Cornell University Libra “iii CORNELL UNIVERSITY LIBRARY University of Montana State University University of Montana Studies Series No. 1 Geology and Economic Deposits of a Portion of Eastern Montana By Jesse Perry Rowe, Ph. D. Professor of Geology, The State University, Missoula and Roy Arthur Wilson, B. S. Assistant in Geology, The State University, Missoula Missoula, Montana 1916 Entered at Missoula, Montana, as second-class matter under act of Congress, August 24, 1912. University of Montana State University University of Montana Studies Series No. 1 Geology and Economic Deposits of a Portion of Eastern Montana By Jesse Perry_Rowe, Ph. D. Professor of Geology, The State University, Missoula and Roy Arthur Wilson, B. S. Assistant in Geology, The State University, Missoula Missoula, Montana 1916 Fig. Fig. Fig. Rig. Fig. Fig. Fig. Fig. Fig. Fig. 10. Fig. 11. Fig. 12. Fig. 13. Fig. 14.. Fig. 15. Fig. 16. Fig. 17. Fig. 18..- Fig. 19. Fig. 20. Fig. 21. Fig. 22. Fig. 23. Fig. 24. Fig. 25. Fig. 26. Fig. 27. Plate I. WAND Op oo bh Plate II. Plate III. Plate IV. ILLUSTRATIONS Page Page Page Index Map Characteristic Prairie of the Fort Union Formation................ 5 Yellowstone River Near Savage, Montana................. 6 Powder River Near Mizpah, Montana..uw.. oo ec.ceceteececceceeeceeeeee 7 Erosion of Pierre Shales 16 Colgate Sandstone Near Glendiven.u........cetceeeeeeeeeeeeeeececteee el T Lance Formation on Cedar Creek Anticlime..uu.....0.eececenn--- 19 Log-like Sandstone Concretions of Lance Formation.............. 21 Badlands of the Lebo Shale. Member... eecee ce eeeeeeceneceeoees 22 Sandstone Pinnacle 24 Log-like Sandstone Concretions of the Fort Union Formation..25 Badland Erosion of Fort Union Formation.........-.eccccccsceccesoe-- 26 Terrace Gravels Tongue River Near Miles. City View on South Limb of Cedar Creek Anticline.. 3 View Showing Block Jointing of Lignite........... one BA View of Rock Fused by Burning Coal Bed........ ...86 View of. Ground Underlain by Burning Coal Bed........:............ 37 A Butte in the Lance Badlands AO View Showing Weathered Coal Bed.............0....0..2.. View of Lignite Bed of the Fort Union Formation. View of Butte Capped by a Burned Coal Bed........ Diagram of Cedar Creek or Glendive Anticline.... Pine Hifls of the Fort Union Formation... 54 View of Fort Union Prairie 55 View of Lance Badlands 56 View of Lance Badlands Showing Alkali Incrustations.......... 57 Generalized Columnar Section of Upper Cretaceous and Lower Tertiary Deposit in Montana, Wyoming, and the Dakotas 11 Map Showing Areal Geology of Region Considered.................- 15 Geologic Section of the Region 30 Columnar Sections Showing Rtratgraphis Relations of Coal Beds ‘ 44 The University of Montana The University of Montana is constituted under the provisions of Chapter 92 of the Laws of the Thirteenth Legislative Assembly, March 14, 1913 (effective July 1, 1913). 3 The general control and supervision of the University is vested in the State Board of Education. The Chancellor of the University is the chief executive officer. For each of the component institutions there is a local executive board. MONTANA STATE BOARD OF EDUCATION 8. V. STEWART, Governor - - Ex-officlo, President J. B. POINDEXTER, Attorney General - Ex-officlo | H. A. DAVEBR, Supt. of Pub. Instruction - - Ex-officlo, Secretary S. D. LARGENT (1916) J. BRUCE KREMER ~ tibiae W. S. HARTMAN (1916) c. H. HALL (1918 JOHN DIETRICH (1917) LEO FAUST (1919) A. L. STONE (1917) W. H. NYE (1919) EDWARD C. ELLIOTT, Chancellor of the University ene” University comprises the following institutions, schools and depart- 8: THE STATE UNIVERSITY at Missoula. Established February 17, 18938, and consisting of: The College of Arts and Sciences, The School of Law, The School of Pharmacy, The School of Forestry, The School of Journalism, The School of Music, The Summer Session, ! The Biological Station, (Flathead Lake) The Extension Service, The Graduate Department. FREDERICK C. SCHEUCH, Acting President. THe S14 1e COLLEGE OF AGRICULTURE AND MECHANIC ARTS at ozeman, Established February 16, 1893, and consisting of: The College of Agriculture, The College of Engineering, The College of Applied Science, The College of Industrial Arts, The School of Home Economics, The School of Mechanic Arts, The School of Agriculture, The School of Art, The Secretarial Course, The School of Music, The Summer Session, The Agricultural Experiment Station, The Agricultural Extension Service. JAMES M. HAMILTON, President. THE STATE SCHOOL OF MINES at Butte. Established February 17, 1893. CHARLES H. BOWMAN, President. THE STATE NORMAL COLLEGE at Dillon. Established February 23, 1893, and consisting of: The Two-year Elementary Course, The Three-year Course, The Four-year Course, The Graduate Course. JOSEPH E. MONROE, President. STATE UNIVERSITY EXECUTIVE BOARD J. H. T. Ryman J. M. Kelth : Frederick C. Scheuch INTRODUCTION The principal aim of this bulletin is to present in as clear and con- cise a manner as possible. the general geology of a portion of Eastern Montana with a rather generalized discussion of the economic geology. The field described is bounded by parallels 45 degrees 30 minutes and 40 degrees 50 minutes and meridians 104 degrees and 105 degrees 30 minutes. The area is approximately 6,800 square miles, including all of Wibaux and portions of Richland, Dawson, Prairie, Custer and Fallon counties. uedod siyf Us P@SSnNIsip oes Seprauy) Ueigsod BSUS _ ark oe eS 7 WL) y i is CHAPTER I. TOPOGRAPHY AND GEOGRAPHY Relief. All of the area discussed in this bulletin lies in what is known as the Great Plains province. While the greater part is characterized by rolling prairie, there are marked diversities in topography which are of special significance since they aid greatly in the identification of the various formations represented, each formation having certain distinctive erosive features. At some past geologic age the entire area was an extensive plateau which has been reduced to its present form by long continued erosion, the larger inter-stream divides still showing something of the original level of the country. The present differences in elevation are small when compared with those in other parts of the state. The maximum difference in any given locality is generally 800 feet and over the area aS a whole approximately 1,800 feet. Blue Mountain and the Sheep Mountains, remnants of the original plateau, which have withstood erosion because of the resistant character of their rocks, are the most prominent points of elevation. Although called mountains, the above described elevations are in truth but hills and owe their prominence to the conspicuous way in which they stand out from the surrounding prairie, forming noticeable land- marks for some distance around. The rocks forming the above hills belong to the Fort Union forma- tion, which covers the greater part of the region and is the highest in this geologic province. Owing to its consolidated character, this formation erodes into broad areas of gently rolling prairie with oc- easional stretches of level bench land. These lands are the principal Fig. 2.—Characteristic prairie of the Fort Union formation. “Clinker buttes” in background, formed from resistant masses of rocks fused by burning ccal beds. asf localities for farming, and also support luxuriant growths of grass which make excellent grazing for stock. One peculiar topographic feature largely confined to the Fort Union formation is due to the so-called “Clinker Buttes” which are more or less scattered over the entire area, but most abundant in the southern part. Capped with brilliant red masses of fused shale and sandstone, these buttes rise abruptly from the general level of the prairie and add a pleasing effect to the landscape. At one time the general land surface was at the present heights of the clinker buttes. Through various causes, among which spontaneous combustion and lightning may have been important, the larger coal beds,. which then covered the region on a far more extensive scale than now, were set on fire. The intense heat generated by the burning coal beds fused the overlying rocks into red masses of clinker and slag, in places the heat being great enough to cause melting and flowage. These fused rock masses, because of their induration, offered much greater resistance to erosion than the unfused portions and stand out today either as steep-sloped, isolated buttes, or as small mesa-like hills in those places where erosion has not been as active. Though rolling prairie is the characteristic erosive feature of the Fort Union formation, there are places where these rocks, due to variations in the hardness of the strata, have been deeply cut by waterways whose banks are generally perpendicular. Interspersed among these waterways are areas quite devoid of vegetation, with here and there small buttes capped with resistant ledges of sandstone. The bare slopes and the buttes, which are composed of a very light colored sandstone, stand out in marked contrast to the grass covered prairie. The “badlands,” which roughly outline the exposures of the Lance formation, are undoubtedly the most interesting and unique erosive feature of this region. The most prominent badland areas are along the Yellowstone River and the lower courses of the Powder and Fig. 3—Yellowstone River near Savage, Montana, showing “bpadlands” of the Lance formation. eGo Tongue Rivers. A written description can only give a vague idea of their appearance. The dark colored shales interstratified with more resistant beds of sandstone yield readily to the attacks of erosive agencies. Bare of nearly all vegetation except the most hardy forms, intricately carved and dissected into deep, canyon-like coulees and steep-sided buttes, with hidden holes and vertical slopes which make travel almost impassable and even dangerous in places, these areas are well named. Extending in a broad strip from the southeastern part of the area to a point about fifteen miles southwest of Glendive is a zone of shales which erode into low, rolling hills and broad, shallow valleys. This zone marks the outcrop of the Pierre shale formation, the lowest exposed in this geologic province. In places where the shales are somewhat more resistant the waterways are rather deep cut, but the general erosive features of this formation are broad, gently undulating stretches. These shales are bordered on either side by a narrow strip of light colored sandstones, the strip on the northeast side being some- what broader. This sandstone erodes into rolling hills in the south- east part of the area and into vertical cliffs farther north. ‘The somber colored strata of the badlands which border the Pierre forma- tion contrast strongly with the above described light sandstone stratum, which is known as the Colgate member of the Lance formation. The above formations will be described in detail later. Drainage With the exception of Beaver Creek and Little Beaver Creek in the extreme eastern part (both tributaries of the Little Missouri River), the entire area is drained by the Yellowstone River. This stream has its source several hundred miles to the west and is of considerable size. As is true of all the rivers in this region, the Yellowstone is generally quite muddy; but there are certain times during the fall and winter when its waters are fairly clear. The river is of suffi- Fig. 4—-Powder River near Mizpah, Montana. Spee cient size for the navigation of lighter and smaller boats, but the rather rapid current is an unfavorable factor. The Powder River drains north through the west-central part of the area, joining the Yellowstone River a few miles west of Terry. This stream is next in size to the Yellowstone and has its source in Northeastern Wyoming. During the dry summer season its volume is considerably diminished, but it never becomes dry. The current is quite rapid and the stream is generally very muddy. The Tongue River drains north through the extreme western part of the area, joining the Yellowstone at Miles City. This is the third important watercourse, and is somewhat smaller than the Powder River. O’Fallon Creek, Sunday Creek, Beaver Creek and Little Beaver Creek are important perennial streams, each draining considerable ter- ritory. There are several other perennial streams of less importance, but the greater number of waterways are dry during the summer months. The entire area is intricately cut by small watercourses and coulees, most of these only containing water during heavy rains. Freshets occur occasionally, and streams that ordinarily are but mere brooks become at such times fair sized rivers. Rapid stream erosion is more or less common. This is aided by the general softness of the rocks, and even during the course of a few years marked local changes in topography are not uncommon. Climate and Vegetation The region lies in the semi-arid Great Plains, where the annual rainfall is from 12 to 15 inches. The soil constituents are such, how- ever, that by the dry-land method of farming only a small amount of moisture is.necessary to raise garden vegetables and the common grains. The winters are of average duration and not unduly severe. The farming season extends from April to October. In summer many of the days are fairly hot, but owing to the general dryness of the air, the heat is not often oppressive. At this season, however, fields of grain are not infrequently parched by dry winds or beaten down by hailstorms. The greater part of the area is open prairie and, aside from the native grasses and shrubs, is destitute of trees, except for a few wil- lows and cottonwoods along stream courses and occasional growths of pine trees on the buttes and hills. Stunted cedars grow in the badland areas. Industries and Settlements Farming and stock raising are the principal industries. Since 1906 the field has been rapidly settled, so that at the present time there are very few sections of land that have not been entered by the home- steader. Miles City, Glendive, Terry, Sidney, Baker, Ismay and Fallon are the most important towns of the region. There are numerous vther settlements along the railroads consisting generally of a few houses with a store or two and a post office. In those parts of the area that are more or less isolated from the railroads a ranch house usually serves as a post office. The Chicago, Milwaukee & St. Paul and the Northern Pacific Rail- ways cross the field as shown on the index map (Fig. 1). Aside from the badland areas, the general absence of any marked relief features —3— make all parts of the region easily accessible. Wagon roads are gen- erally found along section and township lines. CHAPTER II. GENERAL GEOLOGY Succession and General Character of Formations. The rocks of this area may be divided for descriptive purposes into two main groups. The first includes the consolidated sedimentary rocks and the second the unconsolidated deposits of gravel, sand and silt. No igneous rocks are present. The subdivision of the consolidated rocks is based both on paleonto- logic and lithologic evidence. Owing to the ease with which the forma- tions erode and to the general scarcity of vegetation, good exposures are fairly abundant, making it possible to work out the general se- quence.of the beds with comparative ease. The pronounced horizontal position of the strata and the general absence of structure make the geology of the district somewhat simple and the problems offered are regional rather than local. With but few exceptions, contiguous for- mations grade one into another and are conformable. In places the demarcation between successive formations is pronounced, but gener- ally the horizon of division is an arbitrary matter. All rocks of this area belong to the Upper Cretaceous and Lower Tertiary systems. In the following brief description, in order of age, it is intended to give a general conception of the prominent characteristics of the con- solidated rocks and to correlate them with the formations of other regions. The Pierre shale formation is named from its type locality in the vicinity of Pierre, South Dakota. The rocks composing this formation are dark grey or somber colored marine shales which weather into monotonous, gently undulating surface with but little relief. The Colgate sandstone, the stratigraphic unit next above the Pierre shales, receives its name from its marked exposures near Colgate, a small station on the Northern Pacific some miles west of Glendive At the present time the age of these rocks is in doubt, and for con- venience they are arbitrarily classed as a member of the Lance forma: tion. The lower seventy feet of this member is composed of sandstones intercalated with shales, while the upper portion is formed of a massive sandstone varying from a light brown to white in color. The Lance formation is so named because of its development on Lance Creek, Converse County. Wyoming. It is composed of alternat- ing beds of shale and sandstones, varying in hardness but prevailingly soft, and generally of a somber color. Interstratified are beds of coal, usually thin and of poor quality. Near the top of the formation are found a few coal beds of better grade, but they are generally lenticular and of local development only. Limonitic concretions are abundant. The Fort Union formation is the highest in the area considered, and is named from its type locality near Fort Union at the mouth of the Yellowstone River. It is composed mainly of massive sandstones with interstratified beds of shale. The sandstones are generally light grey or yellow in color. Over the greater part of the region the formation has no great variation of lithologic characters and comprises one strati- graphic unit; but in the western part of the area the light colored rocks change at the base of the formation into dark colored strata in which shales predominate. This somber zone is known as the Lebo shale member of the Fort Union, and closely resembles the underlying Lance formation both in lithologic characters and erosive features. The overlying light colored sandstones which compose the main mass of the Fort Union formation, owing to their more homo- genous and consolidated character, erode into rolling prairie. Stratigraphic Section. The general character of the formations of the region is shown in the following table: System Formation Member Characterlstics Thickness (Feet) Quaternary Coarse gravel, sand and silt. 0-30 Massive sandstones, white, grey, brown or yellow in | color; clay shale and _ sili- p I cious shale, usually soft; 1,200 numerous coal beds of fair Fort quality. Tertiary Union Formation Dark colored shales and Lebo sandy shales with some sand- Shale stones; concretions abund- 140 4 ant; several coal beds, gen-| erally thin and impure. Dark .colored shales, and brown or grey sandstones| with limonitic concretions. 