i Aes i) a ii tat te siarantal ti (i fe == shalt Hae pte ata 8 Wat st os ee thy f tet th i its ; +, bapa Tacit Hit ‘ the a Hh a divin pa Auten ai ta Hest ry aig ast nae a ne ut nee iit fakin ny" yy A H i 7 : i i ma ahah ea at aa Be ida a mie his te aia i i Th ai i i j id i tear sa Hi tensciltastrs ¢ hh ; e i ea nit i sbgtafetiti tity ai 1 ealsegsesess age ioe a tH ath Byte Bak 8 388s 4 usa ‘ i ini ie bebatatere Lanntesy ia ay eae ih bd if ei i} saat te pot Pott oes Derren Herne. seme nn be y OF * sah apteata Mot hate Saye tase cor ee) anion ub ht inte tee te 4h - ayyet ee “4 ae ieee tatneettat trabbabes ya hs if 9F) a us ets Eons} %% 2 ENP @POLOGY Digitized by the Internet Archive in 2011 with funding from University of Illinois Urbana-Champaign http://www.archive.org/details/reportofgeologicO2unit lo NIGWE ADIWHSVM 7 SCNVI-GVE FN904 PROFESSIONAL PAPERS OF THE ENGINEER DEPARTMENT, U. S. ARMY. No. 18. RHEPORT OF THE GEOLOGICAL EXPLORATION OF THE FORTIETH PARALLEL MADE BY ORDER OF THE SECRETARY OF WAR ACCORDING TO ACTS OF CONGRESS OF MARCH 2, 1867, AND MARCH 3, 1869, UNDER THE DIRECTION OF BRIG. AND BVT. MAJOR GENERAL A. A. IIUMPHREYS, CHIEF OF ENGINEERS, BY CLARENCE KING, U. S. GEOLOGIST. ro + VOLUME II. UNITED STATES GEOLOGICAL EXPLORATION OF THE FORTIETIL PARALLEL. CLARENCE KING, GEOLCGIST-IN-CHARGE. DESCRIPTIVE GEOLOGY. BY ARNOLD HAGUE AND 8. F. EMMONS. SUBMITTED TO THE CHIEF OF ENGINEERS AND PUBLISHED BY ORDER OF THE SECRETARY OF WAR UNDER AUTHORITY OF CONGRESS. ILLUSTRATED BY XXVI PLATES. WASHINGTON: GOVERNMENT PRINTING OFFICE. L380. 7 TABLE OF CONTENTS. CHAPTER I. Rocky IVE OSINETUAGIN Sigeree = = rere sets enal-fotais icine tase) ain tite ele cecs ye are oie SECTION I. COLORADO RANGE. By ARNOLD HAGUE....... II. LARAMIE PLAINS. By ARNOLD HAGUE.....-.. Til. MEDICINE Bow RANGE. By ARNOLD HAGUE IV. NoRTtH PARK. By ARNOLD HAGUE..-........-. “V. Park RANGE. By ARNOLD HAGUE........... VI. CRETACEOUS REGION FROM Como TO SEPARA- TION. bY ARNOLD HAGUE. ....Geological Survey of Canada, 1863, 501. LARAMIE HILLS. 17 however, is black, and unctuous to the touch: it contains but little iron- pyrites. Deposits of sufficient size to be worked are said to occur, if the mineral were only of the requisite purity. The following analysis is by Mr. R. W. Woodward: LG nigah e604 Key oy Sear ene area Se en ee ee La Rate: JN Oe oa a ee ee ee eee 11.50 JE voinntoyuisy ele: G Sesser aay ae, eer 17a. Mona OUShOM QC acter teens oe pale ao 0.24 TEAR GOVS Lect arn es NS See pees nce eee eee 1.44 Loon sel pees tere, wale eee ete ee eee = oie SUL ana aly 5. ellowish-nedisan (Stom Greys seer aera er ease ee erate eee 100 fect. 6G. Massive-red. sandstone fan... 2.0 ssa5 ee so stele acm Serer oh toate oe nee creer ayers 300 feet. 7. Coarse red sandstone, with conglomerate...... 2.2.2.2... c 0 eee eee cee 100 feet. 650 feet. It is evident, from the foregoing sections, that sandstones form by far the greater part of the entire series of strata. Even the conglomerates, shales, clays, and earthy beds, which occur interstratified throughout the formation, appear more or less arenaceous, and are really closely allied to true sandstones, only showing considerable diversity in texture and mechanical conditions. The conglomerates would appear to be much more prevalent near the base of the series, especially south of the Cache la Poudre, where they lie resting directly upon the Archean body, and carry considerable detrital material, derived from the lower rocks. Shales and impure clays, which, in thin layers, may be found anywhere from the base to the summit, characterize the middle formations; while the upper beds are usually finer-grained, with an earthy appearance, and carry thin beds of limestone and gypsum. EASTERN FOOT-HILLS. 35 In the more compact and massive sandstones, transverse bedding forms a very prominent and marked feature of the Triassic Red Beds. Tn addition to the well-defined limestone beds, many of the horizons in the upp*r impure sandstones appear highly calcareous, which is, however, not evident to the eye, but which, when they are treated with dilute acids, is indicated by a brisk effervescence. A typical specimen of the red sandstone in our collection, taken from near the entrance to the Big Thompson Cajion, and not far below the top of the Triassic series, presents a fine-grained fria- ble texture, a deep red color, with a shaly, laminated structure. A fragment from this rock was subjected to chemical analysis by Mr. B. E. Brewster, with the following result : BS ICEOn Cre eee ee Pe tak: SiR eet tee a ne Tap PLT eee pee etd Seal tee, ee b 2.15 USES Gog re ee ee Pe cant Bs 5 est Wey ete 6.07 ORCS Ip hee ome rere aes eee ee Payee at SO eke 4.40 EO Asc es etter Meee ia 2 Pot) 2.38 Sn rr ee eee ae Tt CEPT) s conn Yer 12) 640 Hem ete ance a se 9.00 VV AL CTR Ram rssh eeese cca ts is a ac Lea 2 1.62 Insoluble residue ras ee aa Gy a Ss ey FAN it aah ct: Rae eee eee eee ea Ce 5.69 101.12 This analysis shows the presence of a large amount of soluble carbon- ates, including a high percentage of dolomite, and a considerable admixture of argillaceous materials. So large a proportion of carbonate, with no trace of gypsum, occurring in a formation in which many of the horizons are marked by the presence of heavy deposits of the latter salt, is not without considerable interest. Underlying the shaly sandstone a short distance, occurs a deep red bed, somewhat coarser-grained, still more friable, and without any laminated structure, which, upon being treated with acid, gave no indication of the presence of soluble carbonates. Laminated impure shales, from a well- 36 DESCRIPTIVE GEOLOGY. marked horizon on Horse Creek, and interstratified between coarse sand- stones, were found on examination to contain a considerable amount of calcareous substance, which is probably, like the bed from the Big Thomp- son, largely dolomitic. They have a fine-grained arenaceous texture, with a deep reddish-yellow color. The limestone beds in the Triassic series along the base of the Colorado Range are, in many localities, well marked, but do not appear to be per- sistent over very wide areas; at least, they were not always observed in crossing the formation, but, where noticed, always preserved the same gen- eral characters. Two distinct beds, but a few feet in width, are recognized. They are clearly defined north of the Chugwater, and are shown in the sec- tion already given, where the lower bed consists of a compact, siliceous, granular rock, and the upper one a bluish-white cherty band with a brittle fracture, not unlike the characteristic limestone stratum so persistent in the Jurassic beds. The deposits of gypsum, which form so marked a feature of the Red Beds, are largely developed in many localities along the base of the Colorado Range. They occur in irregular beds, interstratified in the deep red sand- stones and shales, and, although they may possess a wide range throughout the entire horizon, are much more prominent in the upper formations. Although quite distinct, and separated from the limestone strata by interven- ing sandstone layers, the heavier gypsum beds are usually at no very wide interval from the limestones. The deposits vary in thickness from 2 feet up to at least 25 feet; the heavier bodies frequently occurring as broad beds, with a considerable thickness in the centre, but thinning out toward the edges. The gypsum occurs both massive and crystallized; is nearly white in color, with a light reddish tinge, and frequently reddish-gray from the admixture of slight impurities; the lustre is usually pearly, and often brilliant. The compact massive gypsum, where not discolored by exposure, is generally snow-white, presenting a striking contrast with the brick-red dolomitic shales. Gypsum from Red Valley, near the northern end of the Laramie Hills, is quite pure, possessing very nearly the required theoretical composition, an analysis yielding— EASTERN FOOT-HILLS. oF Sunnate ol lieu ene tere ee Wee (S11 Wintel ne ee ere WEP ea ee, PAPA 9032 No organic remains have yet been found in the Triassic formations east of the Colorado Range. JURASSIC FORMATION.—Intimately connected with the Red Beds oceurs a series of loose friable sandstones, limestones, marls, and impure clays, pre- senting great variety in color and texture, and passing from one to the other by almost imperceptible grades. The entire series is usually composed of fine-grained material, and the clays and marls show a more or less laminated structure. These beds have been referred to the Jurassic age. While the prevailing character of the Triassic beds is a deep red sand- stone, the overlying Jurassic rocks possess a slightly reddish tinge, but are marked by beds of orange, purple, and lavender-colored strata, which con- sist largely of argillaceous material. Narrow beds of limestone oecur in both formations, while gypsum, which forms so prominent a feature in the Triassic, is found in thin, narrow seams, in what would appear to be several different horizons in the Jurassic. The line which has been adopted as forming the boundary between the Triassic and Jurassic periods is a somewhat arbitrary one, not being based upon any broad structural or paleontological evidence, or, indeed, upon any clearly-defined lithological distinction. Where the upper forma- tions of the Triassic period consist of the deep red compact sandstones, and the overlying strata are friable light-colored beds, the line of separation has been taken at the summit of the more massive red series. In many places, however, the top of the Red Beds possesses a yellowish shaly char- acter, shading into the upper series, rendering a sharp line of separation somewhat difficult to establish. As the upper beds, however, are much more easily decomposable, they have suffered considerably from erosion, and have been worn out into longitudinal depressions, or valleys, between harder layers of rock, forming a somewhat marked physical feature of the ridges, and at least offering a convenient division between the two series of rocks. No organic remains sufficiently well preserved for specific deter- 38 ; DESCRIPTIVE GEOLOGY. mination have been obtained from Jurassic rocks east of the Colorado Range; but as characteristic strata have been easily recognized, which are very persistent wherever the Jurassic occurs, and below which, on the Laramie Plains not far above the Red Beds, well-defined Jurassic fossils have been identified, it seems, for the present at least, desirable to place the line of separation at the top of the compact red sandstones. In thickness the rocks, which have been referred to the Jurassic, vary from an extreme development of 250 feet, down to less than 75 feet. They attain their greatest thickness in Colorado in the region of the Big Thonip- son Creek. To the northward, their average width may be taken at 200 feet. In Wyoming, along Lodge Pole and Horse Creeks, they appear less clearly defined, being in part obscured by loose soil, and in part resembling the Triassic beds. Here they are probably represented in some places by only 75 feet of strata. Still farther to the northward, they expand again to at least 150 feet, showing most of the characteristic beds to be found in Colorado and Western Wyoming. The lowest beds in the Jurassic series are usually reddish-yellow sandstones and shales, passing into lighter- colored beds, partaking more and more of a marly nature, with inter- stratified beds of hard sandstone and some limestones. The upper beds are friable sandstones interstratified in compact gray and cream-colored marls, with varying proportions of lime. In several localities, these cal- careous marls would seem to be represented by well-defined beds of lime- stone. At Box Elder Creek, near where the stream leaves the mountains, the following section was made across the Jurassic beds: 1. Fine friable sandstone. 2. Gray marls and clays. 3. White marls. 4. Yellowish calcareous sandstone. 5. Cherty limestone. 6. Orange sandstone with light-colored clays. 7. Gray marl with purple and reddish-brown bands of clay and thin layers of sand. 8. Gray arenaceous marl. 9. Reddish-yellow friable sandstone. It represents a section of rock strata of from 200 to 250 feet in thickness. EASTERN FOOT-HILLS. 39 The limestones of the Jurassic strata appear to be of two distinct kinds; the one a hard, dense, cherty rock, carrying a considerable amount of silica, and usually of a characteristic gray color; the other less compact, exhibiting greater variety in color and texture, and in most localities probably dolomitic. A specimen from the latter limestone, collected at the base of the mountains, near the old “Denver and Laramie Stage Road,” shows a rather more than usually compact texture, and a pinkish-gray color. Scattered through the rock are numerous small crystals of calcite. Under the microscope may be seen minute grains of silica and crystals of quartz. This rock was subjected to complete chemical analysis by Mr. B. E. Brewster, with the following result: parce ee eee ee eve EN eat Da Ws 2.95 Alumina and ferric oxide....-..-..----- 0.79 0.54 1 OTben sats me Soe Rea AA ar ee eee ae ee AES aX0, 29.89 VPRO TIO SIR ereeene ay oS PP Ge ee AS BS ars 19.31 19.36 MOUs AMCs pOLassame eee mee ee Be 0.38 0.28 (SAT DOMIGHAC eee Aer enh ered 9 45.05 45.14 Vn LCT e pe: Sete see M eset oo Sele ere 5 aa 135 1.30 99 55 99.46 Combining the carbonic acid with the bases we have very nearly the composition of true dolomite. (Canbonate. Ginkine 22 Fo. 6 seeec cae ah > e 53.40 53.02 Carbonate of magnesia......---.....-.- 40.55 40.66 93.95 93.68 Daxora Oretaceous.—Although the base of the Jurassic is fre- quently obscure, the upper horizon, which has generally been adopted as separating the series from the Dakota sandstone of the Cretaceous group, is usually well defined and easily recognized along the entire base of the mountains. The Dakota beds are essentially a sandstone formation, and as they are usually hard and compact, frequently almost a quartzite, they form a well-defined horizon. Lying between the easily-eroded Jurassi¢ 40 DESCRIPTIVE GEOLOGY. marls and clays below and the overlying blue shales, clays, and soft erunibling rocks of the Colorado group above, the Dakota beds are usually a conspicuous feature in the ridges which form the foot-hills of the main range. Where the Paleeozoic and Mesozoic rocks are found resting upon the Archean masses at a highly-inclined angle, the Dakota group may usually be traced, forming the uppermost member of the steeper ridges; but where the angle is low, they frequently pass under, and are concealed by the overlying Tertiary beds. In approaching the mountains from the Great Plains, the Dakota beds are especially prominent, as they form the outlying member of the series of upturned sedimentary beds, which rise so abruptly above the plain; for, although the overlying Colorado group is perfectly conformable, they never occur high up on the long ridges, which form a sort of barrier between the level country and the mountain region beyond. As already mentioned, the Dakota is a sandstone formation, and pre- sents but little variety in its physical features and composition along the base of the mountains. The bottom of the series is usually a fine conglom- erate, passing into a hard brownish-yellow sandstone, distinctly bedded, and in many localities showing the presence of considerable iron scattered through the rock. Occasionally, the iron will appear in segregations, form- ing rounded patches or blotches of iron stain on the yellow sandstone in a very striking manner. Frequently, the upper members of the group are a loose friable rock, many of the beds being nearly a white sandstone. Thin seams of black clay, rich in iron and occasionally some carbonaceous mat- ter, although somewhat rare, occur all through the upper part of the group. Along the Laramie Hills the Dakota formation in places develops a ten- dency toward a shaly structure, but in general they are characterized by massive beds. In thickness, the Dakota group may vary from 200 to 300 feet. The following section in Northern Colorado will serve to show the general character of the lower member of the Cretaceous series, especially as represented between the Big Thompson and Cache la Poudre: EASTERN FOOT-HILLS. 