Zurich, 61 Landslides, 61, 82 Lateral stresses, 62, 81, 82 Lawson, A. C., 69 LeConte', J., 64, 67 Leith, C. K., 21 Lesley, J. P., 35 Leverett, F., 34 Limonite, 51 Little Elk Creek, 43 Little Falls, N.Y., 55 Local structures, 36 Localization of forces, 78 Location and area, 22 Lockport limestone, 25 Logan County, Ark., 52 Logan River, Utah, 60 Lorraine beds, 25 Lower Kittanning clay, 36 Lowville limestone, 25, 26 Luther, D. D., 27, 29, 56 Lyons quarry, Chicago, 52 Magnetite, 51 Manheim, N.Y., 48 Manlius limestone, 25 Manlius, N.Y., 59 Marcasite, 51 Marcellus, 59; shale, 25 Maryland, 16 Mathews, E. B., 12, 16 Mathews, G. F., 69 Maumee Lake, 35 McGee, W. G., 37 McKean County, Pa., 80 Meadville, Pa., 4, 34, 38, 39, 59, 61 Medina sandstone, 25 Michigan, 3 Middle Kittanning coal, 37 Middlesex black shale, 28 Miles Grove, 42 Mill Creek, Ohio, 62; Pa., 24 Miller County, Mo., 54 Miller, W. J., 26, 36, 54, 63 Minor folds, 3, 17 Mississippian, formations, 18, 25, 27, 31; period, 68 Mono Valley, 61, 64 Monoclinal, 14, 15 Montana, 57 Nature and origin of stresses, 76 New Brunswick, 69 New England, 69 New Scotland beds, 25 New York, Chicago, St. Louis R.R., 4? New York, 41, 55, 69, 80 Niagara, Gorge, 26; folio, 60; limestone, 13, 52; quadrangle, 27 Nipogon Basin, Canada, 48 North East, 4, 8-14, 43, 74 Northeastward tilting, 78 Nova Scotia, 37 Ohio, 3, 30 Oklahoma, 3, 17 Olcott, N.Y., 27 Olean conglomerate, 31 Oheida conglomerate, 25 Onondaga, County, 56; limestone, 25 Ontario, 37 Open anticline, 9, n Orangeville shale, 25, 27 Ordovician, 18, 25, 26 Origin of folds and faults, 48 Oriskany beds, 25 Orton, E., 30 Oswayo formation, 27, 31 Ouachita Mountains, 67 Outline, i Overturned anticline, 7 Paine Creek, 7, 46, 47 Painesville, 4, 10 Parallel folds, 37 88 STUDIES IN MINOR FOLDS Passage shales, 28 Pegmatite, 17 Peneplain, deformed, 80, 82; Harrisburg, 33,81; Schooley, 33; Worthington, 33 Pennsylvania, 3, 35 Pennsylvanian, 18, 25, 31 Periodicity of crustal movements, 76 Permian, 18, 68, 82 Physiographic history, 33 Piedmont, 12, 1 6 Pirsson, L. V., 59 Pleistocene, 32; age, 68, 82; tilting, 78, 79 Porphyry, 17 Portage, beds, 25, 27; group, 28 Post-glacial, age, 82; changes, 35; deposits, 32 Post- terrace, 82 Potter County, 80 Powell, J. W., 1 6 Pre-glacial, 68 Pre-Pleistocene, 82 Presque Isle, 32 Pressure, gas, 63; ice, 49 Prospect, N.Y., 26, 63, 72 Prosser, C. H., 6, 27, 29, 30, 31, 32, 57 Provo delta, Utah, 60 Purpose, 3 Pyrite, 51 Quartz-monzonite, 17 Quaternary, 32, 37 Quebec, 67 Queenston, 26; shale, 26, 27, 41, 60 Radial shortening, 57, 58 Reagan sandstone, 18 Recumbent fold, 8, 9, 82 Reeds, C. A., 14, 17, 19 Relation to larger movements, 78 Remsen quadrangle, 26 Residual