500 ‘ Contains a few local beds of Tertiary Lance lignite. OF Formation Cretacéous White, brown, and yellow sandstones. Lower part in- 175 ter-stratified with shale. Colgate Sandstone . Drab or dark shale with! ‘ Cretaceous ee caleareous concretions and 300+ © Selenite crystals. Regional Correlation In the study of the Upper Cretaceous and Lower Tertiary deposits of Montana and adjoining states, although equivalency between larger groups of strata is now well established, there are still certain minor subdivisions whose regional relationships are in question. In addition, large masses of strata in one region too homogenous for subdivisions are differentiated in other regions into distinct lithologic units. The true relationships of these formations can only be worked out by eareful tracing and detailed study of their changes from point to point. In the following discussion the relationships and the correlation of the various stratigraphic subdivisions as recognized by different authors are considered. The accompanying table (Plate I.) shows the principal characteristics of the Upper Cretaceous and Lower Tertiary deposits in different sections of Montana and adjoining regions. 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YAO + NOINS] LHOY + i 4Y5Y 0 ABE aalscoyy 4S OOO SAOYS PAYLYOIIIU P SEUQS pPPYoS aAISSOL/ NOIN/) Lexa! AYWILYI EE, LOM CPN NULLS TIA PNELNOS NYFLOVF VNVLNOLY TVHLNID VALLNOLY WHLNID LSI ONINOAM? VOM] CRN] WNWINO],| 30 SLISOdIC] UWL] AMO] P SNOIWLIUD) Wada] 40 NOLLISG CIZMWSINIC) J awd a |; ee The Dakota sandstone, a coarse cross-bedded sandstone, in places conglomeritic, is extensively exposed in Wyoming and the Dakotas, where it forms the base of the Upper Cretaceous deposits. In eastern and south-central Montana, however, the Dakota sandstone is ap- parently absent, the overlying marine sediments of the Colorado group grading conformably into the Kootenai beds of the Lower Cretaceous. The absence of these basal sandstones from the above areas indicates either a probable hiatus at this horizon or that the sediments laid down during the Dakota time are here represented by beds not easily separable from the Colorado shales. The Colorado group, which is the basal strata of the Upper Creta- ceous over the greater part of Montana, is generally composed of dark colored marine shales interstratified with sandstones and limestone. In Montana this group comprises one stratigraphic unit which is known as, the Colorado shale. Further east and south in the Dakotas and Wyoming the strata of this group are differentiated into two prominent lithologic units, the Niobrara limestone and the Benton shales, the last named comprising several formations. The rocks of the Colorado group are unexposed over the area considered in this article. Overlying the Colorado group is a series of sedimentary beds that Occupy a vast area over the Great Plains. The horizon of these beds in eastern Montana is marked by the Pierre shale, the lowest formation exposed in the area here described. The marine shales of the Pierre formation with the overlying Fox Hills sandstone, which has an ex- tensive development in North and South Dakota, constitute the Mon- tana group. The rocks of this group vary considerably in thickness and character, but in all sections exhibit features in common, consist- ing in general of sandstones and dark shales, the latter being especially characteristic. In Wyoming and Colorado the Montana group is represented by the Lewis and underlying Mesaverde formations. The Pierre shale is ex- tensively developed in these states, and it is held by Stanton* that the Mesaverde and part of the Lewis also belong to the Pierre. In central Montana this group is divisable into four formations: The Eagle, Clagett, Judith River and Bearpaw. The Eagle is mainly a massive sandstone, in places coal-bearing. The Clagett is composed of shales and sandstones of marine and brackish-water origin. The Judith River is prevailingly of fresh-water origin. Above the Judith River beds is the Bearpaw formation composed of dark colored marine shales, closely resembling in lithologic characters the Pierre formation as developed in the eastern part of the state and into which the above described lithologic units gradually merge as they are traced eastward. While the Fox Hills sandstone, the highest member of the Montana group as developed in the Dakotas, is represented by beds in eastern and central Montana having the same stratigraphic relation, definite correlation of these beds is still in doubt. In the central part of the state the Cretaceous section is normal to the top of the Bearpaw shales. Resting on these shales are transitional beds of dark colored fresh and brackish-water sandstone and shale which occupv the horizon of the Fox Hills formation. Owing to the few fossils which have been eollected in this formation, which is known as the Lennep sandstone, its true relation to the Fox Hills is in doubt. In the area considered in this paper the Colgate sandstone. at present arbitrarily classed as a member of the Lance formation, occupies the stratigraphic position of the Fox Hills. While the base of this sand- *Willis, B., Index to the Stratigraphy of North America; Prof. paper, U.S. G. S., No. 71, P. 681. —12— stone member shows a gradual transition from the underlying Pierre shales, grading from shales and sandstones in the lower part to a massive light colored sandstone at the top, thus showing a lithologic similarity to the Fox Hills, the evidence of the fossil leaves indicates that the greater part of these rocks are of later age. Species of plants identified by Knowlton as of Tertiary age have been collected to within seventy feet of the base of these sandstones, showing that the Fox Hills, if present, must constitute the interval between the lowest point where these plants are found and the top of the Pierre shale. Only a careful and detailed study of this sandstone. member over its exposures and a tracing of the beds from the known Fox Hills of North Dakota to this area, as far as this may be possible, will definitely solve the problem. And if it should prove that the lowest part of the Colgate sandstone is of Fox Hills time, these sandstones, according to paleontological evidence, will consist of two distinctive stratigraphic units, the lower of Cretaceous and the upper of Tertiary age. Overlying the Colgate sandstone and the Fox Hills of other areas are a series of somber-colored strata which comprise the Lance forma- tion. This formation is widely distributed in eastern Montana and adjacent regions. ‘The type locality is in Converse County, Wyoming, where the beds attain a thickness of several thousand feet. As can be seen from the correlation chart, the lithologic character of the Lance does not vary greatly over the areas of exposure. However, in Mon- tana, there is a general change in the color of the strata from dark to light hues as they are traced westward. Owing to the fact that there is considerable question as to the proper position of the Lance formation in the geologic column, some geolo- gists ascribing it to the Lower Tertiary and others to the Upper Cre- taceous, the formation is here classified as transitional. To whichever system the formation may be ultimately referred, it is certain that it has the same stratigraphic position as the Laramie beds of Wyoming, corresponding in great measure to the upper portion or perhaps to the whole of the latter formation. The present differences of opinion as to the age of the Lance beds are based on both lithologic and paleontologic evidence. Dr. Knowlton* places the Lance as the lower member of the Fort Union formation, which is of Tertiary age. He bases his evidence on the fact that over a third of the plant species collected in the Lance beds are common to the Fort Union. Reasoning from the stratigraphic viewpoint Knowlton mentions certain marked unconformities between the Lance and the underlying Fox Hills strata as exposed in Wyoming and the Dakotas, these unconformities being angular as well as ero- sional. Where these unconformities occur, the Fox Hills sandstone is in places nearly absent or very thin, seeming to indicate a time in- terval of some magnitude between the uplift and erosion of the Fox Hills beds and the deposition of the Lance sediments. The rather general distribution of these unconformities, two minor ones being also noted at the base of the Lauce as exposed on Cedar Creek, Mon- tana, tends to show that the break was more than local. In addi- tion, the above author argues that the fact that the Fox Hills fossils in places extend up into the Lance beds does not necessarily indicate that they lived at the time the Lance sediments were deposited. It is possible that during the time of encroachment of the Lance waters, fossil shells were eroded from the marine Fox Hills beds mingling *Knowlten, F. H., The stratigraphic position of the Lance formati (“Ceratops beds’): Jour. Geology, Vol. 19, 1911, pp. 358-876. = —13— with the then living fauna of the former formation. Furthermore, Knowlton states that the absence of any unconformable relations be- tween the Lance and the overlying Fort Union formation and the fact that with a few local exceptions there is no reliable lithologic demarca- tion, indicates an inseparable association with the Fort Union beds. Dr. Stanton, on the other hand, contends that while unconformities exist they are local, and that channeling would be natural in the change from marine to brackish-water conditions, making it possible for marine genera to be commingled more or less with brackish-water forms. This fact and the presence of dinosaur remains, especially Triceratops, in the Lance beds leads Dr. Stanton to class this formation in the Upper Cretaceous.* The Fort Union formation is the best known and the most extensive of those considered in this discussion. It covers a great part of eastern and portions of central Montana, western North Dakota, and eastern Wyoming, extending northward into Canada. This formation over the above areas is generally characterized by massive grey or light colored sandstones interstratified with darker shales and clays. Beginning near Terry in eastern Montana and increasing in thickness as they extend westward are a series of dark colored andesitic shales and sandstone which form the basal member of the Fort Union formation. West of the Crazy Mountains this member and all the underlying formations down to the Hagle sandstone, the basal formation of the Montana group, merge into a series of dark colored shales and sand- stones composed mainly of andesitic material which constitute a single lithologic unit known as the Livingston -formation. This formution is overlain by the lighter colored Fort Union beds which here have a thickness of 4,000 feet or more. *Stanton, T. W., Age and stratigraphic relatioris of the “Ceratops Beds” of Wyoming and: Montana: Proc. Washington Acad. Sci., Vol. 11, 1909, pp. 289-293. —14— AREAL GEOLOGY MAP OF PORTION OF EASTERN MONTANA DESCRIBED IN THIS PAPER Scace in Mites 29 Flate I i * ‘ LEGEND és a, Z Fort Union Formation. se Lebo Shale Member : é : | \ Lance Formation +" | ¢ Colgate Sandstone Member oe Pierre Shole —15— DETAILED DESCRIPTION OF FORMATIONS Pierre Shale. The distribution of the Pierre shale over the area considered in this discussion is not very extensive when compared with that of some ‘of the overlying formations. Its exposure, which is due to a rather marked uplift known as the Cedar Creek anticline, is characterized by a ribbon-like outcrop, varying from two to four miles in width, that enters the state near the east central part of Fallon County and trends northwest, terminating on the west side of the Yellowstone River a few miles southwest of Glendive. Since only the upper 300 feet of this formation is exposed any esti- mate of its thickness is necessarily a’ rough one. In adjacent regions the total thickness varies from an average of 1,500 feet in North Da- kota to 700 feet in central Montana, and would probably be the mean of these figures in the area here described. However, it is likely that the formation includes a considerable thickness of beds beneath the lowest horizon that reaches the surface. Owing to the lack of any comprehensive exposures and the general concealment of the rocks by erosive material, only a generalized state- ment is possible regarding the lithology of the beds. The exposed strata consists of soft jointed shales showing but little variation in texture. The color of the rocks is predominately a dark grey with occasional blue and purple tints. In places the weathered shales are stained with iron oxide and a few limonite concretions are present. Crystals of selenite are scattered over the eroded surface Fig. 5.— View cf Cedar Creek, showing characteristic topography of the Pierre shale forrnation. The stream here follows the apex of the Cedar Creek anticline. The white escarpment in the distant enews marks an exposure of the Colgate sandstone. of the formation, and where present in any quantity attract the eye from a considerable distance as they glisten in the sun’s rays. The upper part of the formation is characterized by numerous calcareous concretions, varying from a few inches to as much as four feet in diameter. They are generally light grey in color and are often cut by sty gas a network of calcite seams. Many of these concretions contain in- vertebrate fossils. The rocks of this formation indicate extensive sedimentation with some fluctuation in depth but with marine conditions prevailing. The invertebrate fossils are all marine forms. The writer found Inocerami, Scaphites, Baculites and Lucina to be the most abundant genera both as regards distribution and number. One peculiar fact noted in the study of the Pierre shale areas was the marked predominence of the genus Inoceramus over all other inverte- brate forms, a feature which was not confined to any one locality but seemed to be quite general. From collections made along the ex- posures of this formation in the area here described, the following species were found to be the most characteristic: Inoceramus sagensis, Owen. Inoceramus cripsi, Morton. Inoceramus sp. Scaphites nodosus var. brevis, M. and H. Scaphites nodosus var. plenus, M. and H. Secaphites nodosus var. quadrangularis, M. and H. Baculites ovatus, Say. Baculites compressus, Say. Nautilus dekayi, Morton. Lucina subundata, M. and H. Lucina occidentalis, M. and H. Margarita nebrascensis. M. and H. The Pierre shale formation can hardly be confused in the field with any other. The dark colored uniformly eroding shales with an entire absence of sandstone strata, and the numerous fossil bearing, calcareous concretions make it easily recognizable. Colgate Sandstone. The areal development of the Colgate sandstone within the district is the smallest of any of the stratigraphic units here considered. Exposed by the same uplift that brings the Pierre shales to the surface, the Colgate sandstone borders the former formation in a narrow belt on the southwest limb of the anticline and in a somewhat broader belt on the northeast limb where the dip is more gradual. Fig. 6—Badlands near Glendive, Montana. ‘The white stratum marks the Colgate sandstone. The overlying dark strata belong to the Lance forma- tion. On the right of the picture can be seen slumping of the strata. —17— On the northern end of the Cedar Creek anticline near Colgate this sandstone has its best exposures and development. From a thickness of 180 feet at this locality the beds thin out somewhat as they are traced southward along the anticline. The upper 40 feet of this member is characterized by a massive white sandstone which stands out prominently from the overlying dark colored Lance strata. Underlying this is about 70 feet of brown, rather coarse sandstone that in places contains numerous fossil leaves. The lowest 80 feet of strata is composed of dark grey sandstone interstratified with numerous seams of shale. In the southern part of the Cedar Creek anticline the lower shale and brown sandstone beds of this member seem to disappear, the massive, white sandstone that forms the upper part seeming to rest directly on the Pierre shale. As the outcrops in this area are not continuous and are concealed more or less by vegeta- tion and erosive material, the structure can not be worked out in detail. _As previously stated, because of the present doubt as to the age of these rocks, they are arbitrarily classed aS a member of the Lance for- mation. The fossil flora found in the central, brown sandstone horizon of this