41 1 - Yellow sandstone .u..< «cc. secie ee adcc ure oaeees Be ens Weekes "| 100 feet. 2. Coarse sandy beds, with frequent clay-seams........-..-..-..--..---.- : 4, Coarse yellow sandstone, passing into fine conglomerate............... 3. Yellowish-brown sandstone, almost a compact quartzite....... - : } 200 feet. 300 feet. So far as examined, none of the sandstones would appear to carry any carbonate of lime. A specimen of the Dakota sandstone, taken from the prominent outlying ridge north of Wahlbach Spring, presents a fine-grained, even texture, with a yellowish-brown color. Under the microscope, with a high power, many of the quartz grains indicate the presence of glass cavi- ties, while between the sand particles may be recognized an occasional fragment of hornblende, and a green mineral, which appears to be chlorite. This rock was subjected to chemical analysis with the followine result: } $ RICH ee ets SS cs oat Me ee ee ee ee! 95.46 SAINT S601 0 age gee a a mh SS! NTE Mn Ais 2.69 ies ia CS op a6 2g aay ae oa Reo rc sn eey Uy a ee lee 0.18 Nbintie Seer ee, ge ere ees, Bee eo 0.14 MICS Tat we Ae. ae Se eee RT nat ee et 0.06 modaamel notassae- 0c ee ee 0.25 OMG eee eee ee ee ence eh. Bot 1.18 99.96 Cotorapo Creraceous.—The Colorado group embraces the Middle Cretaceous highly fossiliferous rocks, which overlie the Dakota sandstone, and, in turn, underlie the sandstones of the Upper Cretaceous. The group is essentially composed of clays, shales, marls, limestones, and some impure sandstone, which, taken together, form a well-defined series of beds, and include the three divisions of the Middle Cretaceous, of the section made by Meek and Hayden along the Upper Missouri River: the Fort Benton, Niobrara, and Fort Pierre beds. Along the east base of the mountains, within the belt of our exploration, but more especially in Wyoming, it would appear almost impossible to indi- cate, with any degree of*accuracy, the divisions of the series, although their 42 DESCRIPTIVE GEOLOGY. presence is very easily recognized, and in many localities either one or the other bed occurs well developed. The Fort Benton beds, which lie next the Dakota, and are only exposed along the base of the abrupt ridges, consist of dark plastic clays, at times distinctly bedded, and frequently occurring as thinly-laminated paper-shales. The lower beds are always more or less arenaceous, with interstratified beds of purer clay, while the upper beds sometimes carry thin seams of argilla- ceous limestone, which, in many places, cannot be distinguished from similar beds in the Niobrara. In general, the beds are very uniform in lithological character. Along the Laramie Hills, they are somewhat difficult to recog- nize, as they are usually concealed by loose soil and débris from the steeper ridges to the westward, but in Colorado they may be traced for long dis- tances in well-defined north and south lines. The Niobrara beds, which occur just above the Fort Benton, although much thinner, are, from their characteristic colors and peculiar mode of weathering, more easily recognized. Where the upper beds of the Fort Benton are calcareous, it is generally difficult to separate the two series below the yellawish-white chalky limestone strata, which is so pronounced a feature in the Niobrara. They consist mainly of marls and impure limestone. The lower members are bluish-gray argillaceous limestones, with occasional beds of lighter-colored clays, all of them highly fos- siliferous, passing up into yellowish-white or cream-colored marls, associated with gypsum, which occurs frequently in thin, irregular layers, and at times disseminated through the marls. Not far above this occurs a second bed of somewhat deeper yellow marl, which at the top passes into bluish-gray, soft, earthy beds. The base of the Fort Pierre group is usually much obscured by over- lying soil, and at times the lithological aspect of the beds blends so com- pletely with those of the underlying Niobrara that any separation seems impossible ; but in other localities the dark, almost black, thinly-laminated clay affords a very distinct horizon. The entire group, like the Fort Benton division, is made up of clay-beds, with but little variety in general character. The lower clays carry considerable amounts of carbonaceous material, and along the seams and cracks incrustations of gypsum and alkaline salts. EASTERN FOOT-HILLS. 43 Some of the richer carbonaceous shales along the Laramie Hills have been examined for coal, but without success. The upper beds are somewhat lighter in color, usually harder, and perhaps more siliceous. Only at one locality north of the railroad, about 2 or 3 miles north of Horse Creek, do the Fox Hill beds, overlying the Fort Pierre, occur, and even these may prove to be the extreme upper members of the lower division ; everywhere else the Pliocene Tertiary strata conceal the beds above the black clays. In Colorado, however, the Fox Hill beds extend along the base of the range, and the junction between the Fort Pierre and the next series above may be well seen. Here the more prominent clay-beds pass into coarse argillaceous sandstone, of a dirty brown color and crumbling texture, but carrying well-defined fossils, characteristic of the group. The Colorado beds, like all the other sedimentary strata along the base of the range, vary somewhat in thickness, and appear to reach their greatest development in Colorado. No accurate measurement of their thickness was made at any one place, and it would. be somewhat difficult to do so, as the group is more or less covered with soil, and the dip extremely variable. The lower members of the Fort Benton beds incline uniformly with those of the Dakota; but, owing to their plastic, flexible nature, are likely to fall away to the eastward with a lower angle, which is not recognized until we find the more prominent marls of the Niobrara lying inclined at a reduced dip. The Fort Pierre clays would seem either still more flexible, or else, removed farther from the main range, to have undergone less pressure in the uplift of the strata, for we find the beds extending far to the eastward, flat- tening out with low angles, frequently inclined at from 3° to 5°. In this way, they change from an abrupt angle of 30° to nearly horizontal strata, rendering an estimate of their thickness a matter of some difficulty. Their thickness may be best obtained by measuring at different points, where well exposed, the several members of the group, and from these esti- mating the entire expansion. Probably the extreme development of the Colorado beds will be found not to fall much short of 1,000 feet in Colo- ~rado; while, in Wyoming, the thickness will be somewhat less, but over the greater part of the area the upper members of the series are never seen. 44 DESCRIPTIVE GEOLOGY. The following section was not made at any one point, but will serve to show the character of the entire group: —_ . Interstratified beds of clay and sands, in many localities the clays ) predominating, and in others the sandy beds....... oes x s 2. Dark arenaceous clays ....-.-..--..+- ee rere pert P Spt eae 3. Grayish-black carbonaceous clays and shales............ ..-.- 5 4, Bluish-gray, soft, earthy beds.........-....-. Big eRe ae: Nae 1 5. Mellow amarlssand limestone, 4.2. ee 0s eres ts Pere oo state oir 6. Wihitish:oray marlisc.. < sis 2 sccte pier cia ee nena alee se eres Sct oe 7. Yellowish-white and cream-colored marls and limestone, with ‘ 100- 200 feet. Oy SUM ae peeve Ques asoe sa Bet Pe carte g Oe nee Sy TOM COLO RCC Hy et] Seyi e oer teeta seals c= 1 ee | 9, Blue argillaceous. limestones. 222 2. sees awe dese ee) see aoe \ 10. Clays, with tine ‘calcareous bed§... <.0. ss srcce scsese sees seas 11. Dark plastic clays, with arenaceous and ferruginous layers. .... 600-1000 feet. Although the beds show considerable variety, it is evident that they possess, in general, simple features; the lower members being clay-beds, more or less arenaceous, passing up into calcareous beds, and, still higher in the series, again becoming clays, with sands near the top. It would be difficult in most places to separate the Fort Benton from the Fort Pierre group by lithological distinctions, but for the striking pecu- liarities of the brilliant-colored marls of the Niobrara; where the latter are concealed, and the beds are not defined by their organic remains, it becomes almost impossible to say to which series certain beds may belong. ‘The Fort Benton clays, lying next the rigid sandstones of the Dakota, are easily eroded into broad shallow depressions, with the gray and yellow marls of the Niobrara forming low ridges to the eastward. ‘These marl-ridges, although very insignificant in proportions, form. one of the most. striking features along the foot-hills, affording excellent geological horizons in tray- elling rapidly over the country. In many places, they show great. persist- eney, and may be followed without any break in their continuity for many miles. They rise usually but a few feet above the surrounding level, stand- ing out in a most marked manner above the dull earthy clays, presenting EASTERN FKOOT-HILLS. 45 their steepest sides toward the range, and falling away gently to the east- ward. Farther to the eastward the Fort Pierre clays produce a second slight depression, or valley, but frequently somewhat broader and less regu- lar, showing low benches and occasionally isolated mounds or hills. Organic remains characterize several well-determined horizons in the Colorado group east of the mountains, and in many localities appear very abundant, especially the genus Ostrea, which frequently forms almost entire beds, held firmly together by a little calcareous cement. They show, how- ever, but little variety in species. Many of the fossils in our collections are too poor for specific identification. The following forms, which ocenr in the Colorado group, have been found east of the mountains: Inoceramus problematicus. Callista Deweyi. TInoceranus difformis. Anisomyon sexsulcatus. Inoceramus Barrabini. Pinna restituta, Inoceramus mcurvus. Scaphites nodosus. Ostrea congesta. Baculites ovatus. Ostrea ————? Ammonites ———? In addition to the above, many of the clay-beds are characterized by numerous small finely preserved fish-scales, lying parallel to the plane of stratification. Descriptive GeoLocy oF THE Eastern Foor-niiis.—In giving the general geological and lithological features of the main formations, which make up the belt of sedimentary beds along the base of the range, but little has been said of the details in any one locality, and scarcely anything of the structural peculiarities of ridges, showing the relations which they bear to each other, and to the older Archzan rocks. In the following pages some of the more salient features will be pointed out. Beginning with the northern end of the Laramie Hills, the first occurrence of Paleozoic and Mesozoic rocks south of the Laramie River is found at the head of Bush Creek, in a small bay-like depression in the Archean rocks, known as Red Valley, just where they jut out to the eastward from the main body. Here the Paleo- zoic series rise somewhat abruptly in a short isolated ridge, while the Trias- sic beds are the only Mesozoic rocks represented. The latter lie nearly 46 DESCRIPTIVE GEOLOGY. horizontal, covering the greater part of the valley, and resting directly against the granite. he chief point of interest in the formation centres in the immense deposits of gypsum, which appears to underlie the entire valley, just below the surface, cropping out everywhere and in many places, over wide areas, actually forming the surface itself There would seem to be sev- eral beds, varying in thickness from a few feet up to 15 feet. It is mostly a clear white pure gypsum, and, if required for market, could be mined on a most extensive scale. On the south side of this projecting mass of gran- ite, and north of Reschaud Creek, the Paleeozoic rocks crop out for a long distance, dipping to the southward at a highly inclined but uniform angle. In places, the Mesozoic beds may be found overlying them, but the Tertiary beds lie high up on the range, concealing the greater part of the strata. South of Reschaud Creek, the Palaeozoic limestones, standing at an angle of 70° to 75°, extend in a nearly north and south direction for 3 or 4 miles, in an unbroken line, until reaching a small tributary of the Chugwater. In the region of the Chugwater, the outlines of the Archzean body are very irregular, and the sedimentary beds circle around the mass, conforming closely to all the lesser details of the older rocks. Just north of where the Chugwater leaves the Laramie Hills, the Archzean mass projects to the eastward, and the later beds, influenced by the underlying body, turn in like manner in the same direction, present- ing a great variety in structural features, and marked by abrupt changes in strike and dip. Within 7 miles, in an east and west line, the entire series of ridges, from the base of the Paleozoic to the top of the Colorado group, describe a complete letter Z. Immediately to the south of the Paleozoic beds, already mentioned as lying to the southward of Reschaud Creek, the entire series of beds occur, standing at a very high angle, and striking nearly due north and south, until just before reaching the Chug- water they turn off slightly to the southeast, curving around the Archeean mass. Here the Paleozoic strata appear very prominently, rising nearly vertically, and reaching to the summit of the Archean body. The Triassic and Jurassic strata, although inclined at a high angle, do not extend high up on the flanks; but the Dakota sandstone, on the contrary, rises in a very bold manner, forming a high wall, dipping at an angle of 85° from the EASTERN FOOT-HILLS. 