stresses, 77 Rhinestreet black shale, 28 Riceville shale, 25, 27, 31, 38 Richmond beds, 25, 26 Rise in temperature of rocks, 49 Rochester, 36; shale, 25 Rocky Mountains, 48 Rogers, G. S., 57 Rondout waterlime, 25 Russell, I. C., 37, 61, 64, 67 Salina, 25, 56 Salisbury and Atvvood, 67 Salisbury, R. D., 4 Sandusky, 3 Sardeson, F. W., 63, 64 Schneider, P. F., 41, 48, 55, 56 Schooley peneplain, 33 Schuchert, C., 27, 76 Schoharie grit, 25 Sedimentary rocks, 18 Seneca Lake, 28 Shaler, N. S., 54 Sharon conglomerate, 25, 31 Sharpsville sandstone, 25 Shenango beds, 25 Shortening by drop of hypothenuse, 56 Siebenthal, C. E., 54 Sierra Nevada Mountains, 48 Silurian, 1 8, 25, 69 Simpson formation, 9, 19, 20, 36 Sixteen Mile Creek, 14, 24, 43, 44, 74 Smallwood and Hopkins, 23, 50, 59, 61 Solway well, 56 St. Elizabeth limestone, 54 St. John, New Brunswick, 69 Stone, R. W., 33 Stony Island, 13, 65 Stresses due to crystallization of lime- stone, 53 Structure, 19; general, 35; local, 36 Summary, 81; of minor folds, 21; of origin of folds, 67 Sunbury shale, 27 Swanville, 45 Sycamore limestone, 18 Syenite, 26 Sylvan shale, 18 Symmetrical anticline, 4, 5, 45 Syncline, 13 Synclinoria, 14 Syracuse, 48, 56, 81 Table I, 18; II, 25; III, 27; IV, 28; V, 80; VI, 80 INDEX 89 Taconic Mountains, 67 Terraces, 24; deformed, 70, 71 Thirty Mile Point, 64, 70 Thompson's Ledge, 25, 32 Tilting of lake region, 78, 82 Topography of area, 23 Transverse folds, 38, 40 Trend of axes, 39, 40, 80 Trenton Falls, 36, 53, 59, 63 Trenton limestone, 25, 26, 36 Trumbull County, 30 Tully limestone, 25 Twenty Mile Creek, 24, 34, 37, 38, 47 Types of folds, 4, 19 Udden, J. A., 63, 64 Uneroded top of fold, 70, 71 United States, 60 United States Geol. Surv. folios, 48 Unsymmetrical anticline, 5, 6, 40, 42, 45, 46 Upland, 24 Uplifts, 8 1 Upper Devonian, 28 Uptilting of lake region, 78, 79, 82 Utica shale, 25 VanHise, C. R., 9, 16, 21; 51 Van Horn, F. R., 50, 62, 70 Vanuxem, L., 36, 37, 53 Vertical pressure, 59 Viola limestone, 18 Wallis, B. F., 18 Walnut Creek, 24, 34, 38, 46, 61 Warren folio, 30 Washita River, 20 Weathering, a cause of folding, 52, 82 West Canada Creek, 73 West Virginia, 3 Westfield, 4, 35, 39 Wheelock, C. E., 55 White, I. C., 23, 25, 27, 28, 29, 30, 31, 4i, 63 White, T. G., 36 Whittlesey, 35 Wichita Mountains, 3, 14, 17, 21, 67 Width of folds, 76 Willis, Bailey, 7, 9, 43, 46, 76 Wilson, A. W. G., 48 Wisconsin, 3 Wiscony shale, 28 Wood worth, J. B., 41, 69 Worthington peneplain, 33 Wyoming, 12, 13 14 DAY USE RETURN TO DESK FROM WHICH BORROWED EARTH SCIENCES LIBRARY This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. LD 21-40m-10,'65 (F7763slO)476 General Library University of California Berkeley