member are, according to Knowlton, of Tertiary age. A feature quite uncommon to this region and one which serves to complicate the question of the position of this member in the geologic column, is the occurrence of three unconformities at the top of the white sandstone stratum. The most marked is found along the bluffs southwest of Glen- dive, where the Northern Pacific Railway traverses the Colgate sand- stone for some distance. The diagnostic feature of this member is the upper 40 feet of white sandstone which is easily recognizable and present along the greater part of the anticline. Lance Formation. The areal development of the Lance formation is large, and owing to the badland topography into which this formation erodes, there are many good exposures. The formation outcrops in a broad belt along the northeast limb of the Cedar Creek anticline and in a somewhat narrower belt along the southwest limb. Large areas are present in the Yellow- stone Valley west of Fallon, the formation also covering extensive stretches along the Powder and Tongue Rivers and their tributaries. Because of the general absence of vegetative covering an excellent oppor- tunity is afforded for the study of the lithology of the beds in detail. Good outcrops occur in all parts of the exposed areas, so that horizontal variations may be traced, though not continuously. Since the transition from the true Lance beds to those of the over- lying Fort Union formation is very gradual, it is impossible from present knowledge to assign any exact thickness to the Lance formation as exposed in this region. Furthermore, the change from the dark colored strata of the Lance to the light colored rocks of the Fort Union is. characterized by a horizon of demarcation that varies greatly over the’ area, making any estimate of thickness based on lithologic characters | only approximate. Considering the present known range of Triceratops, the type vertebrate fossil, and the somber color of the beds, as determin- ing factors, an average thickness of 500 feet a be ascribed to these strata. The lithology of the Lance formation is well illustrated by the follow- ing descriptive section measured in Sec. 7, T. 3 N., R. 55 HE. The measure- ment is from the upper part of the formation and the beds are desenivett in descending order: ‘ Ft. In. Light colored, massive sandstone..........22.22---2--22:e:ceeeeeeseeeeeeeeeeeeeeete 10 # Base of Fort Union formation. Carbonaceous horizon characterized by seams of dirty lignite varying from 1 to 2 feet in thickness, separated by thin S@amsS Of SHA] C..cos. sevcessvecsevcpcesseesev ec cectt eaaai veces eeeeataneede ie 10 6 Sandstone, soft, grey.... . Shale, carbonaceous - : Aste SB OY ss ccocececnass dena hoacenenn Sorat erecta eaneeens mae 1 : Shale, carbonaceous -.... Sandstone, yellowish Seam of brown, ferruginous concretions........2..2.....:-:::ecceceeceeeeee eee Shale, dark grey...... 4 Sandstone, hard, light brown... Sandstone, soft, grey, cross-bedded. TING, SERA cok cia pan sania ca escce es ppemuaschceedceamstaacadcsadedpssousuanstecteneceuad Shale, carbonaceous Sandstone, grey, cross-bedded g Sandstone, massive, yelloW...........2-22.2-----::ccccccccecceeeeeseeeceeessceececnecceees Shale, brown Lignite 2 ROTEL, VCS accaechecevaalosncechocssceusssasusspsnun ssaecdssenueSnewdsanmsausauscssan ae: » bo: Or: 100 9 The Lance formation consists chiefly of dark colored shales, but inter- bedded with these is a considerable proportion of sandstone. As can be seen from the above section, the shale and sandstone strata are more or Fig. 7—Exposure of Lance formation on south limb of Cedar Creek anticline. less uniformly intermingled in the upper part of the formation, but as the lower limit is approached the sandy material becomes more rare and the bulk of the sediment is made up of shale and clay. Of the shaley rocks which predominate in the Lance beds, the most abundant variety is a dark-greyish mudstone which weathers into a ={9— coarse, loosely aggregated clay-like material that forms a sticky “gumbo” when wet. The general color of the shales varies from grey to nearly black, dependent upon the amount of carbonaceous material present. Beds of carbonaceous shale are common and these often contain inter- stratified seams of lignite. The shale beds of the upper part of the formation seem, on the whole, more arenaceous than those of the lower horizons. The sandstones of this formation vary considerably in lithologic char- acters. They are generally characterized by rather coarse, unconsol- idated material that is readily susceptible to weathering. Beds of more indurated sandstone occur, however, and when present they project as prominent ledges from the softer strata. The color of the arenaceous rocks varies from a dirty white to a dark grey, the latter predominating. Occasional beds of light yellow sandstone, resembling in color and litho- logic characters the arenaceous material of the overlying Fort Union formation, are present, especially in the middle and upper horizons. Intercalated. with the shales and sandstones are seams of coal, vary- ing from a few inches to several feet in thickness. The coal is generally a low grade lignite, although in places it exhibits semi-bituminous qual- ities. The beds are practically all lenticular and of local development. Where the beds are of any thickness they are usually interstratified with seams of bone and clay and often the lignite grades vertically into earbonaceous shale. Among the features characteristic of the Lance sediments is the fre- quent occurrence of brown, ferruginous concretions, generally limonitic, which occur promiscuously distributed through the strata or as thin local seams. (See Fig. 27). In addition to these limonitic concre- tions, which are ordinarily but a few inches in size, nodules composed of pyrite and sandstone are not uncommon. The latter rarely exceed two inches in diameter, are quite spheroidal in shape, and frequently indi- vidual specimens are joined together. A peculiar feature of both the Lance and the overlying Fort Union beds is the occurrence of log-like sandstone concretions which are as much as four feet in diameter and sometimes thirty-five feet in length. These concretions are generally oval-shaped in cross section, and while they sometimes attain the length mentioned above; they are more often found as disconnected blocks that have broken at right angles to their length. While the surfaces of these concretions are at times iron-stained, they generally have the color of the sand of which they are composed. Usually they occur alone, but in one case the writer found a mass of these concretions heaped indiscriminately together at the base of an unconsolidated sandstone bank. (See Fig. 8.) These masses of sand- stone differed from the ordinary shape in that they were nearly circular in cross section and had peculiar disc-like .bulges on their surfaces. At present, the origin of these peculiar log-like concretions is in doubt, but the theory has been advanced that they may have ‘been formed by the percolation of chalybeate waters along restricted channels at the time the sandstones in which they are found were laid down.* The abrupt change in the character of the rocks, both laterally and vertically, indicatés that the Lance formation is of fresh water origin. Sandstones merge into shales and shales into clay over short horizontal and vertical distances and cross bedded sandstones are common. These facts indicate sedimentation varying from lacustrian to fluviatile. Fur- thermore, the general varied chafacter of the coals and the frequent *Todd, J. E., Loglike concretions and fossil shores: Am. Geologist, Vol. 17, pp. 347-349, 1896. —20— Fig. 8—Log-like sandstone concretions of the Lance formation. occurrence of partings in such beds show that conditions favorable for their formation were often disturbed by marked changes in deposition at these points. Microscopic examination of the materials making up these rocks shows that they are primarily composed of quartz and kaolin with the frequent occurrence of flakes of muscovite and biotite. Undecomposed feldspar is not uncommon in the coarser sandstones. Ferruginous mate- rial is generally disseminated through the rocks Fossils are not abundant in the Lance formation as represented in the area here considered. A few plant remains are found which Knowlton has identified as of Tertiary age. Invertebrates are rare and the only forms found by the writer were a few Campeloma and Viviparus near the upper limit of the formation. Dinosaurian remains are not uncommon. They seem to range more or less through the entire vertical extent of these somber beds but are more abundant- in the lower horizons. The genus Triceratops is the most common and may be classed as the type vertebrate. Taken as a whole, the life of the Lance time was such as would be found in areas undergoing fresh water sedimentation. Of the diagnostic features of the Lance, the most useful in dis- tinguishing this formation are the somber color of the strata and their general erosion into badlands. The formation, however, does not always erode into badland areas, but gives rise in places to a topography similar to the Fort Union. This lack of relief is not due to the character of the rocks but rather to their location in areas where erosive agencies have less opportunity to work. Moreover, while badlands are typical of the Lance, they are not entirely con- fined to this formation, as the Lebo shale member of the Fort Union beds is characterized by the same topographic feature. In addition, the Lebo shales are somber colored, making it rather difficult to dis- tinguish this member from the Lance formation in the field. In those places where the Lebo shale is exposed, however, the Lance beds generally form the valley floors and show considerably less Stes relief than the former. This fact and the somewhat lighter color of the Lance formation in such areas, coupled with the absence of andesitic material which is characteristic of the Lebo shale member, are criteria which will aid as distinguishing features where the two formations happen to be present. Fort Union Formation The Fort Union formation has the greatest areal development of any stratigraphic unit here considered. The largest area constitutes all of the country lying between the Lance formation as exposed on the southwest limb of the Cedar Creek anticline and as exposed along the Powder River. The area second in extent includes all of the district north of the Lance strata which outcrop on the northeast limb of the above anticline.’ A small, isolated patch. covering a num- ber of square miles, lies some miles east of Miles City. Other areas occur north of Miles City and in the southwestern part of the district. This formation attains a maximum thickness of 1,190 feet in the northern part of the area, although it is generally somewhat thinner. The unusual thickness of these beds in the above mentioned region is due to the elevated points known as the Sheep Mountains and Blue Mountain which are made up of Fort Union rocks. As previously stated, the passage from the Fort Union to the under- lying Lance formation is of such a nature that there is difficulty in fixing a precise boundary. The boundary has been placed at the horizon where the light colored strata begin to preponde rate markedly over the dark colored beds of the Lance. This line of demarcation, however, does not everywhere lie at the same horizon, so that any division based on purely lithologic grounds is open to considerable error. Over the greater part of the area the horizon of transition is marked by a persistent bed of lignite which is arbit ravily considered as the delimiting line. Fig. 9—Badlands of the Lebo shale member of the Fort Union formation. The lenses of white sandstone are characteristic of this member. In the foreground at the bottom of the coulee can be seen a coal bed. —22— While in general the Fort Union rocks possess a greater uniformity in texture and are lighter in color than the underlying Lance forma- tion, the basal portion of the Fort Union formation in the western part of the area is characterized by a zone of shale and arenaceous strata that closely resembles the Lance both in color and erosive features. This zone, owing to certain distinctive lithologic features, is known as the Lebo shale member of the Fort Union formation and has already been briefly mentioned. R This member makes its first appearance near Terry and is ex- posed from here westward along the lower courses of the Powder and Tongue Rivers and in a belt on either side of the Yellowstone River. In the western extremity of the district the Lebo shale at- tains a maximum thickness of 340 feet. The Lebo shale member has been studied in detail, by G. Sherburne Rogers* of the United States Geological Survey, and a brief summary of his investigation is given in the following paragraph. Except for numerous and irregular layers of ferruginous concre- tions and a few beds of rather hard and massive sandstone, the rocks are soft and incoherent shales, prevailingly grey in color. The general absence of yellow and white strata permits an easy differentiation from over and underlying formations. The beds are very irregularly deposited changing rapidly both horizontally and vertically. Scattered throughout the member are numerous lenses of white sand, varying from 200 to 2,000 feet in length and from 10 to 60 feet in thickness. All thése lenses are cross-bedded and frequently lie at oblique angles to the surrounding strata. The rocks are in great part derived from andesite and are generally of a tuffaceous character. Plagioclase set in a chloritic groundmass is abundant and fragments of hornblende, augite and quartz are common. It is supposed that the material of which the Lebo shale member is comprised has been transported from the region west of the Crazy Mountains and that this member is an outfingering of the Livingston formation which has a similar lithologic composition. The cross- bedding, great irregularity of depositions, and angularity of the grains point towards a fluvial origin. The peculiar lenses of white sandy material are possibly the ancient stream courses. : Overlying the Lebo shales, where they are present, and the Lance formation over the rest of the area, is a mass of yellowish to grey sandstone, shale and clay which comprises the main bulk of the Fort ‘Union formation. The fresh water origin of these beds is indicated by the variability of the strata both vertically and horizontally, although, on the whole, they show considerably more uniformity of texture and are more consolidated than those of the underlying Lance formation. The following generalized section as given in Bulletin 471 of the United States Geological Survey will serve to show the common characteristics of the formation. The section begins at the top of Blue Mountain in the northern part of the area and ends at the base of the formation as exposed at the headworks of the reclamation pro- ject on the Yellowstone River. Ft. In. Sandstone and clay, brown to dark grey, the sandstone coarse grained and massive, in beds 10 to 15 feet thick.... 210 Lignite 2 4 Sandstone and clay, ash grey to yellow *Rogers, G. S., The Little Sheep Mountain Coal Field, Dawson, Custe: da Rosebud counties, Montana: Bull. U. S. Geol. Survey No. 531, pp. 168-172, —~23— Lignite 2 10 Sandstone and clay, grey to yellow in alternating beds ........ 12 ee MGTETILCE? acancescseesecctes cone . 4 5 Sandstone and clay, yellow to ash grey 00.....0..2220..22scceeceeeeeeeee 141 i Lignite .. 4 8 Sandstone, clay, and sand, yellow to ash grey ........0...22.....--- 93 a Lignite 9 1 Sandstone and clay, dark gray, cross-bedded ............2-200---- 127 a Lignite -t 8 Clay, Shale, and sandstone, grey to yellow ........2.20....-.2--::0--++- 89 a TIE A os sieaeaccseey epranbnccaes censure ee ats vaonnsreanves 4 Oo 2 Clay, and sandstone, grey to yellow, the sandstone in beds up: to" 5: feet Chick) 2x2t nese ee ae ia aete eerie se cont eesess 89 Lignite 4 Clay and shale, grey, in many places carbonaceous with impure lignite in their SeamMS -......0....22...22.1:2220ceeeeeeeeeeeee eee 97 E Lignite 4 5 Clay and sandstone, grey to yelloOW ..........2....22...-:0:-2:seeeeceeeeeeeee 44 os Lignite 4 6 “Clay and sandstone, Srey -...-.