47 range. Beyond this sandstone wall, the Colorado group occurs, exhibiting its dark shales and yellow marls. This sandstone wall is cut by the Chug- water, showing a fine section through the entire Dakota beds. Immediately south of the Chugwater, the Dakota beds make an abrupt break, turning to the southwest, and, in a very short distance, are found with an almost due east and west strike. Indeed, all the beds turn in a very abrupt manner, inclined at high angles; the Red Beds standing at 55° to 61°. Where the strata are found with an east and west trend, the dip falls off to 25° and 30°. Here the Paleozoic limestones, with a uniform dip of about 25°, are found north of the stream. The Red Beds, which are well exposed, occupy a very consider- able area, forming a small valley, through which for a short distance runs the Chugwater, the stream having cut for itself a narrow channel in the upper red clays. A section across the Red Beds at this point has already been given. On the south side of the Chugwater occurs a long, low ridge, rising but a few hundred feet above the stream, at the base of which the Jurassic marls and limestones crop out, while the top is formed of the Dakota sandstones, which incline gently toward the plains, and are overlaid unconformably by the Pliocene Tertiary beds. Just west of the wagon-road occurs a somewhat prominent hill, in which is exposed a good section of the Jurassic beds. The base of the hill on the north side consists of the intensely red Triassic rocks, overlaid by the Jurassic marls, which in turn are capped by the Dakota sandstones, form- ing the summit of the hill. Immediately beyond the hill, the entire series of strata again trend to the northward, which, standing at a high angle, are seen compressed into a comparatively short space. They trend at first north and south, then east and west, and again curve around to the north and south, forming approximately three sides of a square scarcely 3 miles in width. The Paleeozoic limestone extends high up on the flanks of the Archean, while the Triassic and Jurassic rocks have been worn away by erosion, in many places several hundred feet, but the harder and more rigid Dakota beds are seen forming a somewhat prominent ridge. The clays and marls of the Colorado group occupy the centre of the square, but are largely concealed by loose soil and gravel. At the entrance to the 48 DESCRIPTIVE GEOLOGY. Chugwater Canon, the Carboniferous limestone presents a very marked appearance, rising almost out of the plain, for several hundred feet, in a nearly vertical position, its beds considerably metamorphosed. On the summit of the ridge occurs a limestone bed, altered into a fair variety of white marble. A short distance south of the Chugwater, the Palaeozoic and Mesozoic rocks become depressed, and disappear entirely beneath Tertiary gravels that have been referred to the Wyoming Conglomerate of the Bridger basin. On the Chugwater, some 4 or 5 miles below where it cuts through the wall of Dakota sandstone, already mentioned, occurs a very considerable development of all three subdivisions of the Mesozoic rocks. Unfortunately, but little time was allowed for the examination of this group; it would appear, however, to derive its chief interest from the fact that, so far as known, it forms within the belt of our exploration the only occurrence east of the Laramie Hills of an outcrop of pre-Tertiary rocks, isolated from the main ridges of uplifted beds along the flanks of the range. Where they first rise above the surrounding Tertiary beds, the stream has cut a channel through the dark ferruginous shales of the Fort Benton division of the Col- orado Group, beyond which the stream runs through a narrow passage in the Dakota sandstones. On the north side of the river, the Dakota beds form quite a high ridge, dipping at an angle of 25° to 30° to the south- ward, the entire series of rocks trending approximately east and west. North of the Dakota ridge occurs a narrow valley of the Jurassic marls, with the characteristic low ridge of Jurassic limestone in the middle, in turn under- laid on the north side of the valley by the Red Beds, which appear in a low bench, the bright red soil marking the outlines of the formation for long dis- tances, So far as observed, this group of upturned beds only extends for 3 or 4 miles, beyond which the Niobrara ‘Tertiary covers all the older rocks, and the Chugwater widens out into a broad open valley, with high bluffs and walls of horizontal sandstone. Returning to the foot-hill ridges, the entire series are again seen coming to the surface just above the northern tributary of Horse Creek. From EASTERN FOOT-HILLS. 49 here southward to the main valley, a distance of 3 miles, all the character- istic beds, from the Paleozoic to the top ef the Colorado group, are weil represented. The most striking feature, however, is the bold, abrupt bluffs of the Paleozoic rocks, which rise above the top of the Triassic beds from 50) to 600 feet, exposing almost sheer cliffs of Carbonircrous limestone to the eastward, the strata standing at an angle of 76°. Three streams from the range nearly equally divide the limestone into three somewhat isolated ridges with curiously eroded summits, whose trend is a few degrees west of north. Numerous forms of the genus Productus are found here, but the only one that could be determined was P. semireticulatus. The Triassic beds are, in general, fine-grained, somewhat shalv and argillaceous, with both deep red and reddish-yellow beds. The Jurassic occupies a shallow depression at the base of the Dakota sandstone, which, in many localities, appears to form a hard capping to the more easily eroded marls below. Overlying the Dakota occur somewhat thin exposures of the lower dark clays, in turn overlaid by yellow and blue marls carrying immense numbers of the genus Ostrea, which, in places, almost makes up the rock. The upper dark plastic clays which carry numbers of Baculites ovatus would appear to be well developed, cropping out in low ridges and in occasional mounds and hillocks. Not far north of Horse Creek, the dark clays of the Fort Pierre division of the Colorado group are overlaid by a series of beds, which crop out in long narrow lines, rising only from 12 to 15 inches aboye the plain. They consist of yellowish-brown sandstones, with seams of arenaceous clay, and soft friable sandstone with ferruginous concretions, and some vegetable remains. On the geological map, these beds have been referred to the Fox Hill group, but they may possibly belong to the upper members of the Fort Pierre. The only fossils found were a number of species of the genus Ostrea ; but none of them have been specifically determined, and may not be characteristic of either formation. The beds strike considerably west of north, and dip 25° to 30° to the east. They occupy a very small area, deriving their chief interest from being, so far as observed, the highest members of the conformable series of pre-Tertiary strata east of the Lara- 4D@G 50 DESCRIPTIVE GEOLOGY. mie Ifills. A short distance east of here they pass under the Tertiary plains. Between Horse Creek and Wahlbach Spring, on the Cheyenne Pass road, the complete series of foot-hill strata are represented, presenting a rather uniform appearance. ‘The Palzeozoic rocks form high abrupt blutis, somewhat resembling those to the northward, less bold and. striking, but with the same cliff-like face to the eastward, having a dip angle from 55° to 65°. At the base of the cliffs, red clays and soils reveal the Triassic sand- stone, although they fail to show the unbroken continuity of well-defined strata that is exposed farther to the northward, while the Jurassic, which here would appear to be quite thin, is indicated by a slight depression in the formations, and a light clayey soil, capped by a light yellow sandstone. In the Colorado group, the subdivisions are not always well defined, while the upper, or Fort Pierre beds, frequently lie concealed beneath the Tertiary. Where the Fort Pierre beds are exposed, they have suttered considerable erosion, and have been cut through by numerous streams and ravines, which offer good sections of the dark carbonaceous shales and clays. Just north of Wahlbach Spring, the lower sandstones and limestones occur inclined at 15° to 20°, possessing a much lower angle than the same strata to the northward. The relations of the several horizons to each other and their structural features are shown in the upper section at the bottom of Map I, east half: in the section, however, the Niobrara Ter- tiary beds jut up against the Dakota sandstone. A short distance to the northeast, the overlying Tertiary strata have been eroded, and the yellow o immense chalky marls of the Colorado group are well exposed, carrying quantities of the genus Ostrea. Interstratified in the marls are thin layers, varying trom 6 inches to 3 or 4 feet, of bluish clay-slates, with some eypsum. Between the Cheyenne Pass road and the north branch of Crow Creek, a broad elevated table-mountain extends out from the main Archean range to the eastward. On the east side it rises abruptly in nearly perpendic- ular walls, for nearly 800 feet, while toward the main range it presents a blutf nearly as precipitous, being separated from the granites by a deep canon, which heads near the divide of Lodge Pole and Crow Creeks. This EASTERN FOOT-HILLS. Bl table-mountain consists entirely of sandstones and limestones, including probably the entire Paleeozoic series found along the east base of the range, but here lying in a nearly horizontal position. The cause of this somewhat singular occurrence of the lower sandstones and limestone may be found in an irregular mass of rough granite, lying to the eastward of the table, which has elevated them to their present position, a level nearly equal with the summit of the Archaan body. Upon this mass of Archean granite rest the later sedimentary rocks, dipping westward toward the main mountain-range, with an angle at their base of 8° to 10°; while on the opposite side of the table, the same beds dip 5° to 6° to the eastward, forming a slight synclinal fold or depression, with the beds of the summit lying nearly horizontal. In addition to this synclinal, the limestones at both the north and south ends possess a gentle inclination toward the centre, producing a basin-shaped top. Upon the table, but little soil re- mains; young trees were entirely absent, yet there were numbers of decay- ing and dead ones of older growth, which still remained standing. This is the only instance observed where any disconnected mass of Archean granite occurs, lying to the eastward of the sedimentary foot-hill formations, which accounts undoubtedly for this singular and exceptional structure of the strata. It is the only instance observed of a synclinal fold in Paleozoic strata east of the mountains, and it is exceedingly interesting in dynamical geology to notice how the entire series of lower sandstone and limestone has been elevated in a horizontal position, and in a manner so entirely at variance with the upheaval of the same formations everywhere else along the mountain-slope, for a distance of one hundred miles. In this upheaval, the Mesozoic formations do not appear to have taken part, and they are nowhere recognized in the immediate region of Table Mountain. Kast of the mountain, the structure is entirely obscured by either Tertiary or Quaternary deposits, which rest close up against the granite. It would appear highly probable that the comparatively low dip of the beds just north of Wahlbach Spring were in some way connected with this outlying body of granite, because a short distance still farther northward the beds occur at a high angle, and to the south of North Crow Creek, beyond the lower end of Table Mountain, they again exhibit a somewhat steep dip. 52 DESCRIPTIVE GEOLOGY, Between the north and south branches of Crow Creek, along the base of the range, the Paleozoic strata are the only rocks that occur well exposed, the later Tertiaries encroaching upon the upper members. Just south of the north branch occurs one of the high ridges or bluffs which is so characteristic of the Laramie Hills above Wahlbach Spring, the eastern face exposing the grayish-white Carboniterous limestone, which is always found at the top of the series, and may be so easily recognized for long distances across the plain. On the upper side of South Crow Creek, the beds of the Coal-Measure series stand nearly vertical, the lower rock, con- sisting of a hard compact conglomerate, separated from the red, dense granite by a short narrow canon. The Paleozoic rocks appear quite thin, and it would seem as if the lower members were wanting; at least, the con- glomerate closely resembles an interstratified layer high up among the Carboniferous limestones. In one or two localities, limestones may be seen lying in isolated patches directly upon the granite. Tertiary beds form the banks of Crow Creek Valley, extending close up to the Carboniferous rocks, resting directly against the conglomerate formation, and, as these upper Tertiary strata consist of a hard coarse sandstone almost a conglom- erate, the two sets of beds, lying nearly at right angles to each other, pre- sent a somewhat singular appearance. South of Crow Creek, the horizontal Tertiary rests directly on the Archzean. Crow Creek has worn out for itself a broad valley, which has cut down deeply into the Pliocene strata, in several places exposing remnants of Mesozoic formations, probably Cretaceous, but which lack distinctive characters sufficient to determine their horizon. A pure white, soft, friable rock, cropping out near the stream-bed, indicated a strike of north 35° east, with a dip of 20° east. Occasionally: the steep Pliocene bluffs along the valley will be cut by ravines and eullies, at right angles to the creek, formed by the washing-out of the soft sand rocks during heavy rains and torrents, leaving near the mountains the lower unconformable rocks exposed. In one of these ravines, Capt. A. L. Varney, U.S. A., procured some Rep- tilian remains, imbedded in hard, dusty-eray sandstone, which Prof. O. C: Marsh identified as belonging to Lesfosaurus, a genus of Mosasauroid Rep- EASTERN FOOT-HLLLS. 