----c-c-ceecececceceeeceececeesecceeeeeeseeeeeeeees 39 2 Lignite aS 8 1,188 9 While for the most part the rocks of the Fort Union formation are soft, completely incoherent beds are rare. In general the clay, shale and sandstone are more or less uniformly distributed through Fig. 10—An isolated sandstone pinnacle of the Fort Union formation. The capping of resistant brown sandstone has protected the underlying softer rock from complete erosion by the winds and water. the formation and the predominance of any one of the above kinds of rock is usually local. Although not common to the formation, occasional beds of a rather hard, brown massive sandstone occur, and it is to this more resistant material that the curious isolated pinnacles —24— which are occasionally found in these areas may be attributed, the resistant capping of sandstone preventing the underlying softer roek from being completely eroded. The shales vary in color from light yellow to dark grey, but the prevailing color is ash grey: pink and purple tints occur though not common. They are generally arenaceous, especially the lighter shades, and grade from this sandy material into typical clay or mudstone. Seams of carbonaceous shale are occasionally interstratified, but are not nearly so common as in the underlying Lance formation and Lebo shale member. In places selenite crystals are associated with the shale rocks, and while at times well developed, they are not abundant. The sandstones, although generally soft, are more consolidated.than those of underlying formations and erode more uniformly. The pre- dominant color is some shade of yellow, but light grey and dark grey strata occur. The beds vary in thickness from thin seams to as much as 80 feet and are generally characterized by horizons of shaley material. Ccncretion: of hard. brown sandstone, limonite and, more rarely, pyrite are irregularly di-tributed through these strata. The log-like concretions feund in the Lance formation are also common in, Fig. 11—Log-like sandstone cencreticns of the Fort Union formation. '-the unconsolidated sands of the Fort Union. While these cylindrical ‘concretions resemble closely tho:e of the Lance sandstones, they differ 'in their greater length and in the general absence of the brown color. Traverse joints arc usually well developed. _The strata of the Fort Union are primarily made up of arkosic and quartzose material, with the feldspar generally kaolinized. Frequently associated with this material are flakes of muscovite. The more indurated beds show cross-bedding, ripple marks and mud cracks. These features and the occasional presence of conglomeritic material indicate fluvial deposition: Coal beds are common in this’ stratigraphic unit, the coal varying from brown lignite to sub-bituminous. The greater uniformity of the strata and the better quality and more'constant character of the coals —25— indicate that conditions of deposition were less varied than during the Lance time. ; The Fort Union beds contain an abundant fossil flora represented by nearly 400 species. The genera most typical to these rocks are Sequoia, Populus, Viburnum, Taxodium and Platanus. Among the fossil invertebrates Campeloma, Viviparus and Unio are the most typical, and especially the genus Campeloma. In some localities where erosion has been more active the writer found these last named shells literally covering the bare slopes. The vertebrate fossils, which are rare, are represented primarily by species of fishes and turtles. The distinctive characteristics of this formation are the general light color of the strata, and their uniform erosion into areas of rolling prairie. (See Fig. 25.) In places, however, these strata when Fig. 12—Badland erosion in the Fort Union formation, in which the ab- sence of somber colored strata is a distinctive feature. less homogenous erode into badland areas. Such areas. are usually of little extent and may be readily distinguishea from the bad- lands of the Lance formation and the Lebo shale member by the conspicuous absence of somber colored strata, the light colored sand- stone and shales forming a marked and distinctive topographic fea- ture. (Fig. 12.) Clinker-butte areas are also distinctive of this formation. Quaternary Deposits As mentioned in the introduction to this chapter, the unconsolidated deposits consist of gravel, sand and silt laid upon the eroded surfaces of the older rocks by stream action. They are accumulations of small area as compared with the stratified rocks and have not been so thoroughly studied nor so completely mapped, since they are of less economic importance. For purposes of description they are classed, on the basis of origin and mode of occurrence, as terrace gravels and valley alluvium. The terrace gravels occur upon benches or dissected remnants of —26—- former plateaus at various levels up to as much as 1,000 feet above the present valley bottoms. The character of these deposits of water- worn material and their elevated position show that they are the work of streams in past stages of topographic development. As they me- andered over their valley floors, these streams deposited their loads of sediment, and at various intervals of time uplifts or other causes brought about conditions favorable for rapid down-cutting, leaving remnants of old flood plains to mark the former valleys. : In the present state of knowledge it is impossible to classify these terrace deposits satisfactorily on the basis of age. It is probable that they date in great part from times since the early Pleistocene. This deduction is based on the fact that the extensive burning of the coal beds, which resulted in the formation of the present areas of clinker- buttes, took place, in great part, before the ancient plateau began to suffer extensive dissection from stream erosion or probably some time after the beginning of the Pleistocene. That the deposition of these terrace gravels was primarily subsequent to the period during which this burning took place is evidenced by the occasional presence of waterworn pebbles of clinker. The areal distribution of these gravel deposits is roughly determined by the present courses of the larger streams, but some of them occur a considerable number of miles from any prominent stream. The highest gravel found is in the northwestern part of the district near the south base of the Sheep Mountains. The great bulk of these de- posits, however, rest at elevations of 100 to 600 feet above the present valley bottoms. Occasionally they lie on distinct terraces but gener- ally, though probably once separated, they mantle the -slopes -con- tinuously through a vertical distance of several feet. The most ex- tensive deposits are found in the area north of the Cedar Creek anti- cline and along the lower course of the Powder River. The occasional distribution of these terrace gravels in one horizon over rather exten- sive areas indicates broadly meandering streams, a condition which would not be unnatural when the ancient plateau was in its earlier Fig. 18—Terrace gravels exposed on a country road near Miles City. 297 == stages of dissection. Local cases, seeming to indicate abnormally rapid deposition of these gravels, are found and may imply that during some previous time, probably during the retreat and the melting of the Pleistocene glaciers, the streams were much swollen and heavily laden with detritus. The terrace deposits consist mainly of layers of waterworn pebbles, usually oval shaped. The thickness of these layers varies from three to 30 feet and the interstitial filling is usually quartzose material in the form of fine gravel, sand or silt. In the study of these areas nearly all the types of rocks more resistant to disintegration were found, of which the following were most general in distribution: Sandstone (highly indurated), chalcedony, vein quartz, quartzite, quartz conglomerate, hornblendic granite, granite porphyry, felsite, basalt, gabbro, and gneiss. Aside from some material of local derivation, the pebbles bear signs of having been carried a long distance. Considering the abundance of metamorphic and igneous material, they very likely have come, in great part, from the mountainous areas to the west. The writer made one interesting observation which seems to have some bearing on the question of the origin of some of these gravels. On a small chalcedony pebble was found the fairly distinct impression of the dorsal valve of a brachiopod belonging to the genus Spirifera, probably Spirifera cameratus. This fossil is typical of the Carboniferous limestones of the Rocky Mountains. The nearest exposure of Carboniferous strata to the area here considered is in the Big Horn Mountains to the south- west, and it is possible that a great deal of the material associated Fig. 14—The Tongue River a few miles above Miles City, showing the characteristic alluvial deposits and broad flood plains of the larger streams. with the pebble containing the above described brachiopod impression may have come from these mountains. The alluvium that forms the present valley floors is for the most part of geologically recent age and has been deposited by means which may be seen in operation today. It is chiefly composed of a sandy —28— loam derived from the rocks over which the streams run. Owing to the general softness of the rocks, valleys develop fairly rapidly and the large streams have rather broad flood plains. Alluvial benches occur along many of the more important streams. A great number of the watercourses have alluvial deposits along their lower courses which seem disproportionate in size to the present streams. Such deposits indicate a former period of rapid erosion which was checked through lowering of the stream gradient or from other causes, and now, with more mature development and increased drainage area, the creeks are again cutting into this material and transporting it to the river. —29— : syo4 sKu2 09 S//OS B/t4t9O4¢G BY UaEO PUD. SUOILAUD i a TWHS SSR | A S144 A#M) uo snouegyissog yyyn| FOOL e om soeup BUIZOINPUN AJ $UED __S8/OYS Y4OP WO qQoug FINA Bas x ios Apa “SIG SUlffod|. , SSBES Gin _PeyzR IES ao sa Ajybnos pus S4yri7 Hin 240g | push“ gisbou Powis tse | SLOG >I 48 lhl. spaq apsuby ede oS OP LAB{S Ajpuenbeuy S/12$* oso; ‘suoles2uU02 eruoun;| OSS pone, S 33 SBIDAMSYL KSBUOLSPUOS Aoub PUB, UMOIF| —OGf Fs JINV 8 33 ue snbpe1o/o ‘spuD/pEg Isa0YS ApUDS pes0jO2 YsOQ = kK Ss WUNIALY/O AG -. ROS RTOHES BE ‘ -Ours ‘Spag ayiuby jo2o; ‘aUuas = Patercs suey 4cd/a0xa| pind ahoptia cee pon'sayo OP E-O Se SYIOS B/l4O4{S 'SPUOC/POEG |Kouos pus ‘cejpys pasoj02-4410G Db a Ss 5 |S syl0os Apues zi NOLLYHeO/ . 2 noin77. | 2 | “Spuo/pog Re < OLIYM, {0 SLEUO JOUOISVIID : OOF LYOJ Ayyonb UD JO SUIDES /202| ~OOF fz SHANG 4EHUY2 PUP SHIY PUY | choyeuuny ‘sepys Kptios Pub AWOL pa.ien02-sso.ib. buy Ol |sauayspuos pasopar4yby BAISSOL! . Se2pu4a pub S40q4 KE//O| Ee ay L/!S '(PUOS| OF-O ne y es S 70S GNY AHdve 0 of: LIFI M| NOLLOIS | -90d0L 40 YILIVYYHD SOF e YILIVUVHD SINMOHLLIY VNW 709 me OHO oe NOIDFA FHL #924006 = YOU [8/295 OA NOILIAS GAZI IVEAN FO LL PAF PY] —30— Structure Removed from all areas of mountain uplift, the strata of this region have been but little affected by deformative movements and lie nearly in their original position of deposition. Their general approach to hori- zontality, however, is modified in places by gentle undulations and by a marked fold that occurs in the eastern part of the area, and is known as the Cedar Creek or Glendive anticline. The Cedar Creek anticline is especially prominent since it occurs in a region where the beds lie nearly horizontal. The position of this uplift and the relations of the formations as exposed by it have already been discussed. The anticline is asymmetrical with the steeper dips Fig. 15—View on the south limb of the Cedar Creek anticline. The strata here, which belong to the Lance formation, have ‘a dip of 17 degrees. which are on the southwest limb, varying from 30 degrees on the northern end to about 10 degrees at the point where the anticline passes into North Dakota; showing a gradual flattening out of the fold as it is traced southeastward. The strata, however, resume their horizontal position very quickly on the southwest side of the anticline and pass into a very gentle syncline, the westward termination of which is roughly determined by the Powder River. On the northeast limb the beds dip at a much less inclined angle, generally under four degrees. The structural feature of the northeastern part of the area is governed by the anticline, the strata having a very gentle slope to the northeast, except in the northern most part where the dip is nearly due east. While the influence of the anticline extends to unknown depths, the rocks, due to their general softness, have yielded readily to the folding movement, so that over its entire extent there are no signs of faulting or fissuring and but few cases of local warping. Aside from the above described anticline the deformation of the rocks is confined to gentle undulations. Frequently the water courses occupy very gently synclines over a whole or a part of their. course, and some of the more elevated points owe their origin to slight anti- clinal or dome structures. The northwestern part of the region is a 31— continuation of the eastern limb of an anticline which has its axis some miles west of the area here considered, the beds dipping gently eastward. The predominance of incoherent beds precludes faulting on any noticeable scale, although a few small faults having displacements varying from one to fifteen feet were noted. Slumping is common, espe- cially in the badland areas (See Fig. 6) and along the larger stream courses, and in many places coal outcrops are concealed for consider- able distances through this cause. A few of the harder and more . compact sandstones show block jointing, but such phenomena are quite local and due rather to inherent qualities of the rock than to any deformative movements. Geologic History The geologic history of this area will be best understood by consider- ing the record of events during Upper Cretaceous and Lower Tertiary times of the entire geologic province of which this district is a part. The varying conditions under which the formations of this region were deposited may be shown by considering each in order of age. At the beginning of the Upper Cretaceous, North America was divided by an epicontinental sea which extended from the Gulf of Mexico to the Arctic Ocean, and covered what is now the site of the Great Plains and the Rocky Mountains. It was during this extensive submergence that the sediments of the Colorado series were deposited. The emergence of a large land area to form an extensive coastal plain extending from northwestern Canada to New Mexico characterized the beginning of the Montana epoch. Over this coastal plain the sediments, which were derived from upland regions to the west, were accumulating about as fast as the bottom sank, the area maintaining a halting attitude, now above the sea and now below it, so that sea, marsh and river plains alternated in sequence. It was on this coastal plain that the Eagle, Clagett,’ Judith River and Bearpaw formations were laid down. During this time marine conditions still ‘prevailed to the east of the coastal plain; and in the deeper waters which extended into eastern Montana the’ Pierre shales were deposited. The close of the Montana epoch was markéd by a general retreat of the epicon- tinental sea. In the Rocky Mountain area certain spots were effected by deformative movements and ‘the Big Horn Mountains, Black Alls, and the Front Range of the Rockies had their birth. — “ The Laramie epoch, the upper part. of which is represented’ by the Lance formation in Montana, witnessed the almost complete withdrawal of the sea to its present basins. If it is possible that the lower part of the Colgate sandstone is an extension of the Fox Hills formation, although the present evidence seems to point to a distinct fresh water origin, sucb a fact would indicate that the shoaling sea in which the Fox Hills sediments were deposited extended into eastern Montana. The Lance formation, however, and a great part, if not all, of the Colgate sandstone, are of fresh water origin and record the transition from the predominating marine conditions of the Montana epoch to the continental deposition of the Tertiary. The coal beds, cross-bedded sandstones, and great variability of the strata, both vertically and hori- zontally, indicate conditions of sedimentation in which marshes, river - plains and lakes alternated. At the beginning of the Fort’ Union stage the Rocky Mountains had begun their development as a system, and volcanic activity was a —32— notable feature. The erosive material from these mountainous areas was carried by streams to find lodgement in the low-lying regious to the east, some on river plains and some in lakes. The wind ‘also as- sisted in the transportation of erosive material. The presence of thick beds of lignite indicates extensive and long-continued marsh conditions. On the whole, the conditions of sedimentation during the Fort Union stage, as represented in Montana, were distinctly conti- nental with frequent variations between fluvial, lacustral and eolian deposition. In general, the life both faunal and floral, differed much from that of today, but by the end of the Fort Union stage had begun to assume modern aspects, especially in the case of the flora. The Dinosaurs and other peculiar animal forms which characterize the Mesozoic era extended well up into Lance, but the close of the Fort Union stage found them practically extinct and the ancestral forms of our modern mammals predominating. On the whole, the climate during the Upper Cretaceous and Lower Tertiary times was milder in the area here considered than it is today, a fact which is borne out by the presence of palms and other semi-tropical plants in the formations present. At no time has the area suffered any great deformation, and the general conformable sequence through great thickness of strata, together with the gradual transitions from one formation to another, indieate an accumulation uninterrupted by orogenic movement. The Cedar Creek anticline and the gentle undulations which occur in various parts of the regions had their origin after the deposition of the Fort Union strata, since these- are effected in the folding. The rocks of this region have undergone practically no alteration since their deposition. The development of the present topography has largely been de- termined through stream. erosion. and through variations in the hard- ness of the strata. The winds, however, have already played an active part in erosion, especially in the badland areas. The general uniformity of elevation of the higher points.of the district has already been noted and strongly suggests the former existence of a nearly level _ surface or plateau which has. been dissected by streams into the present relief features. The factors which have determined the location of the principal streams are not clear. .As previously stated, some of the streams occupy synclinal valleys over a portion of their courses, but it is usually only the younger water courses that are mastered by such slight inequalities, the general softness of: the rocks and the average low gradient having allowed much lateral cutting. The ter- races on the sides of the more important, valleys mark halts in the progress of down-cutting, during which the streams devoted them- selves to widening their valleys, and the steeper slopes connecting such terraces mark intermediate periods of more rapid cutting. CHAPTER III. COAL AND LIGNITE DEPOSITS General Character of Coals The district under discussion comprises one of the largest fuel bearing areas in the world. Although the coal is in the main a lignite, in many places of poor quality, it constitutes a vast fuel resource which will eventually become of great value. In general, the coal of this area is classed as lignite,.but much of it possesses the physical characteristics of subbituminous coal. The 93. true lignite is brown in color and shows in many cases the original grain of the wood from which it was derived. Upon weathering, how- ever, the woody appearance of the fresh lignite disappears, the color changes from brown to black, the original tough texture is lost and the material becomes brittle, assuming a shiny luster. The subbitumi- nous coal, or black lignite, is compact and brittle, has a vitreous luster and breaks usually with a conchoidal fracture. Some of it shows a Fig. 16—A lignite bed in the Lebo shale, showing block jointing of the coal. woody structure, but this is much less common than in the brown lignite. All of the coals have a brown streak. A pronounced block jointing is a characteristic feature of the coal beds where well exposed, and the joint planes are usually clear cut and regular. The beds are occasionally interstratified with thin seams of charcoal and in addition often contain inclusions of gypsum and sulphur. Their purity is further impaired by seams of bone and car- bonaceous shale. a feature which is common everywhere. The weath- ered outcrop or blossom of the coals is black. Upon exposure to the air the lignite undergoes rapid disintegration, finally breaking up into small particles. While some of the coals seem to resist weathering better than others, their commercial value, on the whole, is reduced because of their poor stoking quality. In burn- ing, the lignite decrepitates rapidly, produces but little smoke, and is characterized by a yellow flame and a strong bituminous odor. The composition of an average air dried sample of the lignite of this area is represented in the following analysis: = 34-— MO iS€Ure 2. -c0scsssceveecveccanwenntaseracts 21.96 Volatile MAGE secs 31.45 Fixed Carbon oo..ceccccccsescccccccssee: 87.91 PYOXiIMAtEC ...20-.cccccccencececssensecscceeesteees Fy ro gent eeccsesscsseteetevecteeveccenssens 5.41 GMA CG. nscececcecesvccchcccecaccccnttensneccers! Carbon NUCPOSOEN. s.s.ccccccesccsticccsicecssereseedenss ORY GON. weccecsceseses.ccscsedestacetecscciseee Heat Value .........--...--ccceccesccseecensees CRORES seat dition elton seve British thermal units Coal beds are distributed at varying intervals through the entire section of exposed Lance and Fort Union strata. Many of them are too thin to work, or, if of sufficient thickness, are too impure to be of commercial value. In general, the coals of the Lance and Fort Union formations differ in character. The former vary greatly in thickness from place to place, contain numerous partings and indicate varied conditions of de- position. The coals, which are generally black in color, exhibit but little of the woody texture characteristic of the true lignites. The coals of the Fort Union, on the other hand, are more uniform in thick- ness and contain fewer partings. The woody texture is generally visible and the coal is more lignitic than that of the Lance, showing that the conditions during its deposition were more stable and that the coal-forming material suffered only slight changes while accumu- lating. The lignites of the Lebo shale member closely resemble those of the Lance formation, both in physical and chemical properties and in occurence. ‘True lignites, however, as well as the subbituminous coals, are not confined entirely fo any one formation. While in general the coal beds are more or less lenticular, some of them persist over many miles of outcrop, a feature which is especially true of the Fort Union rocks. However, rapid horizontal variation in the character and thickness of the beds is common, a fact which makes their correlation on any large scale rather difficult. - Over the field generally the coal beds vary in thickness from a fraction of an inch to as much as 30 feet, but beds exceeding 10 feet in thickness are comparatively rare. Although seams of coal are distributed from top to bottom of the Lance and Fort Union formations, the more important beds are confined in a broad way to certain horizons. The frequent association of logs and blocks of silicified wood with the coals indicates that they were formed from logs and tree trunks rather than from finer accumulations of vegetable matter. The burning of the lignite along the outcrop in many places and the resulting effect of the fused masses of overlying shale and sandstone upon the topography has already been mentioned. The occurrence of masses of reddish rock, capping many buttes and lying on the slopes of the valleys, is a noticeable feature in many parts of the area. The fact that the greater portion of the burned beds occur in the Fort Union formation has been attributed to the greater liability of the coals of this stratigraphic unit to spontaneous combustion, but it is —35— Fig. 17—-A mass of shale and sandstone which has been fused into a slag- like mass by a burning coal bed. possible that it may be due in great part to the difference in thickness of cover over the coal beds of the two formations, since in those places where the Fort Union strata have been completely removed by erosion from the underlying Lance formation, the coal beds of the latter stratigraphic unit have been extensively burned where the cover is slight. The burning of these coals has been ascribed to various causes, of which the agency of man, lightning, and spontaneous combustion due perhaps to the oxidation of the included pyrite seem the most probable. When once ignited the coal beds burn back until slumping of the overlying strata prevents further access of air. Usually the underlying rocks are but little effected, the burning progressing far- therest back at the top of the bed. Where the cover is greater than 20 feet the beds will not ordinarily burn more than 75 feet back from the outcrop, but where the cover is under 12 feet the burning extends over a large area. The fused rock is generally reddish in color, but yellow, grey, green, black and purple varieties are not infrequent, the different shades being attributed to the varying degrees of oxidation of the iron. Mag- netite is frequently formed and in certain clinker-covered areas has a marked effect on the compass needle. The character of the clinker depends upon the degree of heat to which it has been subjected. The slightly baked material retains much of its original structure. It is usually light red in color. The material that has undergone higher degrees of heat has a smooth, compact texture and generally shows flowage lines, while in certain cases the heat has been intense enough to fuse the rock into a glassy, vesicular slag that is usually black in color. The more highly fused rocks, owing to their resemblance to voleanic lavas, have been erroneously called scoria. At the present time coal outcrops are burning in a number of places over the area. (See Fig. 18.) The major portion of the burning beds noted by the writer were located in young and deep-cut waterways. 56 In such localities masses of earth from the overlying rocks had slumped over the coal, and intermixed with the earth were large quan- tities of bright red ash. For some distance back from the outcrop the ground had yielded more or less and was full of large cracks through which thin wisps of smoke could be seen rising and the heat and odor of gases could be detected. Fig. 18—View showing ground underlain by a burning coal bed. Wisps of smoke are rising thru the cracks, but are not visible in the photograph. Distribution and Description of Coal Beds. The great bulk of the coal of this area occurs in the yellow beds of the Fort Union formation. The Lebo shale member and the Lance formation also contain numerous beds but the underlying exposed measures are barren. One small coal seam attaining a maximum thickness of two feet and having a horizontal extent of several hundred yards outcrops in the Colgate sandstone about four miles southwest of Glendive, but no other lignite is known in this formation. Coals of the Lance Formation The Lance is the lowest lignite-bearing formation exposed in this area. Seams of lignite are distributed throughout the formation but they are all lenticular and vary so greatly in thickness and quality that they are, as a whole, of but little value. In that portion of the district lying north of Glendive the exposures of the Lance are confined to a narrow band bordering the Yellowstone River. The few thin seams that occur in the formation at this point are in general inaccessible and of too poor a quality to merit further . description. The exposures of this formation along the northeast limb of the Cedar Creek anticline contain numerous beds of lignite, but the only locality at which these attain any noticeable thickness is at the heads of Cabin and Cedar Creeks. In this area two rather persistent coal-bearing 37— zones occur, one near the top of the formation and the other from 150 to 200 feet below the first zone. The coal of the upper zone is of better quality, one bed which varies from 12 to 42 inches in thickness outcropping over a distance of several miles. In both these lignite horizons the beds are more persistent than usual, but with the excep- tion of the above described seam, they rarely exced 30 inches in thick- ness. Several miles northeast of Baker three small beds, separated trom each other at distances of about 40 feet, are exposed in the badlands. At one point the middle bed attains a thickness of nearly four feet of good lignite but retains this development for only a short distance along its outcrop. The lower and upper bed do not exceed three feet in thickness, and contain partings of shale and bone. Aside from the above described coal beds the exposures of the Lance on the northeast limb of the anticline contain no lignite of any importance. Along the southwest limb of the anticline the Lance beds are exposed in a very narrow strip extending from the Yellowstone River to. the vicinity of Baker, at which point the outcrop widens considerably -owing to the decreasing dip of the strata. The exposures along the more pronounced portion of the escarpment contain numerous seams of lignite, but the only locality in which they have any appreciable value is an area lying just south of the point where Cabin Creek crosses the anticline. At this locality several beds outcrop persistently over a number of miles. With but one exception they are under three feet in thickness, although comparatively free of the partings of clay and bone that usually characterize the coals of this formation. The best developed seam shows five feet of fair lignite at one point of ex- posure, but this development is local as the outcrop decreases rapidly both in thickness and in quality when traced either way from the above point. A bed of coal which roughly marks the horizon of transition from the Lance to the Fort Union formation can be traced continuously from the vicinity of Baker to near the point where the exposures of the Lance along the southwest limb of the anticline leave the district. Throughout this area the coal lies near the upper limit of the dark- colored strata that lithologically are classed as Lance, and hence is described as belonging to that formation, although in outcrops to the north this same bed is found near the base of beds that are apparently of Fort Union age. On Pennel Creek, where that stream crosses the rather steeply dipping strata of the escarpment, this bed is exposed, showing seven feet of rather poor lignite. Southeast from this exposure the outcrop is marked by clinkers to a point two miles northwest of Baker, where it swings westward to follow the badland escarpment along the north side of Sandstone creek. The exposures along this escarpment are characterized by carbonaceous shale, in some places exceeding 10 feet in thickness, intercalated with thin seams of coal and bone. At the point where the bed dips under Sandstone Creek two miles west of Plevna, the coal has improved greatly in quality and 12 feet of fair lignite is exposed. After crossing Sandstone Creek the outerop has a general southeast trend and is marked for the most part by clinkers. In places, however, from three to five feet of poor lignite is exposed. At the head of the South Fork of Sandstone Creek, some four miles northeast of Willard, the bed shows a great increase in thickness, and at one exposure. where it is being mined by local ranchers, over 17 feet of fair lignite outcrops. South of here the bed is generally. concealed by grass cover, and with the exception of occasional clinker areas cannot be traced. _ Aside from the above described bed no important lignite seams occur —38— in this area, but among the numerous seams exposed at lower horizons several attain local thicknesses of four feet or more. The most per- sistent of these beds is exposed along the base of the badland escarp- ment on the north side of the stream, which it crosses at Rlevna, fol- lowing the south bank close to water level to a point just west of Baker where it disappears under cover. The only place at which this bed is of any importance is an exposure on the south side of Sandstone Creek, one mile east of Plevna, where it contains four feet of fair lignite. Elsewhere the coal is burned or very impure. South of Baker lies an extensive area of clinker-capped buttes but, aside from a few thin seams, no coal of value occurs. In the region south of Brackett Butte several beds are found, one of which attains a local thickness of four feet of good lignite. 7 The Lance formation is exposed along the borders of O’Fallon Creek. from the vicinity of Ismay southward for a distance of some 30 miles. A bed of lignite outcrops extensively near the top of the formation and is probably the same one that is exposed to the east of this area and which serves roughly as a marker between the Lance and Fort Union rocks. Immediately south of Ismay the bed is characterized by thin seams of lignite ranging through a vertical distance of 35 feet. Most of these beds do not exceed a foot in thickness, but about 12 feet from the base of the section 54 inches of coal with two partings is exposed. As this carbonaceous horizon is traced southward, the upper part develops into a bed of fair grade lignite averaging 54 inches in thickness. In the vicinity of Spring and’ Pine Creeks this bed decreases in quality, an average section over this area showing from two to five feet of lignite intercalated with seams of bone and shale. At the points where Miles City and Lame Jones Creeks join O’Fallon Creek, this bed improves somewhat in quality, showing two feet of pure lignite at certain exposures. Over other portions of this vicinity the outcrop of the bed is marked by carbonaceous shale con- taining thin lenses of lignite and by local areas of brick-colored shale and clay where the coal has been burned. Southward from the above area the outcrop is characterized in many places by baked and red- dened clay and shale. Unburned exposures vary from three feet to: seven feet in thickness, and while generally the coal is very dirty and contains partings, local exposures show from three to five feet of fair lignite. Other beds outcrop in lower horizons of the Lance as exposed along O’Fallon Creek, but only attain developments of importance in the area lying between the points where Miles City and Ash Creeks join the above stream. The lowest bed lies practically at the water level of O’Fallon Creek and shows a maximum thickness of 46 inches of good lignite near the mouth of Ash Creek, although in areas to the north and south of this exposure the quality of the coal is depre- ciated by partings. Between the highest bed of the Lance and the above described coal, two beds outcrop on the east side of O’Fallon Creek just north of the point where Lame Jones Creek joins the above stream. The upper bed which averages over four feet in thickness contains good lignite, but its quality is somewhat marred by partings. The lower bed is of better quality and attains a maximum thickness of five feet of fair lignite with a 12-inch parting near the middle of the section. This development, however, is quite local. Aside from the above described areas the Lance formation in the region considered in this discussion contains no coal deserving of description. As previously stated, the exposures of this formation along the Powder and Tongue Rivers and their tributaries, as well as the —39— Yellowstone, generally occupy small areas along the bottom of the streams and are not extensive. The exposures along these water- courses contain no important lignite beds and while outcrops are Fig. 19—A butte in the Lance badlands. The dark seams encircling the pase are thin coal beds. These Le retarded the erosion of the overlying sandstone. numerous the coal is usually of poor quality and the beds too thin to be of any value. But few of the