53 tiles, abundant in the Upper Creaceous sandstone, through the Smoky Hill country, in Kansas. But a short distance north of the railroad oceurs an outlying ridge of Carboniferous limestone and the lower red sandstones, with a strike a few degrees west of north and a dip 30° to the eastward. Here the lower red- dish-gray sandstones, with the red arenaceous limestone, are well developed, overlaid by massive blue limestone, passing up into light-eray arenaceous limestone. Along the immediate line of the railroad, the Pliocene formation extends up to the Archzean body, but directly south the same red sandstones and blue limestone, which occur to the north, come again to the surface, with a dip of only 20°. The limestone has been quite extensively quarried for burning in kilns, and is said to furnish a very good quality of lime, which formerly supplied Cheyenne, Greeley, and the neighboring towns. Coal-Measure fossils were found here: among them were identified— Productus cora. Athyris subtilita. Still farther southward, the Palaeozoic rocks oceur inclined against the Archeean gneisses at an angle of 8° to 10°. They appear to form an unbroken line nearly down to the old Denver and Laramie stage-road, presenting an abrupt wall to the main mountain mass, but falling off with gentle slopes toward the plain. The outlines of overlying formations are somewhat poorly defined, owing to the very irregular erosion of the Pliocene and accumula- tions of Quaternary deposits. South of the Wyoming and Colorado bound- ary-line, all the formations, including the Tox Hill beds, are well exposed, dipping at various angles; the Dakota Cretaceous capping the higher and more prominent ridges, and the Colorado Cretaceous, with the upper sand- stones, stretching far out upon the plain. The uplifts of Carboniferous limestone would appear to terminate near where Box Elder Creek turns and breaks through the sedimentary ridges, and from there southward the Triassic Red Beds occur lying next the Archean, 54 DESCRIPTIVE GEOLOGY. South of Box Elder Creek, the Triassic beds extend westward for 3 or 4 miles, in a broad, level table, occupying a depression in the Archean body, overlapping it in a nearly horizontal position, dipping not more than 2° toward the southeast. Lying next the granites occurs a bed of coarse, reddish-gray conglomerate, which underlies the bright red sandstone. It was not recognized to the north, but it is probably only a conglomerate bed in the Triassic, which here, owing to its position next the granite, appears somewhat coarser and more largely developed East of the Triassic table, the Upper Mesozoic formations dip with varying angles toward the Great Plains, showing considerable disturbance and displacement of strata. Where the stage-road enters the mountains, 3 or 4 miles north of Park’s Station, the Cretaceous and Jurassic beds are seen occupying a small re-entering bay or depression. On the inside of this bay, the Jurassic and Dakota Cretaceous curve around, so as to form nearly a semicircle, while the softer beds of the Colorado group fill the central portion. On the north side, the (e) Colorado beds strike north 45° to 50° east, with a dip 25° to 30° to the southwest; while, on the south side, they present an equally steep dip to the north, forming a sharp synclinal, with the strata contorted and pressed together in a very marked manner; the yellow and cream-colored marls forming so conspicuous a feature of the Cretaceous that the structure is easily made out. The light-colored chalky marls are here characterized by a great abundance of the genus Ostrea, associated with gypsum. From Park’s Station southward to the Cache la Poudre, the ridges present a very uniform appearance, with a strike approximately north and south, and a dip varying from 15° to 25° east; the Dakota Cretaceous forming an almost unbroken wall of hard reddish-brown quartzite. Along the base of this wall, all the subdivisions of the Colorado group may be recognized, especially as the Niobrara beds are well developed, although it would seem impossible to define either their lower or upper limits, as the calcareous beds, which are their characteristic lithological fea- ture, pass gradually into both the Fort Benton and Fort Pierre divisions. The Fort Benton and Niobrara beds at Park’s Station dip from 16° to 18° east, while the Fort Pierre clays to the eastward rapidly flatten out, indi- EASTERN FOOT-HILLS. DD cating dips of 6° to 8°. Near Park’s Station were found the following Colorado Cretaceous forms : TInoceramus problematicus. Inoceramus difformis. Inoceramus Barrabini. The lower geological section, which is given at the bottom of Map I, and crosses the Colorado Range and the Cretaceous plain, cuts the ridges nearly at right angles between 3 and 4 miles south of Park’s Station. Here the Red Beds form a well-marked sandstone ridge, with a dip of 20°. The Jurassic marls, with the included limestones, occupy a slight depression at the base of the more elevated Dakota Cretaceous, which here consists of a hard, compact sandstone, inclined also at 20° toward the plain, but present- ing an abrupt wall on the western side. The Colorado group oceupies a valley along the base of the Dakota wall, with the lower formation clearly represented. From the Cache la Poudre to the Big Thompson, a distance of 15 miles, the ridges: present considerable uniformity, both in structural and lithological characters. The general strike is a few degrees west of north, with an average dip from 18° to 22°; both the Red Beds and Da- kota sandstone maintaining the same average dip. Proceeding southward, the Red Beds would appear to increase gradually in thickness, and to dis- play more variety in color toward the top, with a loose friable texture. The valley, or depression, which has been already mentioned as frequently lying between the Triassic and Dakota sandstone, and occupied by the Jurassic marls, here becomes a marked topographical feature, arising in part, no doubt, from the sandy, easily-eroded summits of the Red Beds, and in part from the nature of the rigid quartzite wall of Dakota sand- stone. The Dakota beds here attain a development of nearly 300 feet, which is their maximum thickness east of the mountains. At the base of the Dakota the conglomerates, which form so persistent a feature, are exposed in a hard compact quartzite rock, which is overlaid by sandy saccharoidal beds, in turn overlaid by a second quartzite. All the divisions of the Colorado group crop out to the eastward, but with the same litho- > 56 DESCRIPTIVE GEOLOGY. logical and structural features that have already been noted north of La- porte. The bluish-gray argillaceous limestone at the base of the Niobrara. serves as an excellent geological horizon, as it crops out so characteristi- rally and is so persistent. A specimen of this rock, a fine-grained variety, in which were im- bedded numerous specimens of the genus Inoceramus, was submitted to chemical analysis, yielding— Carbonate tet tmesua Sa a. oe ee, ee 65. 93 the residue indicating a fine blue clay. The lower members of the Col- orado group maintain approximately the dip of the Dakota beds. An observation made just below Laporte gave, for the yellow chalk marls, an inclination of 16°, while the upper, or Fort Pierre beds, rapidly flatten out, and occur with as gentle a dipas3° to 5°. The upper member of the Fort Pierre beds consist largely of arenaceous material, which renders it difti- cult to separate them from Fox Hill beds. The line which has been fol- lowed, however, lies at the base of a long, low ridge of red ferruginous sandstone, which stretches far out upon the plain. In these lower beds, which have an earthy yellowish-brown color and loose texture, were found: Scaphites nodosus. Pinna restituta. Inoceramus Barrabini. Tnoceramus mecurvus. Callista Deweyi. Anisomyon sexsulcatus. Ostrea ? characteristic Fort Pierre fossils, which have likewise been found on the Upper Missouri River near the boundary line between the Fort Pierre and Fox Hill formations. Several miles before reaching the Big Thompson Creek, a long spur or ridge of Archwan schists projects out, with « slight angle to the south- west, away from the main body of the Colorado Range, becoming gradu- ally lower, finally passing away under the plain. As the great body of Archzean rocks still lies to the westward, this spur causes a sort of depres- SASTERN FOOT-HILLS. Sy sion, or bay, in the outline of the crystalline mass, around the lower end of which the sedimentary ridges curve somewhat sharply, following closely the Archsean boundary in all its details. In this way, the narrow spur forms an anticlinal axis, with sedimentary beds dipping away on both sides, On the east side, the beds maintain the same dip as found to the north- ward, averaging from 18° to 25°. Below the end of the Archean body, the sedimentary ridges extend, with an anticlinal structure, for some distance farther southward, the Dakota sandstone still forming the outer member of the upturned ridges. Near where the Dakota beds curve around to follow up along the west side of the Archean spur, they dip at 45°. The westerly- dipping beds have a general trend of north 35° 40! west, with all the forma- tions standing at a much higher angle than upon the east side, many beds reaching nearly a vertical position, with the Dakota sandstones standing out from the main ridge in a most prominent manner. About 2 miles north of the crossing of the Big Thompson, the Mesozoic formations again curve round, this time doubling upon themselves, and resting directly on the main Archean Range in a manner similar to the outer ridges next the plain, The beds thus dipping eastward from the main range form, with the westward side of the anticlinal fold, a broad synclinal valley within the bay or depres- sion in the Archean body, which at its broadest expanse measures nearly 38 miles across, gradually narrowing toward the north, until terminated by the coming together of the opposite sides of the fold in a continuous ridge. The Colorado Cretaceous, influenced by the abrupt bending of the underlying formations, circles around the Dakota sandstone, but, so far as observed, does not run up into the northern end of the synclinal, the Dakota forming the upper member exposed. At least, on the east side of the recess, Dakota beds occur standing nearly vertical, and on the west side inclined at a much gentler angle, with a considerable intervening space, so covered with accumulations of Quaternary material as to obscure all the underlying formations, concealing, if present, the Cretaceous marls and clays. Where the Big Thompson Creek cuts through the outer or anticlinal ridge, it affords a very good exposure across the Red Beds, Jurassic, and 58 DESCRIPTIVE GEOLOGY. Dakota Cretaceous. The Red Beds are characterized by heavy, coarse sandstones, of an intensely deep red color at the base, shading off into light yellowish-red near the top, both the Triassic and Jurassic carrying the lime-beds well developed, as limestone and gypsum. In the Triassic, calcareous layers, more or less mixed with sand, would appear to occupy a thickness of 50 feet; both the thin beds of limestone, which charac- terize the Red Beds in other places, being well developed, rising above the sandstone in low, regular ridges. The gypsum deposit, a nearly pure white bed, here attains a thickness of at least 25 feet, interstratified in dark red sandstone. In the Jurassic, the light gray cherty limestone, which is a persistent and well-marked feature in the horizon, crops out in a bed nearly 10 feet in width, both underlaid and overlaid by fine, light marls. The line of separation between Triassic and Jurassic, at best an arbi- trary one, based chiefly upon lithological distinctions, is, at the entrance to the Big Thompson Canon, somewhat difficult to follow, as the marly beds show a considerable vertical expansion, passing down gradually into the Red Beds. Cretacrous Priains or Cotorapo.—The clays of the Fort Pierre division of the Colorado group become more and more arenaceous toward the top, and, by imperceptible gradations, pass up into well-defined sand- stones of Upper Cretaceous strata. This great development of sandstone embraces both the Fox Hill and Laramie divisions. It covers by far the greater part of the area east of the Colorado Range, extending beyond the limits of our exploration both eastward and southward, forming the characteristic feature of the Great Plains. The lower member, the Fox Hill sandstone, lies directly upon the Fort Pierre clays, whose upper beds, flattening out to the eastward, are rarely seen inclined at a higher angle than 5° or 6°, with an average dip of 2° to 4°. The junction between the two formations is generally marked by a low ridge, 2 or 3 feet in height, presenting a mural face toward the mountains, but sloping off gently east- ward. This wall, which constitutes a somewhat striking physical feature, is formed by the soft clays having been eroded into shallow longitudinal valleys, and irregular depressions and basins, partially filled in with detrital accumulations, leaving the overlying harder sandstone at a slightly higher CRETACEOUS PLAINS OF COLORADO. 59 elevation. Although these sandstones form by no means an unbroken ridge, they may be traced without cifficulty from Box Elder Creek far to the southward of the Big Thompson. From the top of this low ridge, the Fox Till beds extend out upon the plains with considerable uniformity, falling away gently from the main range, and presenting a nearly level monoto- nous surface. Where best observed, they have in general a north and south strike, agreeing approximately with the conformable sedimentary ridges, which rest upon the Archean body. Along the immediate base of the range, they afford but little variety or interest in structural features, lying at low angles, nowhere deeply cut by drainage-courses, with the surface planed down, and generally covered by vegetation. Along the base of the Laramie Hills, the Fox Hill beds are buried beneath the unconformable Tertiary strata, and first reach the surface just below the boundary-line of Wyoming. Here the exposures are quite irreg- ular, owing to the uneven erosion of the overlying beds and a somewhat local disturbance of the strata; but south of where Box Elder Creek enters the plains, the lower members of the series may be recognized. Tn the low sandstone ridge mentioned above, organic remains, charac- teristic of the boundary between the Fort Pierre and Fox Hill divisions, may be collected in many localities.. They were especially abundant just east of Park’s Station, about a mile north of the Cache la Poudre, and between 3 and 4 miles southeast of Laporte. From the latter locality were collected numerous well-preserved specimens of the genus Inoceramus, including Inoceramus Barrabini, associated with Ammonites. The Fox Hill beds east of the Colorado Range are characterized through- out by great uniformity in texture and physical habit, and consist of a coarse sandstone formation, showing only variations in color from reddish- brown to reddish-yellow. They are usually massive, carrying hard, com- pact beds, interstratified between layers of a more friable nature, which, in the uppermost beds, offer exposures far less numerous than in the lower, and have therefore been less searched for organic remains. We have at the top of the series several hundred feet of strata, whose beds have furnished no fossils; at least to our parties. ‘The thickness of the Fox Hill strata is some- 60 DESCRIPTIVE GEOLOGY. what difficult to estimate, owing partly to the impossibility of determining its upper limit, and in part to the uncertainty of dip. The Fox Hill strata pass by imperceptible gradations into the Lar- ainie series, offering no well-defined line of separation, both formations from top to bottom consisting of coarse sandstone. It is difficult to eive a detailed description of either division which is not in some po- ‘sition in the series applicable to the other. In general, however, the two formations present some distinctions which are recognizable in the field over large areas. In the Fox Hill beds, the sandstones are usually more massive, with great uniformity of texture through a wide vertical range. The Laramie beds indicate more variety, with changes, within certain limits, from coarse to fine layers, frequently interstratified with seams of argillaceous sandstone and of pure clay. In color also, the beds show more changes from rusty-yellow to deep red, with well-marked layers of whitish-gray sandstone. Another characteristic is the occurrence of beds of loose quartz-grains, like beach-sand, lightly held together by some ferruginous cementing material. The Laramie formation possesses even a less dip than the Fox Hill, and over wide areas dips only from 14° to 2°; in many places it appears perfectly horizontal. In one or two localities, far to the eastward, the beds indicate a slight inclination to the westward toward the mountains, as if there had been a gentle oscillation in the move- ments of the beds producing a wave-like structure. Such a structure, however, was not clearly shown. The Fox Hill and Laramie formations taken together have been roughly estimated, in Northern Colorado, as measuring 3,000 feet in thickness, allowing some 1,500 feet to each. In the Laramie formation occur the great deposits of coal, which have proved of such great economic, value to Colorado, They form one of its most distinctive features, as in Northern Colorado, at least, no beds of coal have as yet been found in the Fox Hill beds. Through a wide vertical horizon, thin seams of coal and carbonaceous clays appear to crop out in the more elevated banks and ridges, but no attempt to give the numbers of such seams or their true positions in a section has as yet been made with any degree of accuracy. Within the limit of our survey, 10 coal deposits of any great value have as yet been opened, although a munber of tunnels CRETACEOUS PLAINS OF COLORADO. 61 have been run, and shafts sunk in several localities without success; all the important deposits of workable coal lie to the southward. Near what has been taken as the base of the Laramie formation oceurs a bed of friable red sandstone, which may be followed for a long distance by a low bank or ridge. In:this red sandstone, numerous beds of coal and carbonaceous clays outcrop, many of them having been more or less ex- plored. At the extreme northern limit of the Laramie formation, about 18 miles southwest from Cheyenne, and 5 or 6 miles west from Carr Station, on the Denver Pacific Railroad, in this same red sandstone, occurs one of the earliest discoveries of coal in Colorado. It derives some interest from its being the most northern occurrence of coal, before the beds pass under the Tertiary deposits. At this locality, the sandstones dip between 10° and 12° to the eastward. A tunnel has been run in on the coal for 100 feet. The seam of coal between the im- pure clays measures 3 feet. The following section was made along the bank, beginning at the top: 1. Reddish-yellow sandstone. 2 LER AC COUR OlAVice seit ae tain Ree vero. tinsel Ss Pte 3 feet. Ome DIUCICAY Ase ates tera Di hsus/ Ets Mcte ole teRore ayes ven aie yep ee vinveins cosa eerie Sie ee on 1 foot. a ee ease ee eee as tees Phd Ee mh Sea 3 feet. >. Black clay and impure Coal... .eccef Ciacne eset acavdncces oeccdactece.. 1 foot. 6. Red sandstone. In the sandstone overlying the coal may be collected large numbers of shells belonging to the genus Ostrea. The coal appears to be of poor quality, and the mine was soon aban- doned. It possesses a jet-black color, a somewhat resinous lustre, but crumbles readily on exposure to the air. Following this red sandstone southward, the coal outcroppings may be traced for a distance of several miles along a shallow longitudinal valley, which trends a few degrees west of south. Along the eastern ridge, usu- ally above the outcropping clay beds, the same shell of the genus Ostrea was found as already mentioned, but very poorly preserved. Still farther southward, the same sandstone rocks would appear fo gradually assume a less dip until they are seen, nearly west of Park’s Station, lying horizont- ally, terminating to the eastward in abrupt escarpments, overlooking a 62 DESCRIPTIVE GEOLOGY. nearly level grassy plain of Laramie sandstone. Still farther southward, the same beds appear to extend nearly to the Cache la Poudre. Along the valley of the river, and upon the adjoining banks, Quaternary beds con- ceal in a great measure the underlying rocks, but the same series of beds would appear to lie upon both sides of the stream. On Lone Tree Creek, near Carr’s Station, may be seen the grayish- white sandstone, which would appear to be characteristic of certain hori- zons in the Laramie formation. Along Lone Tree Creek, Owl Creek, and still farther eastward beyond the limits of our map, beds of undoubted Lara- mie age form the surface of the plains. In many localities, beds of argil- laceous sandstones and carbonaceous clays, with indications of coal seams, crop out; while on Crow Creek, between 4 and 5 miles from its mouth, where it empties into the South Platte, occurs a thin stratum of coal em- bedded in reddish-gray sandstone, which lies approximately horizontal. So far as known, at the time our parties visited this country, in the summer of 1872, no shaft had been sunk in explorations for coal. Since that time, however, numerous openings have been made, which have cut coal seams varying from 2 to 3 feet in thickness, almost always lying nearly horizontal ; but in most eases, the shafts have been abandoned, the coal, besides heing a thin seam, having proved to be of little practical value. All the specimens of coal which were collected in northern Colorado, from the area under examination, and those which were shown to us from the immediate region, presented the same general characters, mostly jet black, with a bright lustre, when pure, and free from clay. Many of them carry thin films of pyrites. All of them would appear to crumble readily when exposed to atmospheric agencies. In regard to the paleontological evidences of the Laramie formation, it may be said that none of the characteristic types of the Fox Hill beds, so abundant all along the base of the mountains, were found by our parties in the upper sandstones. But at several localities forms of marine inver- tebrata, which Prof. F. B. Meek has pronounced as undoubtedly of Cre- taceous age, were obtained from a friable red sandstone which forms the overlying stratum of the Plains. They were found in the high bluff which CRETACEOUS PLAINS OF COLORADO. 63 lies between the Cache la Poudre and Big Thompson, west of Greeley and Evans; the most westerly occurrence being some 8 miles west of the former town in a bed which dips about 1° to the eastward. They also occur on Lone Tree Creek. On Crow Creek, about 4 miles above the mouth, and between the two streams, several miles north of the South Platte. The following species have been identified : Avicula Nebrascana. Avicula cancellata. Cardium speciosum. Mactra Warrenana. Nucula planimarginata. In addition to the above, Prof. J. J. Stevenson' has since obtained from near Evans and Platteville, just southeast from the map, the following species : Ammonites lobatus. Mactra alta. Anchura — ? Still later, members of Dr. Hayden’s® corps have visited the region and collected most of the above species. It is evident that this group of fossils which occurs together in so many localities within such a limited area, in beds of precisely the same physical habit and composition, and lying approximately level, belong to the same geographical horizon. Indeed, the beds may be traced without much difficulty along the Big Thompson and Cache la Poudre Valleys, and then eastward up the valleys of the northern tributaries to the South Platte. These sandstones form the exposed banks along Crow and Lone Tree Creeks, and may be traced northward, passing under the Tertiary of Chalk Bhufts. No organic remains were obtained from the beds in close proximity to carbonaceous clays and coal-layers east of the Denver Pacific Railroad, but the structural relations of the beds is such that there can be no doubt that the thin layers of coal occur under, or rather interstratified in, the red sandstones, with well-defined Cretaceous marine invertebrata overlying 1The Geological Relations of the Lignitic Groups, by J. J. Stevenson, 1875. 2United States Geological and Geographical Survey of Colorado, Washington, 1876. 64 DESCRIPTIVE GEOLOGY. them. Although the Laramie beds in the extreme northern portion of Colorado have as yet furnished no coal deposits of great economic value, the region is of considerable geological interest from the occurrence of Cretaceous types lying in such close relation with the coal strata. Beyond the limit of the map, the Laramie beds were traced. southward, and were found underlying the plains at Denver, and to include the valuable coal deposits at Erie, and the Marshall and Murphy mines north of Golden, extending from within one-half mile of the base of the range far out upon the Plains into Eastern Colorado. The Laramie beds form the uppermost members of the great series of conformable strata that lie upturned against the Archzean mass of the Rocky Mountains; all overlying strata resting unconformably upon the older rocks. South of Chalk Bluffs such beds cover very subordinate areas, and are of but little geological importance. They are always found lying either horizontally or upon some sloping bed of deposition, and fre- quently occur filling depressions, the results of erosion in older formations. Along the immediate base of the range, usually just outside the Dakota sandstone, occur isolated patches of irregular terraces and benches, which consist of coarse gravel and smooth rounded boulders held together by ferruginous sands. To the southward, they are much better developed than in the extreme northern part of Colorado. In the region of the Big Thompson, they form a local but well marked feature, appearing on both sides of the valley, having been cut through by the stream. They reach a development of about 200 feet in thickness, extending southward as far as the Saint Vrain’s Creek. There is no positive evidence as to age of these terraces, and they may be either Pliocene-Tertiary or Quaternary ; but, from their close resemblance to beds east of'the Laramie Hills and to others westward on the Wyoming Plains, they have been referred provisionally to the Wyoming Conglomerate, the latest Pliocene beds of the Rocky Mountains. Out upon the Plains, away from the Colorado Range, the Lara- mie sandstones are in places concealed by heavy accumulations of coarse detrital material of Quaternary age, which consist of finer material than the well-marked terraces, but would appear to be largely derived from their decomposition. Such accumulations form quite a prominent feature in the TERTIARY PLAINS OF WYOMING. 695 region of the South Platte, and on the low blufis between Evans and Greeley, and, indeed, in all the shallow basins worn out of the Cretaceous rocks. They lie scattered over the plains in irregular detached beds, and are not represented on the geological maps. Tertiary Puains or Wyominc.—Between 5 and 6 miles south of the forty-first parallel, the Fox Hill and Laramie Cretaceous pass beneath the nearly horizontal beds of Tertiary strata. Abundant evidence has been furnished that these deposits are, without doubt, the sediments, filling the basins of the ancient fresh-water lakes, which formed so characteristic a fea- ture of the Rocky Mountains through Tertiary times. Hast of the Rocky Mountains, these formations cover a very large area. From Chalk Bluffs they extend completely across the Territory of Wyoming, having been traced far northward into Dakota. To the eastward, they stretch far out upon the Nebraska Plains, their boundary in that direction having never been carefully determined; while westward the Colorado Range formed an effectual barrier. Within the area of our survey occur both Miocene and PHocene strata, with paleontological evidence of their age. To the beds of the Miocene lake-basin, the name, White River, first used by Dr. Hayden, has been applied, while, for the overlying Pliocene strata, the name Niobrara, first suggested by Prof. O. C. Marsh, has been retained. Waitt River Miocenr.—As the White River formation is overlaid by the Niobrara, the outcrops of the former are necessarily somewhat limited, and the difficulty of tracing the outlines of the ancient Miocene lake is greatly enhanced, Within the area of our map, the only opportunity for studying the White River strata is to be found along the south and east face of Chalk Bluffs, where they protrude from beneath Pliocene beds. They rest unconformably upon the Laramie sandstones, and although the Cretaceous lies essentially horizontal, it has undergone considerable erosion before the deposition of the Tertiary, as the latter is found filling the uneven surfaces of the former. The Chalk Bluffs are but the abrupt terminations to the south and east of the Tertiary strata, which rise from the Cretaceous Plains by receding benches and terraces some 700 feet in height. From Cheyenne southward, the surface of the country is nearly level until within a short distance of the VE DAG 66 DESCRIPTIVE GEOLOGY. bluffs, where the country begins to fall off in low, gentle benches of Pliocene strata, overlying the more abrupt cliffs of Miocene which rest directly upon the Laramie sandstones. Along the face of Chalk Bluffs, numerous small streams, ravines, and gullies have worn out good exposures in the soft beds, producing a rugged, uneven surface of clays, marls, and sands. From an examination of the country to the west of Chalk Bluffs, it seems evident that, while the Pliocene shore extended close up to within a short dis- tance of the Archzean foundation of the Colorado Range, the waters of the Mio- cene lake, which undoubtedly stood at a much lower level, were hemmed in farther out upon the plains by the gently-rising Mesozoic beds, the beach-line being formed upon the Laramie formation. On the other hand, the Pliocene beds are found resting uncontormably upon all the divisions of the Creta- ceous, Jurassic, Triassic, and Carboniferous. The Miocene strata are exposed from near Carr’s Station on the Den- ver Pacific Railroad, eastward across Owl Creek, the tributaries of Crow Creek, and stretch beyond our eastern boundary. Between the Laramie sandstones below and clearly-defined Pliocene beds above, the Miocene presents a development of nearly 300 feet in thick- ness, With an altitude for the basin of about 5,800 feet above sea-level. The line separating the Miocene and Pliocene lake-basins is, however, a difficult one to determine, as both formations lie horizontally, and consist of beds with much the same lithological composition, with but little evidences of erosion between the two series. So far as the Chalk Bluffs are concerned, the line of junction between the two basins rests almost entirely upon paleontological evidence. In general, the Miocene lake strata, in their lithological habit, are char- acterized by a rapidly-changing composition, with a considerable variety in texture, color, and compactness of beds, and are noticeable, in distinction from the Pliocene, for the fineness of sediment and absence of heavy layers of coarse, sandy material. They consist of alternating layers of marls and clays, interstratified with thin beds of fine sand, loosely held together with- out any cementing material. Occasionally, beds will be more compact from the presence of ferruginous layers, which make well-defined horizons. In TERTIARY PLAINS OF WYOMING. 