beds exceed three feet in thickness, and rarely are beds of this thickness completely free from partings. Coals of the Lebo Shale Member of the Fort Union Formation The Lebo shale is exposed on the borders of the lower courses of the Powder and Tongue Rivers and along the Yellowstone River to a point five miles east of Terry. The most prominent and persistent lignite bed in this member lies at the base and marks the transition to the underlying Lance forma- tion. Although this bed has not been definitely correlated with the coal which lies at the horizon of demarcation between the Lance and Fort Union strata in the areas previously described, it occupies the same stratigraphic position. While in several places this bed exceeds 15 feet in thickness, its value is greatly lessened by numerous partings, a feature which seems to be common over the greater part of its outcrop. This coal is often characterized by several beds lying within a range of 50 feet vertically. The exposures of this bed along the valley of the Tongue River show no development of any great commercial value. Extensive ex- posures occur, especially in the badlands southeast of Miles City, but the coal contains numerous thin partings which render it unfit for mining. The best development of this coal occurs along the water- courses of Mill, Squaw and Pumpkin Creeks, where occasional ex- posures show from three to five feet of workable lignite. Along the south side of the Yellowstone valley from Miles City east- ward to Cottonwood Creek this bed outcrops more or less extensively, —40— but the coal is of practically no value. Exposures on Cottonwood Creek show a thickness of 10 feet, the coal containing many thin partings. From Cottonwood Creek to the valley of the Powder River the bed retains the above thickness, but the numerous partings persist and the lignite is quite dirty. The bed has undergone considerable burning over this area of exposure. In the vicinity of Zero and Blatchford on the Northern Pacific Railway the bed contains seams of workable coal varying from two to four feet in thickness, and is being mined for local use. Among the best exposures are those occuring along the bluffs. and badlands on the west side of the Powder River valley. North of: Strevell Creek at the base of the badland hills the coal outcrops per- sistently for a number of miles, in places attaining a thickness of 20 feet with intercalated seams of coal, occasionally eight feet in thickness, that are free of partings. The quantity and quality of the coal de- creases north and south of this locality and over the remaining area of exposure on the west side of the river the lignite is of little value. Fig. 20—Badlands of the Lebo shale member of the Fort Union formation. The flat-topped ridge in the foreground is formed from a weathered coal bed which has disintegrated into a carbonaceous shale On the east side of the Powder River the bed outcrops in many places along the lower courses of Sheep, Horse and Locate Creeks, and below the gravel of a terrace that lies a short distance above the river. In several places over this area as much as five feet of good lignite is exposed, but the general absence of complete sections does not allow a detailed study of the bed. The average quality of the exposed coal, however, is poor where Coal Creek joins the river. Exposures along the above creek and for several miles south show a decrease in partings and an improvement in the character of the bed, and in places.seven feet of clean lignite is exposed. Northward from Coal Creek to the point where the outcrop leaves the Powder River valley and follows the south bank of the Yellowstone eastward the bed has an average thickness of five feet with from one foot six inches to four feet of clean coal exposed. At the point where the =A outcrop enters the Yellowstone valley four miles west of Terry the bed has one of its best developments, one exposure showing eight feet four inches of clean coal. From this point to where the bed dips under the Yellowstone River six miles east of Terry the exposures of clean coal vary from two to five feet in thickness. On the north side of the Yellowstone River this coal is extensively exposed and attains its maximum thickness, but the bed contains so many partings that it has little commercial value, aside from a few local developments. From the point where the bed dips under the Yellowstone River westward to Custer Creek the outcrop varies in thickness from five to twelve feet, but the greater part of the bed is com- posed of bone and dirty coal, although lenses of good lignite are not infrequently interstratified, some of these having a thickness of three feet. The maximum thickness of 35 feet occurs on Custer Creek, and while a considerable portion of the bed is composed of dirty voal, bone and carbonaceous shale, one exposure on the west side of the creek shows 10 feet of good lignite free of partings. Other exposures in this vicinity show seams of good coal varying from three to five feet in thickness occuring at various horizons in the bed. West of this area along Harris, Sand, Muster,.and Sunday Creeks the bed varies from five to fifteen feet in thickness, but the coal is in great part impure, the seams of good lignite rarely exceeding a thickness of four feet. These exposures of this bed along the badlands on the north side of the Yellowstone River show extensive burning, but generally the fusion has not been great, the outcrop being marked by baked and reddened clay and shale. While other seams of lignite that attain local developments of im- portance occur in the Lebo shale member, none of them outcrop as persistently nor have the thickness of the above described bed. Of the other seams outcropping at various horizons in this member no de- tailed description will be given since they have but little present value. Several of the beds outcropping in the vicinity of Signal Butte southeast of Miles City have been mined at various times, but the numerous partings make mining difficult and unprofitable. Aside from the bed previously described, no coal deserving mention occurs in the Lebo shale as exposed south of the Yellowstone River. Generally the seams are under three feet in thickness and quite dirty, although occasional local developments are found where the lignite is of some value and is strip-mined by nearby ranchers and homesteaders. North of the Yellowstone River the Lebo shale contains several seams of lignite outcropping at various distances above the main bed that marks the base of the formation, but all of these seams are quite impure, and while some exceed five feet in ‘thickness, the lenses of good coal are too thin to be of value. Coals of the Fort Union Formation The Fort Union is preeminently the coal-bearing formation of this region. Although the abundance of carbonaceous horizons is perhaps no greater than in the underlying formations that contain lignite, the general persistence of the beds over large areas and their average good quality are factors which make the coals of this stratigraphic unit of considerable economic importance. As the Fort Union lignite beds of this region have been studied in detail by the United States Geological Survey and various portions of the field here considered are discussed at length in Bulletins 316, 471, and 531, only a general description of the beds over the field as a whole will be given. For —49— convenience in describing the coals of this formation the field is divided into four districts as follows: The area lying north of the Cedar Creek anticline; the area between the anticline and the Powder River; the area between the Powder and Tongue Rivers, and the area lying north of Miles City and the Yellowstone River. Area North of Cedar Creek Anticline The proportion of lignite present in the Fort Union formation as -exposed north of the anticline is greatly in excess of that of the re- mainder of the region where this stratigraphic unit outcrops. The general good quality of the beds, as well as their marked and persistent thickness, make this portion of the district of special importance as a lignite-bearing region. Among the lignite beds exposed there are ten that are of economic importance over considerable areas; and two of these have value over many miles of exposure. Of these beds, all of which are found in the lower. part of the formation, the principal one lies about 500 feet above the base and deserves a more detailed description since it serves as an aid in correlating the lignite beds of surrounding fields. The first exposures of this bed in the northern part of the area are found along the lower course of Alkali Creek, and show an average thickness of 11 feet of clean lignite. Southward from this vicinity along the borders of the Yellowstone River and the lower courses of Shadwell Creek, and Harpster and Elm Coulees, this bed shows from four feet two inches to 13 feet of lignite, free of partings. Over this area the bed has undergone considerable burning. ‘The numerous outcrops along Smith Creek and its tributaries give a maximum thickness of five feet. _The outcrops that underlie the intricately dis- sected plateau surrounding Devils Canyon are generally burned, but occasional exposures of clean coal show sections varying from seven to thirteen feet in thickness ; the bed, however, as exposed around the rim of this canyon, is generally inaccessible. While the outcrop of this bed along Dry Creek is generally concealed by a grass cover, the numerous clinker rims make it easily traceable. The few exposures of coal over this area show a thickness of from. six to twelve feet. The bed in the Cottonwood Creek area has been extensively burned. The few outcrops show a general decrease in thickness, the average section in this area giving six feet eight inches of lignite, with a minimum of two feet ten inches and a maximum of ten feet eight inches. The exposures of this bed along the head of Box Elder Creek show extensive burning, the few incomplete exposures showing a general thickness exceeding three feet. What is probably the same bed is exposd along Beaver Creek two miles north of Wibaux, where it lies a short distance above water level. The thickness here averages nine feet and the coul is mined for local use. Among the other beds exposed over the area lying north of the anticline the next in importance, designated bed H in the reports of the United States Geological Survey, lies at an average distance of about 100 feet stratigraphically above the coal just described. Al- though the outcrops are not as extensive and are concealed over large areas, good exposures occur along the various watercourses, especially ‘the upper part of Smith Creek and its tributaries. In the vicinity of Benny Pierre Creek in the northernmost part of the area the outcrop of this bed is principally marked by slag, but occasional exposures of clean coal show sections varying from two to ten feet in thickness. In the vicinity of Shadwell Creek the few unburned exposures give —43— an average thickness of four feet. The most extensive outcrops are found along C. S. Creek and the upper part of Smith Creek where the bed can be traced continuously for a considerable number of miles. The thickness over this area varies from two to eight feet of clean lignite. Northwest of Blue Mountain the bed is characterized by isolated outcrops of no special importance. The unburned exposures that are found at the heads of Castle Creek and the East Fork of Cottonwood Creek vary from two to ten feet in thickness. South- ward from this region the bed was not definitely correlated, as it is generally concealed. PLATE IV. Norfn of Yellowstone River Between Powder Between Antichne North of Cedar Creek and Tongue Rivers. and Powder River. Anticline. Beg 4 Dark Shale Beg G Beu F Bed £ lea \ fort Union Formation Bed D Bed C's bed 3 : YL - VLA 7; Lance ames PRON Formation Colgate COLUMNAR SECTIONS Sandstone OF VARIOUS PARTS OF REGION SHOWING STRATIGRAPHIC RELATIONS OF COAL BEDS | Pierre ===} | Shales Note= Aside from bed A, which 1s doubtfully correlated, remaining beds have no Hnown relation, although lettered similarly —44— Aside from the two beds described, important developments of the remaining lignite seams are quite localized, and the outcrops are continuous only over small areas. The comparative thickness and stratigraphic relations of these beds are shown in the columnar section. (See Plate IV.) Bed I, which lies approximately 150 feet ver- tically above bed H, in exposures along portions of Benny Pierre Creek on the south side of Shadwell Creek in the vicinity of Bull Butte, varies from two to six feet in thickness. Other outcrops are found on the north side of Smith Creek in the vicinity of Miller, along the head of C. S. Creek, and around the base of Blue Mountain. Over this area the bed is extensively burned, but the outcrops are con- tinuous over considerable areas, especially in the vicinity of Blue Mountain. The thickness over the above regions varies from six inches to six feet with shale partings frequently present. Of. the remaining beds lying above and below the horizons of those described, the most important developments are found on Cottonwood Creek and in the vicinity of Blue Mountain. The beds in the formation above bed H underlie only the small upland areas of the field, but those lying below bed G probably underlie a great part of the field, although their outcrops are not generally persistent. The thickness of the above seams varies from a few inches to seven feet, but the quality of the beds where they attain their greater size is frequently depreciated by partings. Of the lignite beds found in the Fort Union ‘outcrops west of Wibaux, the only bed of importance is the one lying at the base of the formation. This bed, which has already been described, attains its most marked development in this area three miles northeast of Allard, on the Northern Pacific Railway, where it attains a thickness exceeding 35 feet, of which 24 feet is clean coal. Other beds overlying the above coal attain occasional thicknesses of ten feet, but these de- velopments are quite localized. Area Between Cedar Creek Anticline and Powder River The area of the Fort Union exposures lying etween the anticline and the Powder River, while containing numerous seams of lignite, is not of as great importance as that region north of the anticline, since the beds are generally thinner, contain more partings, and have been burned over large areas. The outcrops are not as a rule persistent and their isolation renders correlation of the beds rather difficult. The greater portion of the beds in this area do not exceed five feet in thickness and many of them are under three feet. The broad areas of rolling prairie which characterize the Fort Union rocks serve to extensively conceal the beds and it is quite probable that the lignite of this region is of considerably greater importance than is evidenced by the exposed sections. Among the more persistent beds of this district, the one showing the greatest thickness of clean coal is exposed on the escarpment of the southwest limb of the Cedar Creek anticline at the head of a tribu- tary of Pennel Creek. This bed, which lies about 200 feet above the base of the formation, shows a thickness of 12 feet at the above ex- posure. The other few exposures along the escarpment give an average thickness of seven feet. Generally, however, the outcrop is burned, but the great fusion of the overlying rocks and the constant thickness of the clinker rim show that coal of good quality underlies the area. A bed which can be traced continuously from the Yellowstone River to the vicinity of Baker outcrops along the southwest escarpment of the anticline about 12 feet above the lignite marking the base of —45— Fig. 21.—View of a lignite bed of the Fort Union formation. The coal seam is 6 feet in thickness and is strip-mined by nearby ranchers and homesteaders. the Fort Union formation. Although extensively burned, frequent ex- posures show a constant thickness of from seven. to eight feet, with the lignite generally quite free of partings. Aside from the above beds, the lignite seams of this district do not attain any marked development, and where they do, these are quite local, the beds quickly thinning or depreciating in quality when traced either way from the point where exposures show the best values. In the vicinity of Knowlton a seam showing six feet of lignite containing two small bone partings, outcrops at the head of one of the numerous deep ravines found here, but the exposure is quite local, the bed having undergone extensive burning over the remainder of the area. ; Area Between Powder and Tongue Rivers The exposures of the Fort Union strata in this district are not as extensive as in the areas already considered, and are confined to the higher portion of the area lying to the east of Miles City, and to the southwestern part of the district. The former area, which covers only a few square miles, is completely encircled by rocks of underlying f>rmation. Beds of value in the area east of Miles City are found at two hori- zons, the lower of which is 160 feet and the upper 300 feet above the base of the Lebo shale member. The greater part of the outcrops of the coals in these horizons is indicated by clinkers. The most important lignite which is found in the lower horizon is characterized by a group of beds that range through a vertical distance of 30 feet. The extensively burned bed occuring in the vicinity of Knowlton is probably correlative with this lignite. The beds of this horizon are three in number, and while the different exposures show some variation in thickness. the general good quality of the lignite is fairly constant. The following is an average section of these coals: S16 Fig. 22—The dark strata capping the butte in the background is composed of the fused material from a burned coal bed. Section in Sec. 3, T. 7 N., R. 50 E.