67 color, the beds are all of light shades; the marls, which form the most promi- nent feature in the cliffs, suggesting the name of Chalk Bluffs. From the exposures in the Chalk Bluffs, along the tributaries of Crow and Little Crow Creeks, Prof. O. C. Marsh has made large collections of vertebrate fossils, now in the museum of Yale College, which abundantly prove the Miocene age of the lower beds. Among the more important of these vertebrate remains obtained from this locality may be mentioned the following: Brontotherium ingens, Marsh ; Brontotheriam gigas, Marsh ; two huge mammals, nearly as large as an elephant, and exceeding in size any other extinct animal from this formation. These animals belong to an entirely new family, which Professor Marsh has designated the Brontotheride. They were as large as the Dinocerata, which characterize the Eocene lake- g. ‘They are most nearly ‘related to the rhinoceros, but co) basins of Wyomin show some characters that indicate close affinities with the elephant. Another large mammal, described from this region by Professor Marsh, was Llotherium crassum, also about the size of a rhinoceros. In these same Miocene beds were also obtained many small mammals, carnivores, rodents, insectivores, and a few fragments of birds. Nroprara Piiocense.—The upper 300 or 400 feet of the Chalk Bluffs consist of Pliocene strata, which cover the entire area of our map east of the Laramie Hills. The surface of the country is undulating, but generally so smooth and covered with loose soil as to present but few good exposures, except along the valleys of numerous streams and on the benches high up near the mountains. The valleys of Crow, Lodge Pole, Horse, and the Chugwater Creeks cut deeply into the underlying strata; the bluffs upon both sides of the streams presenting long continuous walls of bare rocks, from 100 to 200 feet in height. These streams, whose valleys vary from 4 to 4 miles in width, usually erode through the soft strata till they reach some bed of hard, impervious clay, which forms their bottom. The surface of the Pliocene lake-basin, along the immediate base of the range, had an altitude above sea-level of at least 7,000 feet, and perhaps from 100 to 200 68 DESCRIPTIVE GEOLOGY. feet more. For the entire thickness of the deposit, the beds would indicate from 1,200 to 1,500 feet. As already mentioned, the beds are found lying unconformably upon the older uplifted strata and overlapping the area of the Miocene basin. South of the Union Pacific Railroad, they occur abutting against Mesozoic formations; just north of Granite Canon, they lie next the Archean mass; and a short distance beyond, at the mouth of Crow Creek Canon, are found essentially horizontal against nearly vertical Paleeozoic limestones. From Crow Creek northward, as indicated upon the geological map, they may be seen resting directly upon every formation, from the Archzan to the Fox Hill Cretaceous. In general, the lithological aspect of the Niobrara Tertiary strata closely resembles that of the Miocene basin, as already described ; but from the much greater area over which they occur, and from the many widely separated exposures, affords peculiarities not observed in the limited outcrop of the latter. The most prominent feature in the Pliocene, in dis- tinction from the underlying formation, is the prevailing arenaceous nature of the beds, and the greater persistency in character, through a wider verti- cal range. Marls, clays, coarse and fine sandstones, conglomerates, with some nearly pure limestones, make up the strata. Of these, fine, marly sandstones are the predominant beds, while light-colored limestones are exceptional, and would appear to occupy only small areas, probably lentie- war masses, filling shallow basins and depressions. Over the greater part of these Tertiary plains, the upper layers are made up of coarse, hard sandstones, which near the mountains may be called a conglomerate, firmly held together by a fine cementing material, forming a sort of concrete. They consist of fine and coarse pebbles, erys- tals of quartz, and feldspar; chiefly material from the Archzean granites and gneisses, varying in size from a pea up to a foot in diameter. The upper stratum withstands atmospheric agencies remarkably well, and serves to protect the more easily eroded beds beneath. The character of these over- lying conglomerates is well shown along the gently-sloping bench upon which the railroad reaches the mountains, and along the bluffs upon both sides of South Crow Creek, where it not infrequently overhangs, eaves- TERTIARY PLAINS OF WYOMING. 69 fashion, the softer rocks below, which have been washed out by the driving rains and storms. A peculiarity of this upper fine conglomerate is a ten- dency to split up into blocks and slabs from 6 to 12 inches in thickness. Near the mountains, these upper beds lie inclined at from 1$° to 25°, form- ing a most excellent railway-grade. Underlying the fine conglomerate along the railway-bench, and in the valley of Crow Creek, occurs another characteristic bed of the Pliocene basin, which consists of an exceedingly fine, almost impalpable, arenaceous marl, of a light cream color, and free from all inclosed pebbles, which mark both the underlying and overlying strata. Out upon the Plains, the upper stratum is usually somewhat finer than near the mountains, but of essentially the same nature—light-gray and ash-colored rocks, consisting of sands and Archzean pebbles. Below these are alternating beds of marls, clays, and calcareous grits, with thin seams of mud-rocks and belts of fine sand. South of the railroad and west of Chalk Bluffs, the country falls off rapidly, and has undergone a very considerable amount of erosion, carrying away a great part of the Tertiary strata, and leaving a broken, irregular surface, with isolated hills and benches of Pliocene beds, which, where not covered by Quaternary detritus, offer exposures of considerable vertical thickness, but without any special geological interest. Erosion has, in several places, so worn away the overlying Tertiaries within the area colored on the map as Pliocene, between Box Elder and Lone Tree Creeks, as to expose, in a fev localities, isolated patches of the underlying unconforma- ble Cretaceous sandstones. About 12 miles south of Cheyenne, and 3 or 4 miles northwest from Carr’s Station, the effect of erosion upon these horizontal strata is well shown at the ‘Natural Forts”, where the reddish- yellow sandstone is curiously worn away, leaving walls well arranged for purposes of defence. About 5 miles south of Cheyenne occurs a develop- ment of the light cream-colored limestone of the Pliocene basin. As it weathers slowly, it stands above the friable sands that here form the top of the plateau, and appears above the surface in a line of low, conical hills. It has a hard, cherty fracture, and is traversed by thin seams of quartz, which give the rock a somewhat more siliceous appearance than chemical analysis would indicate. Attempts have been made to burn it for lime, 70 DESCRIPTIVE GEOLOGY. but they were soon abandoned, the lime made from the Carboniferous rocks at Granite Canon proving much more desirable. An analysis of the Cheyenne limestone yielded Mr. R. W. Woodward the following : SOLIS, Soe ty eee eee ee eee ee eee 1.49 1.52 Kertietoxide<*20 4h eee eee ay 0.31 Maricanous oxide 2-0) eee ee 0.15 0.15 bith Gop oeve ee a eees eek, ee eee ee ees 54.16 54.18 Moomiesig... = etarcee neces er ee 0.15 0.15 Carbonic acid and water.... --......-.-- 43.68 43.69 100.00 100.00 North of Crow Creek, along the bluffs of Lodge Pole and Horse Creeks, the beds retain much the characteristics of those already described, being light-colored, fine-grained sediments, largely arenaceous, with beds of marls and calcareous grits. The prevailing color of the sandstone is either ash or lavender, many of the beds suggesting fine pumice. ve Pole and Chugwater Creeks, both Po) Near the mountains, between Lod the Tertiary and’ Cretaceous rocks have undergone considerable erosion, producing longitudinal valleys shut in by the Laramie Hills on one side and abrupt walls of Pliocene Tertiary on the other. North of Horse Creek these walls, which are quite regular, and have been designated as Shelter Blutis, rise from 250 to 300 feet in height. They consist of the same light- colored marls as found wpon Crow Creek. In places they are filled with small concretions of the same material, although harder, and carry thin beds of sandstone. The Chugwater is, perhaps, the most characteristic valley, which cuts through the Tertiary basin, the stream running for over 50 miles through a nearly continuous line of bluffs, offering exposures of marls, grits, and clays capped by the hard sandstones. Every few hundred yards the blufts are cut at right angles by ravines and gullies, which offer considerable diversity in form and outline, suggesting the ‘‘ Mauvaises Terres” of Dakota. All through the marls and calcareous grits, but more especially abund- ant in the blutfs along the Chugwater, occur irregular segregations and 5 TERTIARY PLAINS OF WYOMING. ree thin lenticular seams of a jasper-like rock. This rock is traversed by nar- row lines of calcareous spar and silica. It possesses a conchoidal fracture, a homogeneous texture, and in color is always some light shade of gray, green, or brown. In many localities, it is penetrated by dendritic forms of iron or manganese. Moss-agates are found scattered through the marls in small fragments and chips, which have been eagerly searched for by the inhabitants. In one or two localities, they occur so abundantly as to form an article of commerce. They were purchased by a firm of manufacturing jewellers, in Cheyenne, at a fixed price per pound, from which they obtained large numbers sufficiently clear and perfect to polish for ornamental stones. Most of the moss-agates which we see worn are obtained from the Tertiary plains of Wyoming. Many of them show a great variety of colors, with beautiful opalescent tints, and are frequently very rich in the enclosed den- dritie forms of iron and manganese. Fine specimens have been found on the Chugwater, near the crossing, and again on Horse Creek, 12 miles east of the Military Road. Silicified wood is also scattered through the Pliocene basin, but is a less marked feature than in the Tertiary beds of Nevada. As regards the life of the Niobrara basin, invertebrate remains would appear to berare; at least, none were found by our parties. But of verte- brate fossils, Prof. O. C. Marsh has made large collections from numerous localities, especially from the upper beds at Chalk Bluffs, many of the species being identical with those found in the same formation from the ““Mauvaises Terres”. Among the more important species obtained by Professor Marsh may be mentioned the following: Mastodon murificus, Leidy. Elephas imperator, L. Protohippus parvulus, Marsh. Pliohippus, Hipparion. These forms show a warm temperate climate Wyoming Conetomerate.—Clearly overlying the Pliocene- lake- deposits are found, along the base of the range, beds of both coarse and fine conglomerate, which hold a somewhat doubtful geological position. te: DESCRIPTIVE GEOLOGY. They are recognized only on the Sybille Creek and its tributaries, and in the region of the Chugwater and Pebble Creeks; localities where the Pale- ozoic and Mesozoic strata are wanting, the Tertiary formations abutting directly against the Archzean body. They are best developed on the Sybille, where they reach a thickness of 300 or 400 feet, made up largely of Archean material, from rocks found in the Laramie Hills. The forma- tion is cut through by numerous streams, which give it a bench-like terrace formation. ‘That they overlie the Niobrara basin there can be no doubt, and at the same time are older than the loose Quaternary detritus that covers the plains all along the base of the range. Upon the geological map, they have been referred, along with somewhat similar beds on the Big Thompson, although at the latter locality they are not so well developed, to the Wyoming Conglomerate of the Green River basin. LARAMIE PLAINS. 73 SEOrLown Li. LARAMIE PLAINS BY ARNOLD HAGUE. PuysicaL Descripriox.