- Ft. In. TT TTC Cs asc oe wee pteeesss cance seen evens Sie os eee ape ev 7 PORE ERIN YE pas ccs oe scp ps concn eee nse ae op anmcattense ian ae co ee ctiasaea peo ats oe 13 Lignite 3 6 Shale, sandy oe Tal Ste cots oe eee a a oo ee ae waa pce A 6+ The bed of the upper horizon has been so completely burned as to be of little economic value. The only unburned exposure found showed over 12 feet of fair lignite. The fact that this coal only covers a few acres, and is generally inaccessible, makes it of but little im- portance. In addition to the above described beds, other thin seams occur that are rarely of workable thickness or quality. Aside from a seam, lying 70 feet above the top of the Lebo Shales, which has a local development of four feet of good lignite in an exposure near the Hill Ranch, these beds need no description. The second area of Fort Union rocks lies-in the southern part of the district and contains but few exposures of lignite, since the gently undulating prairie forms a topographic feature unfavorable to out- crops. The numerous clinker buttes and mesas that are found in this area, especially in the vicinity of Kingsley, show that at some previous time the region contained coal beds of considerable thickness and extent. At the present time, however, unburned outcrops are practically absent. As the clinker masses generally fringe the higher parts of the buttes and mesas it is probable that but little unburned coal remains. The only exception to this is found in a prominent burned bed that caps the plateau-like divide between Pumpkin and Mizpah Creeks in the vicinity of Kingsley. This bed, which is probably correlative with one of the important lignite seams that outcrops around the base of Elk Ridge some miles to the west, contains several expo- AT sures of clean coal. The outcrops show two benches of lignite separated by a shale and sandstone parting varying from five to thirty feet in thickness. The upper bench of lignite shows from six to fifteen feet of fair lignite at its best development. The coal is mined for local use. While occasional seams are found at lower horizons in this district, they rarely attain workable size or value. Area North of Yellowstone River The Fort Union strata that lie north of the Yellowstone within the area considered, contains numerous seams of lignite that are generally of little commercial value, as the beds, although in many places of sufficient thickness, are frequently interstratified with lenses of bone and dirty coal. The principal outcrops of lignite are found at the heads of Custer and Deadman Creeks and in the extreme northwestern part of the area on the Missouri side of the Yellowstone-Missouri River divide. At the head of Custer Creek the most prominent bed is extensively burned, but exposures of unburned coal occur in some of the deeper cut coulees. Exposures in Sec.:1, Tp. 12 N., R. 48 E. show two benches of lignite separated by a six-foot parting of sandy shale, each bench averaging six feet in thickness. The lignite, for the most part, is of good quality, but thin lenses of bone and impure coal are present in the upper bed. In the vicinity of the above outcrop the bed ‘is burning at the present time in three places. Exposures of the same bed at the head of Deadman Creek give an average thickness of three feet six inches, but rarely does a section show more than two feet of pure lignite. A bed lying at about the same stratigraphic horizon outcrops in several deep waterways on the Missouri side of the divide in the extreme northwestern part of the district. Excepting the exposures in the coulees, this bed is burned or under a grass cover. The thick- ness varies from six to eight feet. Although several small partings occur, the coal is of workable value. With the exception of the above described exposures, no coal of value is found in this area. The other seams that are found rarely attain a thickness greater than four feet and are for the most part made up of carbonaceous shale, bone and impure lignite. Quantity of Coal In addition to the coals described in the Lance and Fort Union forma- tions, there are many other beds that outcrop in this field, but only a relatively small proportion of the coal exposed is of workable thickness. Generally the more valuable coal lands are confined to small areas, and only thorough prospecting will determine their exact boundaries. Since the beds, with few exceptions, are only known along their line of outcrop, nothing is known of their persistence back of this line, so that any estimate of the tonnage present can only be approximate and the percentage of error high. The average specific gravity of lignite is 1.3, therefore a cubic foot of the material weighs 81.25 pounds. A square mile of lignite one foot thick contains 27,878,400 cubic feet and with the specific gravity given above weighs 1,132,560 short tons. The total lignite in any one bed can be estimated by multiplying this tonnage by the area in square miles underlain by the bed, times its average thickness in feet. Under favorable conditions, about two-thirds the coal in a bed can be mined. The following estimate is based on the assumption that all the coal —48— may be recovered and takes into account only beds 36 inches or more in thickness: Estimated Tonnage of Coals in Area Described in This Paper Area— Tonnage, 36-in. Basts. Area north of Cedar Creek anticline ....W......2..2-21:.1:c---- 48,500,800,000 Area. between anticline and Powder River . . 25,080,000,000 Area between Powder and Tongue Rivers . 830,000,000 Area north of Yellowstone River ..........-..---:--:-----eeeeeee-+ z 1,189,832,820 Grand total 75,600,632,820 The figures given above are conservative and, owing to the general extreme variabilities of the coal beds, the limit of error in this esti- mate is high. Considering the large areas over which beds of workable size are concealed, coupled with the probable occurrence of coal in areas where there are no natural exposures, it is reasonable to as- sume that the total quantity of workable coal, through future prospect- ing, will be found to greatly exceed the above estimate. Mining Operations and Development Practically all of the mining in this area is carried on to supply local demands only. The general inferior quality of the coal as com- pared with that of other fields in the state is not favorable to any early development of the coal resources of this region on a large scale. The greater part of the present mining is carried on by home- steaders and ranchers who obtain fuel for domestic use by stripping or undermining the lignite where exposed in cut banks and coulees. There are, however, a few mines, scattered over various parts of the field close to transportation facilities, that have been developed on a small scale. In the S. E.% Sec. 7, T. 22 N., R. 60 E., near the mouth of Benny Pierre Creek, is a small entry from which a little lignite was being mined at the time of the writer’s visit. The fuel from the mine, which is located on a six-foot bed of lignite somewhat depreciated by bony inclusions, is used by the farmers and ranchers of the vicinity. A bed outcropping south of Terry has been mined in a desultory manner to supply local demand but at the present time the open cuts are abandoned. The lignites of this area have been most extensively developed in the vicinity of Miles City. The Kircher mine, six miles northeast of Miles City, is the largest. The coal bed, which here lies some 60 feet below the surface, is reached by a slope and the mining is done by the room and pillar method. The mine cars are pushed by hand to the foot of the slope and drawn up by a gasoline engine hoist. At the time of the writer’s visit three men were employed, and the product of the mine was hauled to Miles City. The average price of this coal is $2.50 per ton. -There are numerous old openings on the coal beds exposed in the vicinity of Signal Butte, but all of them are at present apparently abandoned. Aside from the above described mines, the numerous small entries located in various parts of the field are of little importance. Future Development and Utilization of the Lignites At the present time there are several factors which serve to retard the development of the enormous lignite deposits of this area. The —49— general absence of timber is a great hindrance to underground develop- ment, especially since the soft shale and sandstone rocks of the region require an unusual amount of timber in the mines. In addition, the high moisture content of these coals and the resulting rapid dis- integration upon exposure to the air is one of the greatest difficulties to be overcome in their commercial handling and utilization. The above factors and the tendency of the coal to slack when burned, even after the removal of the moisture, have thus far confined the use of the lignite to small regions near the deposits. The future development of these deposits on any large scale lies in the conversion of the lignites into a fuel free from moisture, which would greatly increase its efficiency and facilitate its shipping. Generally the more important beds are readily accessible, except over small portions of their outcrops, so that the problem of transportation does not present any great difficulties. A long series of investigations has proved that the lignites of the West can be successfully and economically utilized in the manufacture of fuel briquets, thus saving a large quantity of coal and slack that would ordinarily be wasted. E. J. Babcock, director of the Mining Experimental Station of North Dakota, has shown that a plant which would combine the manufacture of producer-gas and the briquetting of the residue lignite can be made a commercial success.. The process consists of “first, the removal of moisture from the coal; second, the expulsion and saving of the volatile gas; and, third, the binding to- gether of the concentrated residue into a strong, durable, and satis- factory briquetted product.”* The producer-gas, which is used in internal-combustion engines has been proved to have a large commercial utilization for power purposes, as well as for heating and lighting. Briquets made from the con- centrated residue produce an excellent fuel which nearly approaches the efficiency of anthracite. One ton of air-dried lignite will produce from a half to two-thirds of a ton of briquets in addition to 8,000 or 10,000 cubic feet of gas. Briquets have many advantages over raw lignite. They can be shipped for considerable distances, the heating value is double that of ordinary lignite, and they do not disintegrate on standing or burning. Moreover, they are superior to ordinary coal since they leave no unburned carbon on the grates or in the ash, and their uniformity in size makes them more convenient to handle. The general scarcity of wood in this area necessitates the use of lignite as a domestic fuel. As the lignite in coming from the mines generally contains over 25 per cent. of moisture, there is considerable loss of heating value and much inconvenience if the lignite is burned in the damp state, especially in large lumps. The lignite to be used in stoves should be broken at the mine to uniform lumps about three inches in diameter and dried. Coal of this size is more convenient to handle and, on the whole, more acceptable for domestic use, the additional heat value obtained thereby resulting in a great saving to the consumer. Considering the immensity of the lignite deposits of this area. their present limited development, and the comparatively simple means by which these deposits can be made of commercial value, they are to be considered as of great economic importance. At the present rapid rate of settlement it will not be a great while until there will be a general demand for some kind of cheap power, owing to the resulting growth *Babcock, E. J., Economic Methods of Utilizing Western Lignites. Bulletin, U. S. Bureau of Mines No. 89. —50— of industries. The successful utilization of these low-grade coals in the manufacture of producer-gas, the superiority of this source of power over steam, and the general absence of water power over the area, are factors which, in addition to their use as fuel, will make lignite deposits of this area of great future value. CHAPTER IV. OTHER ECONOMIC FEATURES Oil and Gas Owing to the nature of the writer’s work while in the field and lack of detailed information as to present development, only a general statement can be made at this time regarding the possibilities for oil and gas in this area. From the discussion of the stratigraphic and structural geology in Chapter II. it can be seen that the part of the region most favorable for the storage of oil is the area bordering the axis of the Cedar Creek anticline. While the structure of this anticline is favorable for the accumulation of oil, this does not necessarily signify that oil will be found, since the fundamental requisite is the presence of an oil- bearing sand, a fact which as yet, to the writer’s knowledge, has not been proved. FiourE 23 — Diogrom of Gerihve anticline,Momt., looking est U.S.GS However, there is, so far, nothing to indicate that oil is absent from ‘this anticline. Due to the general unconsolidation of the rocks and the resulting readiness with which they have yielded to the folding move- ment by which the present anticline was formed, no fissuring or fault- ing has taken place, so far as the writer is aware, and the seepage to the surface of any oil that might be stored in underlying rocks ‘is thus prevented by the impervious strata forming the upper part of the Pierre formation. Owing to this fact, the ultimate presence or absence of oil in the anticline can only be determined through drilling. While a careful correlation of the formations present in the anticline with those of adjacent regions, that have proved to be oil-bearing, would be of material assistance and greatly facilitate drilling opera- tions, there are several factors to be considered. Exposures of the Pierre shale are confined to the anticline, and the underlying forma- tions are concealed over the entire area because of the general hori- zontality of the strata and the absence of pronounced structure. As a result the nearest exposures of oil-bearing formations that occupy the same geologic horizon as the Pierre shales and the strata immedi- ately underlying them, occur many miles from the anticline. This fact, added to the general horizontal variation of the Upper Cretaceous strata in lithologic characters, renders regional correlation somewhat difficult so that the ultimate correlation must rest primarily on paleon- tologic evidence. The development of this field is still in its infancy. Although desul- tory drilling has been carried on for several years, no extensive opera- = tions have as yet begun. At-the present writing the greater part of the more favorably situated land along the axis of the anticline is held by leases, and several companies have begun or are contemplating drilling. The Eastern Montana Oil & Gas Company has drilled four wells near the mouth of Cedar Creek, all of which are producing gas and average about 400 pounds pressure. This company, according to the latest reports, is beginning the construction of a pipe line. The Montana Petroleum Company has started drilling at a point just east of Baker. The Treasure State Oil & Gas Company, with an authorized’ capitalization of $1,500,000, have also begun drilling on their numerous leases. The Kingsmont Oil Company has several leases in the vicinity of Kingsmont, but as yet, so far as known, have not started opera- tions. In addition to the producing gas wells at the mouth of Cedar. Creek, a small flow of gas has been struck in the vicinity of Baker. Information relative to the logs of the various producing wells and the quality of the gas is not available at the.present writing. On the whole, the possibilities of oil along the Cedar Creek anticline are promising, and although the presence of natural gas is not neces- sarily an indication of oil, their common association, together with the ideal structural conditions, make this area worthy of investigation. The finding of oil in paying quantities in this field or in any of the other fields that are at present being developed in the state and the resulting impetus that will be given to commercial development by the addition of this natural resource to those already present makes the question of oil in Montana of much future interest. Artesian and Ground Water As is true of all semi-arid regions, the question of water in this -area is one of considerable importance. Aside from the rivers and a few of their larger tributaries, most of the watercourses are dry during a considerable part of the year. While there are certain sections of the district in which water is present in a greater or lesser quantity during the entire year, there are other areas over which surface waters ‘are absent during the summer months so that the inhabitants are entirely dependent on wells for their supply. Generally, however, water can be obtained by drilling, although there are parts of the region, due to the perviousness of the roeks, in which it is necessary to drill .to a considerable depth before the upper level of the ground water zone is reached. The upper beds of the Fort Union formation are the most favorable ‘for the accumulation of ground waters near the surface, owing to ‘the more consolidated nature of the rocks and the intercalation of