—To the westward of the Laramie Hills extends a broad expanse of open, nearly level country, known as the Laramie Plains, oceupying the depressed area between these hills on the one side and the Medicine Bow Range on the other; the former a comparatively low uniform ridge, reaching scarcely more than 1,500 feet above the plain; the latter a high mountain mass rising between 3,000 and 4,000 feet. On the south, the plains are shut in near the forty-first parallel, the boundary between Colorado and Wyoming, by the coming together of the Colorado and Medicine Bow Ranges, and to the north by the Rattlesnake Hills, an irregular mountain group beyond the limits of this exploration. To the northwest, however, the plains are not entirely rimmed in, the open country stretching for a long distance without any marked geographical boundary. For most purposes, however, it will be well to regard the western boundary of the plains as limited by the Como Ridge, just north of the Medicine Bow Range, and situated at the extreme northwestern corner of the east half of Map I. As thus defined, the Laramie Plains measure at least 80 miles in length by about 30 miles in width. In their broader general features, these plains bear many points of resemblance to the areas of depression along the Archzean Ranges of the Rocky Mountains, which stretch across Colorado southward as far as New Mexico, and have been designated the parks. In the case of the plains, they cover a somewhat larger extent of country, and are not in so marked a manner completely enclosed by high rugged mountains. The Laramie Plains have an average elevation of 7,000 feet above sea-level, varying from 6,800 to 7,300 feet. The town of Laramie, on the 74 DESCRIPTIVE GEOLOGY. line of the railroad, has an elevation of 7,143 feet, or a little more than 1,000 feet above Cheyenne, near the east base of the Colorado Range. The surface of the country is undulating, with slopes so gentle, and with such rounded outlines, that to the eye the greater part of the area seems practically level, and appears only to be broken by long bench-like ridges and oceasional patches of sandstone, which have escaped the general erosion. Over the greater part of the area there is no elevation that could possibly be called a mountain or butte, only hilly ridges 100 or 200 feet above the general level. The Laramie Plains are drained almost exclusively by the Laramie River, which, rising high up in the Medicine Bow Range, enters the plains near Sheep Mountain, flows a little north of east until it reaches the lowest part of the valley on the eastern side, then near Fort Sanders turns and runs north, with a very circuitous course, till beyond the limits of our map. No stream joins the river from the east, and from the south only two creeks, Willow and Antelope, reach the main stream. Numerous streams come down from the Medicine Bow Range on the west, but the Little Laramie, with its many tributaries, alone empties into the main river, the others either ending in Jakes or sinking in sands, except in seasons of long-con- tinued rains. The Laramie River is a fine, clear stream, with a rapid cur- rent and a broad alluvial bottom covered with grass. Rock Creek, at the northern end of the Medicine Bow Range, after running out upon the plain, suddenly turns, flows westward around the oD Como ridge, and joins the Medicine Bow River; the only stream that does not drain toward the Laramie River. . Dotted ever the surface of the plain occur numerous small lakes, lying in shallow basins in the nearly horizontal sandstones. Many of them are fresh-water lakes, or only slightly brackish; while others, especially the smaller ones, are strongly alkaline, and, in dry seasons, completely disap- pear, leaving incrustations of salines, admixtures of carbonates and sul- phates. Deposits of these salts also occur on the plains, which may prove to be of considerable thickness and of economic value. The largest of the lakes is known as Cooper's Lake, lying in about the middle of the plains, just oO oD west of the railroad. It measures nearly 4 miles in length by 2 in width. LARAMIE PLAINS fi) Trees are wanting over the entire area, except along the broad valleys of streams; but the ridges, benches, and plain are well covered with a luxuriant growth of grass, offering fine grazing land. GenEeraL Grotocy.—In their geological aspect, the Laramie Plains are essentially a Cretaceous formation. All the beds of the Cretaceous recog- nized in other localities are represented, from the Dakota well up into the Fox Hill sandstones. The Colorado group, however, covers the greater part of the area, lying in a nearly horizontal position. Along the flanks of the ranges, the entire series of conformable strata exposed in the uplifted ridges east of the mountains occurs bordering the plains, resting unconformably upon the Archean masses at varying angles, but always flattening out toward the valley. Rising above the plain, on the eastern side, occurs the broad belt of Paleozoic strata, which forms the western side of the anticlinal fold, already mentioned as passing over the Laramie Hills. From beyond the northern limits of our map, southward for 55 miles, these Paleozoic beds extend with an unbroken continuity of strata nearly to Harney Station, on the line of the railroad, where they are overlapped by Red Beds, which, from here southward, rest directly on the granite. No- where along the belt of the Fortieth Parallel Survey do the Carboniferous limestones present for so long a distance so uniform a ridge, with as little exhibition of marked flexures or folds, as along the west side of the Laramie Ilills. The strata everywhere dip with great regularity at gentle angles; the highest observed dip being 12°, with an average inclination from 5° to §°. High up on the hills, next the Archeean rocks, the lower red sandstones occur, forming a low wall with a steep face toward the older formation, and sloping gently westward. The surface is usually so worn down, and the inclination of beds so slight, that the ridge effers but few localities for obtaining sections through the entire series. Only in one or two places have the limestones been cut by deep cations, exposing any very great thickness of strata; the cafons just back of Laramie City and Fort Sanders probably presenting the best exposures, with abrupt walls several hundred feet in thickness. The Palseozoic series, on both sides of the Laramie Ills, show very great similarity in lithological habit. At the base occurs the coarse red sandstone, more or less compact, made up of fine quartz-grains 76 DESCRIPTIVE GEOLOGY. and angular pebbles, in places passing into a hard conglomerate; but none of the beds observed exhibit the tendency to form quartzite, as on the oppo- site side of the range. The lower sandstones pass up into granular arena- ceous limestone, with a reddish-yellow color, intercalated with layers of lighter shade, and in turn overlaid by heavy massive blue limestene, reach- ing to the top of the series. Probably, the narrow layers of sand-rock and conglomerate, which may be detected in the blue limestone along the east base of the range, occur here, but the opportunities offered for observing such details are by no means as good. In thickness, the Paleozoic beds west of the hills develop not far from 1,200 feet of sandstone and limestone strata. All paleontological evidence obtained from these beds would tend to show that they belonged to the Coal-Measure limestones. No fossils were found in the upper members of the series, as on the east side; but, on the other hand, in several localities, within 200 or 300 feet of the base, charac- teristic Coal-Measure forms are abundant. Near where the road which crosses the range, following up Sybille Creck from the east, passes over the Carboniferous limestone upon the west side, the beds which are found dipping from 5° to 7° consist of a hard, moderately fine-grained, bluish-gray limestone, carrying through a vertical range of 200 or 300 feet, and reaching at least within 200 feet of the base, the following Carboniferous species: Productus Prattenianus. Productus costatus. : Athyris subtilita. : Near the top of the Cheyenne Pass, in a very similar-looking limestone, and doubtless the same horizon, were found— Productus semireticulatus. Productus cora. Athyris subtilita. Bellerophon ———? Orthoceras ————? Still farther southward, some 5 miles northwest of Sherman, the lower LARAMIB PLAINS. art members of the series are well exposed, the lower red sandstone being quite characteristically but thinly developed, and overlaid by a slightly reddish- gray limestone, possessing a strike of north 20 east, and a dip from 7° to 9° west. Near the base of this limestone were procured the following : Productus Prattenianus, Productus cora. Along the gentle slope of the plain, directly overlying the Carboniferous limestone, occur the Red Beds of the Triassic. The junction between the two formations is not always so readily traced as east of the mountain, the surface-rock being generally concealed by loose soil, and is frequently only to be followed by the distinctly reddish tinge given to the gravel by the overlying sandstone. In a few localities, red sandstone beds stand out prominently in bench-like formations or bluffs, but usually appear planed down, conforming with other strata in the gentle slope toward the bottom of the valley, lying inclined from 4° to a perfectly horizontal position, and offering but few good exposures. The best exposures of the Red Beds, including, as well, those of the Jurassic group, may be seen near Red Buttes on the railroad where the characteristic and peculiar forms produced by erosion on sandstones of horizontal Triassic beds has suggested the name for the station. Over a considerable area, immediately east of the station, sandstones, marls, and clays have been eroded into isolated hills and ridges, exposing the strata in many places for over 190 feet in nearly vertical walls, the heavier and more compact layers preserving the softer and more friable ones beneath from being worn down by atmospheric agencies. Near Red Buttes, the upper members of the Carboniferous are bluish-gray limestones, while the lowest red sandstones lack the intense color usually observed at the base of the series. They have a reddish-yellow tinge, with concentric spots of brick-red, and show more or less of a shaly structure. In compo- sition, they appear to consist of fine grains of quartz, held together by a cementing material of carbonate of lime. All the beds are fine-grained, with occasional layers carrying coarser pebbles, and varying in color, but always exhibiting some shade of red, even up to the top of the Jurassic, the two formations being here very difficult to separate by the lithological 78 DESCRIPTIVE GEOLOGY. distinctions observed elsewhere. In the upper part of the series, the most characteristic Jurassic limestone has a flesh-red color, with a uniform texture mixed with considerable fine, angular grains of sand. No well-defined organic remains were found, but the rock shows indistinct fossiliferous frag- ments. This limestone was subjected to chemical analysis by Mr. B. E. Brewster, with the following result : SHE Ca oe Dice te creak ee OR Oe Ge een BO rede 22.218 Ferric oxide and alumina... 2.2205 25.9-.-.2--- 0. 210 (sive Giese oye. hore Se ee ee eee 43, 237 Maonesiie Ss 2: coc Malte ee ome ee eee 0.150 Carbonivracid: <2 ota ® = eee See Bor oat LA eee na oe eee RA Ee a 0. 140 99. 890 @anbonateron Lite a= eee 76. 748 Varbonate Ol Mmeaonesias 4 2.2.52 eee sea oe Ona cil Along the east side of the plains, the Dakota sandstones may at times be recognized, cropping out above the accumulations of Quaternary débris, sufficiently well defined to trace their horizon. At the north, the beds appear to incline westward, with an angle of 2° to 3°, but become hori- zontal to the south. They crop out just east of Lake Ione, and, so far as observed, are found only on the east side of the Laramie River till within 6 or 7 miles of Laramie City, which they appear to underlie as well as Fort Sanders, and to form the banks on both sides of the river; on the west side rising above the stream in a low rounded ridge, sloping west- ward. The rock is a yellowish-brown, medium-grained sandstone, with some sarbonaceous material scattered through the upper beds. South of the railroad, the Triassic, Jurassic, and Dakota Cretaceous, following the boundary of the Archzean, curve around to the southwest, lying inclined at not more than from 1° to 4° toward the plain. The Tyi- assic Red Beds rest directly on and overlap the granites, jutting up against oD LARAMIE PLAINS. 79 the steeper slopes. All the beds fall away gently, with an occasional ridge formed by a layer of hard sandstone, capping softer beds, which presents a slight escarpment toward the mountain, and through which the streams have cut their channels, exposing portions of the strata. On Antelope Creek, the gypsum deposits are well shown, just north of the Willow Creek and North Park Road, interstratified in dark red sandstone. South of the road, about one mile, bordering the creek on both sides, the light-colored Triassic strata have been eroded into the most fantastic and fanciful forms, which very readily suggest the name, which has been applied to them, of the “ Ruined City”. They are neither as prominent, nor cover so large an area, as the similar formation at Red Buttes, but, on the other hand, are much more compact, with great diversity of outline, and suggest a variety of architectural designs. A short distance west of Antelope Creek, the Triassic rocks occupy a re-entering angle, or depression, in the Archean body, extending some twelve miles to the southward of the Colorado and Wyoming boundary, and westward to the Laramie River. Along the east side of the river is a nar- row strip of Archean rocks, upon which the Triassic beds rest. Here the Red Beds, and probably the overlying Jurassic marls and limestones, are well developed, exhibiting a series of nearly horizontal strata, between 1,000 and 1,200 feet in thickness, with perhaps a very slight inclination toward the north and northwest, the beds dipping away from the Archean body. Facing the plains, the red sandstone presents an abrupt wall from 600 to 1,000 feet in height, showing in a marked manner the variegated beds, with the interstratified marls and limestones; while, on the east side, Sand Creek, which rises on the top of the plateau, cuts a deep, narrow canon through the Red-Bed formation. ‘These exposures exhibit a development of Triassic beds at least 1,000 feet in thickness, and are overlaid by the light-colored Jurassic strata, forming the summit of the plateau. Resting upon the Archeean body, the lowest beds consist of coarse, fri- able, ash-colored sandstone, with pebbles and angular fragments, over- laid by a thin bed of hard, cherty limestone, which is, in turn, overlaid by reddish-gray sandstone. Above this comes the usual persistent red sand- stone, passing up into clayey beds, more or less shaly, developing into a 80 DESCRIPTIVE GEOLOGY. series of argillaceous sandstones, with thin layers of fine red clay and beds of gypsum, the latter varying in thickness from two or three inches up to several feet; the thickest deposit of pure solid gvpsum, measuring 22 feet in width, lying between two beds of hard red sandstone. Nowhere in the region of the Rocky Mountains, within the belt of this exploration, are the gypsiferous deposits so well developed, at the same time so easily accessible from the railroad. Overlying the gypsum series, again occur the red sand- stones, passing up into yellowish-red friable beds, shading off into white sand- stones and marls, which may represent the base of the Jurassic series. Above the white sandstone occurs a bed of fine bluish-gray cherty limestone, which crops out prominently, and probably represents the stratum which is so persistent east of the Colorado Range; this, in turn, is overlaid by white and cream-colored sandstones and marls, passing into brownish beds, with layers of variegated clays and marls, and again overlaid by fri- able white sandstone, which appears to form, in many places, the summit of the plateau. The following section represents more clearly the succession of strata, with estimated thicknesses, beginning with the Jurassic on the summit of the plateau, and descending to the base of the Triassic, which rests on Archean rocks : Jurassic. Ae Lirriblesvnites sanuStonee: s-s1e eee oe nee rere ee eee eer 7 2, Reddish brown sandstone, wita layers of variegated clays and | , j . 