im- pervious material which prevents the water from sinking to any great depth. Furthermore, the greater abundance of vegetation found over the areas in which these rocks are exposed aids materially in retaining the water obtained from rainfall and in preventing the rapid evapora- tion and run-off that is so characteristic of the more barren areas, especially the badlands. In addition, the greater abundance of sandy material in upper Fort Union rocks allows more complete absorption. Although unconsolidated sandstones make up a considerable part of the underlying formations, the general prevalence of shale and clay is conducive to rapid run-off. A factor which is of importance not only in the Fort Union formation proper but in the Lance and Lebo shale as well, is the presence of coal seams. The percolating ground water upon reaching the plane of one of these seams, if it covers a considerable area, is prevented from sinking further and flows along —52— the plane of the coal bed to emerge as springs at points along the out- crops of the coal. Over the greater part of the area the water courses and depressions are favorable places for the digging of wells. Even when dry, the damp sand found along their bottoms indicates that water may be found within a few feet of the surface. The depth at which water may be reached varies, naturally, over different parts of the region with the upper limit of the ground-water zone which is dependent upon the texture of the rocks, their structure, and upon topographic features. Generally, however, it does not exceed 100 feet, although there are places in which it has proved necessary to drill several times this dis- tance. ‘ The greatest precipitation in this area occurs during the months of April and May and during the first part of June. Thunder showers are fairly frequent up to the latter part of August, but from this time until the beginning of winter the rainfall is very small. Many ranchers and homesteaders residing in areas where the surface run-off is rapid, through the construction of substantial reservoirs, are enabled to retain the waters precipitated during the periods of heavier rainfall and thus supply their stock during the dry periods. Over the area as a whole the water is more or less alkaline, the degree of salinity, depending upon the nature of the rock through which the ground-waters flow. Generally the more alkaline waters are found in the areas characterized by the prevalence of shales, which, owing to their more impervious nature, contain a large amount of insoluble salts. Chemical analysis shows the water in this region to be primarily made up of the following salts: Sodium sulphate, magnesium sul- phate, calcium sulphate, sodium chloride, and magnesium chloride. The few artesian flows noted by the writer were confined to the valley of the Powder River and to Cottonwood Creek, a stream drain- ing into the Yellowstone a few miles east of Miles City. Artesian flows occur, however, in other parts of the area, but most of them are confined to the above region, which lies in a shallow syncline, the western limb of which extends southward from the vicinity of Forsyth, and whose eastern limb is roughly determined by the Cedar Creek anticline. Since the axis of this syncline lies near the above anticline, which it roughly parallels, the water supplying these wells has probably descended along bedding planes from higher levels ‘to the west and its source is thought to be along the outcrops of. the underlying Cretaceous rocks in the vicinity of Forsyth. The rise of the strata is gentle, and the resulting pressure at the points where wells have been drilled is small, generally not exceeding 25 gallons per minute. The water, which is very cold, aside from a slight sulphur taste, will compare favorably with that found in the mountainous regions to the west. Limited time did not permit any detailed study of artesian waters in this area. From all indications the Powder River district is the most favorable for artesian flows, not only structurally, but owing to the fact that erosion has removed the greater part of the Fort Union strata, bringing the underlying impervious rocks nearer the surface. Over the other parts of the area considered in this paper the struc- ture is not, as a rule, favorable for artesian water, and at those pvints where it is, the great thickness of Fort Union strata would render drilling rather costly. Much remains to be done, however, in the study not only of artesian water possibilities, but of the geology of the region as a whole, and future investigation will undoubtedly disclose many facts of value on the above question. : —jR— Soils and their Adaptability to Farming In describing and discussing the soils of this area three different types of topography will be considered ; the rolling prairie, the alluvial deposits of the rivers and smaller water courses, and the badland areas. All of these types are characterized by a general scarcity of vegeta- tion. With the exception of occasional clusters of pine trees growing Fig. 24—Pine hills of the Fort Union formation. This type of topography is frequently found in areas where coal beds have undergone extensive burning, the baked sandstone and shale forming a soil favorable for the growth of pine trees. in certain favored localities and a few deciduous trees found along the waterways, this region is treeless. The general vegetation is represented by the more hardy forms and those adapted to semi-arid regions, consisting mainly of grasses, sage, cacti and various shrubs. The present general scarcity of vegetation does not necessarily signify that such conditions have always existed. On the other hand, during past geologic ages this region was the seat of abundant plant life, but climatic conditions which seem to have been ushered in at the close of the glacial period brought about a period of dryness so that the soils ceased to bear forests and those forms of vegetation characteristic of more humid regions. However, aside from the badland areas, the soils prove themselves of usual fertility when properly tilled. This is due in part to the gen- eral level character of the land which permits a rather deep accumula- tion of soil owing to the lessened activity of stream erosion. Those soluble materials necessary to plant life are allowed to accumulate more rapidly than the ground waters can bear them off, and a gradual enrichment of the soil in mineral matter results, a great part of which is in a decayed state and ready to pass into a condition fit for plant - food. Furthermore, the unusual number of cloudless days characteristic of this region and the resultant added sunlight is a factor which aids vegetative growth when the soil is made receptive for such. The limited amount of precipitation necessitates farming by what is - 54 known as the dry-land method, except in the few areas where irriga- tion is possible. By this method the moisture which falls into the soil is retained until it can be utilized by the growing crops. By plowing, packing, harrowing and cultivating the soil at a certain time and in a certain way, successful crops of wheat, rye, flax, corn and other grains are obtained. While the richness of these soils shows a rapid decrease under reckless agriculture, their fertility can be made very enduring by careful farming and proper rotation of crops. The soils of this region vary from those containing a large percentage of sand to those which are predominately clay. Generally, however, they are composed of varying mixtures of sand and clay and may be classified as loams. While tillable soils are found on all the formations represented, the areas covered by Fort Union rock are the best adapted to agricul- tural purposes. This formation and the alluvial bottom lands of the rivers and more important watercourses form the principal farming localities. Owing to its greater homogeneity, both as regards texture Fig. 25.—The prairie, the characteristic erosive feature of the Fort Union formation. The prairie soils are the most favorable for agricultural pur- poses, due to their loamy character and the uniform erosion of rocks from which the soils are derived. and kinds of rock, the Fort Union formation generally erodes uni- formly, thus providing areas suitable for tillage. Furthermore, due to the general predominance of sandy material, the soils are of an open texture and can be easily worked. There is but little trouble from alkali because of the more perfect drainage which prevents the excessive accumulation of surface waters. The materials derived from the disintegration of the Lance beds, which contain a considerable amount of shale rock, and from the Pierre shale formation generally form a stiff clay or “gumbo”. Such soils are extremely difficult to work, and because of their impervious- ness to water contain a large amount of insoluble salts. While these soils are very fertile when properly treated, they are at present farmed 55 but little as land much more adapted to immediate tillage is available elsewhere. : The badland areas are generally inhospitable to agriculture. In these localities, due to the softness of the rocks, and their variations in hard- ness, erosion has proceeded rapidly, so that everywhere are found precipitous slopes and steep-sided coulees. On such surfaces, gullied and channeled in all directions, the soil has but little chance to gain Fig. 26—Typical badlands of the Lance formation. The barrenness of the badland areas is due to the variation in the hardness of the rocks, the resulting unequal erosion preventing soil accumulation. a foothold. The resulting barrenness is not confined to the steep slopes. but extends even to those places favorable for soil accumulation, since the sand and detritus from the eroded surfaces are carried into. the small valleys and hollows, there to form areas unfavorable to all but the most hardy forms of vegetation. Along the bottoms of the larger watercourses of these areas, however, are stretches of alluvium, composed of a varying mixture of sand and clay, which are much more favorable to tillage. As it is true of all more or less arid regions, the soils of eastern Montana exhibit in places an unusual degree of salinity, forming the so-called “alkali lands”. While the alkalies are more or less dissem- inated everywhere, they have a tendency to accumulate in the more arid soils until they become in quantity far greater than in ordinary humid parts of the earth. The reason for this, as has already been explained, is due to the failure of the percolating ground waters to carry off the soluble mineral matter as fast as it accumulates. AS the sodium, potassium and other soluble elements become excessive they manifest their presence on the surface by a white coating which ~ may appear as a thick crust or a powder. This coating is formed by the evaporating ground waters which leave their mineral content be- hind as they pass into the atmosphere. : The soluble salts present in the soils of this area are sodium chloride = 5G (a minute trace), sodium sulphate, magnesium sulphate, magnesium chloride and calcium sulphate. Sodium carbonate, which is very detri- mental to plant growth, is practically absent. The above mentioned salts form the so-called “white alkalies’, and are not in themselves poisonous to plants, but when present in excessive quantities prevent the plants from obtaining their needed nourishment. Generally, in the more sandy soils there is less trouble from alkali accumulation, owing to the freer action of the seepage waters. At times, however, when from various physical causes the seepage waters in such soils become restricted in their movement, they rise to the surface and the resultant rapid evaporation causes an accumulation of alkali which is especially noticeable in depressions and along certain waterways. The more pronounced alkaline areas are associated with soils con- taining a marked percentage of clay, and it is because of this fact that the white incrustations of these salts are found generally asso- ciated with the Lance and Pierre shale formations. Fig. 27—View of the Lance formation on Glendive Creek. The white areas are alkali incrustations. The dark material in the foreground marks a bed of limonite concretions, common in the Lance formation and in the Lebo shales. The reclamation of lands excessively charged with alkali is a rather difficult matter, especially in the “gumbo” areas. Since the flow of seepage waters in such soils is so slow that they are unable to carry away in solution the soluble salts, the only remedy to this condition lies in the establishment of an efficient system of drains. Dependent upon the value and location of the land such a procedure may or may not be worth while. In irrigated areas where an excessive and un- necessary amount of water is supplied to these soils, the seepage waters accumulate to such a degree that the once fertile lands eventually be- come swamps and alkali flats and of no value whatever for agricultural purposes. The prevention of such a condition lies in a careful sys- = 57 tem of irrigation. The water should be used very sparingly and the surface kept under thorough cultivation, thus preventing an excessive accumulation of water in the subsoil and keeping surface evaporation down to a minimum. By these methods the natural fertility of these arid soils can be retained for an enduring period. -FQ— INDEX Page Alkaline Creek 43 Alkaline Soils 56, 57 Allard 45 Anticline, Cedar Creek. 18, 19, 22, 31, 51 Area Between Cedar Creek Anticline and Powder Rivet...............-....------ 45 Areal Geology Map 15 Area North of Yellowstone River. 48 Artesian Water 52 Ash Creek ..... : 39 Baculites 17 Babcock, HE. J. 50 Bad Lands 17, 22, 26, 40, 41, 56 Baker 38 Bearpaw Shale 11, 12 Beaver Creek 8, 43 Benny Pierre Creek 43 Benton Shale ‘ f1, 12 Block Jointing of Lignite 84 Blue Mountain 5, 43 Box Elder Creek 43 Burning of Lignite. 3 35, 36, 37 Cabin Creek 38 Campeloma s 21 Carboniferous Limestone 28 Carlile 11 Castle Creek 44 Cedar Creek 16 Cedar Creek Anticline 18, 19, 22, 31 Claggett Formation 11, 12, 17 Climate 8 Clinker Buttes 5, 6 Coal Beds, Description of. 37 Coal Beds, Distribution of. 37 Coal Beds, Stratigraphic Relations 43 Coals, Character of 33 Coal Creek 41 Coal Deposits 83 Coal, Estimated Tonnage. 49 Coals of Fort Union 42 Coals of Lance Formation 87 Coals of Lebo Shales ‘ :..40 Coal Quantity of .. 48 Colgate Member : a Colgate Sandstone : 9, 10, 11, 12, 17, 18 Colorado Group 11. 12 Colorado Shale : 11, 12 Columnar Sections 44 Concretions, Log-like 20, 21, 25 Correlation, Regional 10 Correlation Chart 11 Cottonwood Creek ... 40, 41 Cretaceous, lower 10, 12 Cretaceous, upper 10, 11, 12 Cc. S. Creek 44 Dakota’ Sandstone 11, 12 Deadman Creek 43 Deposits, Coal and Lignite 33 Development. Mining 49 Devils Canyon 43 Distribution of Coal Beds 387 Drainage 7 Dry Creek 43 Eagle Sandstone 11, 12 Economic Features 51 Elm Coulee 43 Features, Economic 51 Formations, Descriptions of 16 Forsyth 53 Fort Union 5, 6, 9, 10, 11, 12, 18, 18, 19, 20, 22 Fort Union, Coals of 42 Future Development of Lignite 49 Gas ....: 51 Geography 5 Geologic History 82 Geology, Areal 15. Geology, General 9 Glendive 8 Glendive Anticline 51 Glendive Creek 57 Granerous Shales 11 Gravels, Terrace 27 Great Plains 5 Greenhorn JTimestone ~...2..222.22.22-.2-:e-ceeceeeeee eee 11 Ground Water 52 Harpster Coulee 43 Harris Creek 42 Heat Value of Lignite. 235 Hill Ranch AT History, Geologic 32 Index Map : 4 Industries 8 Inoceramus 17 Introduction 8 Ismay 89 Judith River Beds 11, 12 Kingsley 47 Kinsmont Oil Company 52 Knowlton, F. H 18, 18, 21, 46 Kootenai Beds 12 Lame Jones Creek 39 Lance Formation.......-........-22...-----1---+00---0-4 6, 7. 9, 10, 11, 12, 17, 18, 19, 20, 21 Lance Formation, Bad Lands of. 56 Lance Formation, Coals of. 37 Lance Formation on Glendive Creek 57 Laramic Formation 11 Lewis Shale Lod! SD Lebo Shale 10, 21, 34, 41 Lignite Deposits. 33, 34 Lignites, Future Development and Utilization of.........-.----.-.--------10-+ 49 Little Beaver Creek 7%, 8 Little Missouri River q Livingston Formation 11 Lucina 17 Margarita ....- 17 Mesaverde Formation 11, 12 Miles City 8 Mining Operations and Development 49 Mispah Creek 47 Montana Epoch 32 Montana Group 11, 12 Montana Petroleum Company 52 Mowry Shale 11 Muster Creek 42 Nautilus 17 Niobrara 11, 12 North Dakota 50 O’Fallon Creek 8, 39 Oil : 51 Operations, Mining 49 Pennel Creek 45 Pierre Shale. 7, 9, 10, 11, 12, 18, 16, 17 Pine Hills : 54 Plevna 39 Powder River. 6, 7, 8, 18, 41 Proximate Analysis of Lignite. 35 Pumpkin Creek . 40 Quaternary 10, 26 Relief 5 Rogers, G. S 23 Sand Creek ..... 42 Sandstone Creek 38 Sandstone Pinnacles 24 Scaphites 17 Sections, Columnar 44 Section: (Geologic: cesccscctecs.: coc tete eed cae sietee eee ere eee ae tn ae 30 Settlements .... 8 Shadwell Creek 43 Sheep Mountains 5, 22, 27 Smith Creek ..... 43 Soils 54, 58 Spirifer 28 Stanton, T. W. 14 Stratigraphic Section 10 Strevell Creek 41 SUTUCCIKO: .sccssch csc cecci acess wees ee rear es a asec, 31 DUNGAY ‘CLOCK sc cnescccecssienenwercovnassenseehinaseney sata wues ener ee nesseuesssecessevdeessesaccsteccesaad 8, 42 Terrace Gravels suisse deat Visaesitizustcun cg 27 Terry 8, 40 Tertiary 10, 11, 18, 21 Tertiary, lower 10 Todd, J. E 20 Tongue River. 7, 8, 18, 28 Topography sees 0. Treasure State Oil & Gas Company 52 Triceratops 18, 21 Ultimate Analysis of Lignites | 96 Utilization of Lignites 49 Vegetation 8 Viviparus 21 Water, Artesian and Ground 52 Willes, Baily 12 Wyoming, Northwestern ... 8 Yellowstone Rivev..... 6, 7. 8 Yellowstone River, Area North of.. 48 AV CLIGWSTONG Vay? is ses tcc see cane oa att eset ec a arr oe gp 18 Zero GEOLOGY AND ECONOMIC DEPOSITS EASTERN MONTANA