100 feet. thd Sie ee eee een i er er A Ree Serie | 3. Cream-colored sandstones and marls .........2.--..---------+-- J 4; Bluish-cray cherty limeéstone.=2. ...2223ecco2 tases aces as see sees 25 feet. be Griyishawhitexsan ds tones. cac cee cake eit eels Se eee 7) tect. G. Yellowish=red sandston@ 2. 22= - 5c. 22-26. 8 eee IEE ere RO ’ ee 7, Hine Wespted: sAndstOnGH <. yoc2 eso ees Foe ee eee sore 8. Argillaceous sands and shales, with interstratified layers of fine clay and gypsum, including one bed of gypsum 22 feet thick.......-..-- 150 feet. 97 Redscompact sandstones <3 5- a) sen.- sas wee alee oe ee ae 250 feet. 10. Reddish-gray sandstone -..........-.. Sale ter eer ae eee ”) 11. Thin bed of sray cherby limestone cos.22s05 4. ese. eee er ( 225 fret. 12. Course, friable, ash-colored sandstone, with pebbles. ...--..--..--- \ 1, 200 feet. LARAMIE PLAINS. 81 Near where the North Park road leaves the plains, going over to the Upper Laramie River, the Triassic and Jurassic formations jut up against the Archzan, which here occurs as a low, narrow ridge, only 300 or 400 feet in height, forming the east side of the Laramie Valley. On the oppo- site side of this ridge, in the broad river-valley, occurs an isolated body of Red Beds, the sandstones which formerly covered the ridge having been entirely eroded. The exposure of Red Beds, however, is limited, as a short distance below, where the road crosses the valley, the river runs through a narrow gorge in the Archzean rocks, west of Sheep Mountain. Under the characteristic red sandstones is found a narrow layer of blue limestone, only a few feet in thickness, below which occur white sandstones. Returning to the Plains again, we find that east of Sheep Mountain the beds of the Colorado group are the oldest formations found lying upon the Archean body. The older rock-mass presents a steep slope toward the plains, around which curve the Cretaceous clays and marls, with a very varying strike and a somewhat varying dip. The line of junction between the two formations is very much obscured by detrital accumulations from the mountain, the clays presenting few well-defined outcrops. Approach- ing the mountain by the North Park road, the first outcrops that are seen, after leaving the nearly level plain and commencing to ascend the gentle grade, are a series of dark brown and black, thinly-bedded shales and clays, with interstratified layers of impure limestone, more or less mixed with coarse sand. In the limestone occur some fragments of organic remains, but too poor for specific determination, while the purer black clays carry thin seams of carbonaceous material, with occasional coatings and incrustations of gypsum. Underlying these dark clays, which have been referred to the Fort Pierre division, occur yellow and blue marls, which crop out in low, rounded banks, rising but a few inches above the level plain, but which mark very clearly the horizon of the Niobrara beds. They are character- ized by the presence of large numbers of the species Ostrea congesta. Below the light-colored marls, the beds pass down into slate-colored mud-rocks, becoming more and more argillaceous, and apparently losing the calcareous character of the overlying strata; these clay-beds, in turn, are underlaid by 6DG R2 DESCRIPTIVE GEOLOGY. brownish, rusty sandstone, with occasional conerctions of ferruginous mate- rial. All of these beds dip at low angles, varying from 8° to 15°. On the north side of Sheep Mountain, where the Big Laramie River leaves the Medicine Bow Range, running out on to the plain, the entire series of sedimentary rocks again appears resting upon the Archeean body. The Carboniferous beds, here a light blue arenaceous limestone, stand nearly vertical, close up against the metamorphic rocks, and overlaid by the bright-colored Red Beds, with the fine gray marls of the Jurassic indi- cated by their characteristic soil and peculiar topography, lying between two formations of massive sandstone: Every formation, from the Triassic to the Niobrara division of the Colorado group, appears well developed along the south bank of the river, inclined at 16° to 18°. Above the bank, the Fort Benton clays are somewhat concealed by overlying soil; but along the bluffs, where the river cuts the beds at right angles to their strike, the dark, almost black clays, passing up into slate-colored marls, are well exposed. The bluish-gray marls, which characterize the junction between the Fort Benton and Niobrara divisions, and form a prominent feature east of the Colorado Range in the region of Big Thompson Creek, are shown here, carrying the well-known Inoceramus problematicus. On the north side of the river, outcrops of the light-colored marls stand out prominently, but would seem to be quite thinly developed; at least, they rapidly pass into coarse, friable sandstones, which, in long, gentle, bench-like ridges, extend out upon the plains. Between the Big and Little Laramie Rivers, east of Bellevue Peak, the Colorado beds are again found resting on the Archean, as in the case of Sheep Mountain, but are still more hidden by overlying Quaternary deposits. At the northern end of Bellevue Peak, the lower sedimentary rocks again reach the surface, and adhere so closely to the Archean out- line as to form a semicircular wall, with a diameter of less than 4 miles, showing all the formations, from the Carboniferous to the top of the Colo- rado group, curving around the older rocks. The Carboniferous beds lie 1,000 feet or more up on the sides of the mountain, inclined at varying angles, having, where they first appear on the east slope, a dip of 20° to 25° to the northward. ‘These Carboniferous limestones possess a saccharoidal LARAMIE PLAINS. 83 texture, and seem highly arenaceous throughout most of the beds. Of the Mesozoic strata, the Red Beds, Jurassic, and Dakota Cretaceous lie inclined from 20° to 80°. Here the bluish-drab, cherty limestone of the Jurassic forms a prominent outcrop, rising slightly above the enclosing marls. Over- lying the Dakota Cretaceous, the clays of the Colorado group incline at a gentler angle, affording few exposures, except the two low rolls of blue and yellow marls in the Niobrara division. Except in the latter beds, no organic remains were found in the entire series, and these were confined to the genus Ostrea. The Fort Pierre black clays dip away from the Niobrara beds with a slight angle, soon becoming nearly horizontal, and, so far as visited, appear to oecupy the greater part of the circular basin north and west of Bellevue Peak, through which the numerous tributaries of the Little Laramie River run, and as these streams cut but little into the clays, their banks offer poor exposures of strata. It is possible that at the upper end of the basin older rocks than Cretaceous may be exposed resting against the Archzean range. The surface of the basin is quite level, and furnishes excellent grazing-land for large herds of cattle. Along the broad valleys of the Big and Little Laramie River, and, indeed, over the greater part of the plains, long bench-like ridges form a characteristic feature of the region. Except on their edges, the benches present but few outcrops, and these usually exhibit coarse sandstone beds, lying in a nearly horizontal position. In numerous localities, organic remains have been found embedded in the sandstones, and have in every case been sufficiently characteristic to show that the beds belong to the Colorado group, without clearly determining the more limited subdivisions of the series. It seems evident, therefore, from our explorations, that the central portion of the Laramie Plains is underlaid by the Colorado group, and that all the beds may be present, but with more or less of an arena- ceous texture. At the northern end of the plains, between Lake Ione and the Laramie River, in a yellowish-brown sandstone, was found the charac- teristic Fort Pierre species, Baculites ovatus, their surfaces being coated with exceedingly beautiful iridescent hues, associated with the genus Ammonites. At the southern end of the plains, near where the Laramie River crosses the western boundary of the Fort Sanders military reservation, the banks 84 DESCRIPTIVE GEOLOGY. of the stream have exposed some arenaceous black clays, carrying inter- stratified narrow beds of purer sandstones, which dip about 1° to the westward, and in their general habit closely resemble the Fort Benton beds. From these clays were collected Ostrea congesta, and minute but well-preserved fish-scales lying with their broader surfaces parallel with the stratification. On the west side of the river, the Colorado group extends well up on the steeper slopes of the Medicine Bow Range, underlying the railroad as far westward as Como Lake, but affording few characteristic outcrops. Between Lookout and Miser Stations, beds of argillaceous sandstones are well exposed, from which were collected specimens of the genus Inoceramus. A short distance east of Miser, the railroad cuts through a heavy bed of brown sandstone, dipping slightly eastward, remarkable for its concretionary structure. These concretions consist of coarse, reddish sandstone, firmly held together by a ferruginous paste, which causes them to withstand erosion much better than the sandstone in which they are embedded. They occur of all sizes, from a few inches up to two feet in diameter, many of them quite smooth and symmetrical, and having weathered out from the bluff lie strewn over the surface of the plains. About one-half mile west from Miser Station, the railway cuts through another ridge of hard gray sandstone, also dipping east, in which were found large numbers of Colorado fossils, chiefly of the genus Inoceramus, but associated with Ammonites and Saurian teeth, the latter showing the enamel well preserved. Hi Range, overlying the Colorado group, occurs a very considerable develop- s ying ery | oh up, along the gentle nearly uniform slopes of the Medicine Bow ment of Fox Hill beds. The junction between the two series is a very difficult one to trace out, the upper beds of the Colorado group being highly arenaceous, passing into the Fox Hill division of coarse, reddish- yellow sandstone by almost imperceptible gradations; a lithological difficulty enhanced by the absence of all marked physical boundaries, and the very general accumulation of detrital material derived from the elevated ridges of the range, which lies seattered over the slopes. The lox Hill beds, resting unconformably upon the Archzean rocks, extend from Mill Creek, a branch of the Little Laramie River, to the northern LARAMIE PLAINS. 85 end of the range, circling around the spurs of Rock Mountain. They con- sist of coarse, friable sandstones, usually of a reddish-brown or yellow color, with oceasionat layers of harder brown beds; all of them lying nearly horizontal, or with a gentle dip to the eastward. On the south bank of Mill Creek, not far from the Archaan mass, and almost completely surrounded by Quaternary deposits, oceurs a body of brownish-gray sandstone, probably belonging to the Fox Hill group, which carries a layer of rich carbonaceous shales, with thin seams of coal cropping out along the exposed bluff; the shales exhibiting a thickness of 3 feet between the sandstone layers. These sandstones were searched for organic remains, but they only yielded a few imperfect impressions of deciduous leaves. The strike of the sandstone is north 35° to 40° east, with a dip of north 5d to' 607. Between Mill and Cooper Creeks, numerous small stream-beds cut deeply into the sandstones, but offer no exposures of special interest. Cooper Creek, a very considerable stream, which debouches through a narrow canon in the mountains, has worn out by erosion a broad valley in the softer Cretaceous rocks, leaving high banks upon each side, composed of coarse sandstones. The valley for several miles from the mountains is remarkable for the large amount of coarse detrital material, which every- where covers the surface; the large Archzean boulders lying high up on the benches and ridges. At its lower end, the valley cuts through the Colorado beds, which, farther up the stream, are overlaid by the Fox Hill group. This formation apparently extends up to the Archzean ridges; at least, no structural or paleontological evidence was obtained of any later formations, all organic remains collected from a number of localities in the exposed banks being forms decidedly characteristic of the Fox Hill divisions, prin- cipally of the genus Inoceranuts. ; On the ridge, which forms the south side of Cooper Creek Valley, just above the upper wagon-road, and but a short distance from the Archean foot-hills, was found a new species of the genus Axinea, which Professor Meek, in his report, has described under the specific name of Axinea Wyomingensis. 86 DESCRIPTIVE GEOLOGY. Associated with it, occurs the well-known form, Inoceramus Barrabini, which is also abundant in the long sandstone banks on the opposite side of the valley, which dip only 2° to 3° eastward. Between Cooper and Rock Creeks, the country presents but little variety in its general features; the nearly horizontal sandstones terminating in abrupt walls toward the plains. At Rock Creek, the Fox Hill beds are well developed on both sides of the valley, lying close up against the Archean body in flat-topped hills, rising 300 feet above the stream-bed, from which were collected several specimens of the genus Inoceramus. In the Fox Hill sandstones, between Cooper and Rock Creeks, layers of carbonaceous clays and shales occur cropping out along the benches, and carrying several thin beds of lignite. Of these coal outerops, two have been to some extent explored,—the one known as the Cooper Creek, and the other as the Rock Creek coals. The Cooper Creek coal occurs on the north side of the valley, interstratified in a bed of hard clay, which is both overlaid and underlaid by sandstone. The beds have a gentle dip to the eastward. So far as known, no Molluscan remains have been found in immediate connection with the coal strata, yet it seems quite evident that they underlie beds characterized by Fox Hill species. The coal possesses a lustrous black color, a moderately compact text- ure, and resembles that found east of the Colorado Range. An analysis of this coal was made by Dr. F. A. Genth,' of the University of Pennsylvania, with the following result: UVOIS DUT. eg ee 9.28 Wh@lennile! wernt = Ge kee Pt ee 39.12 JET O OTL Se es scot Re, ee 47.04 Ash, yellowish> Drews © 4.c-)- cereus pee ee eee le 4.56 100.00 Tt contams 1.38 per cent. of sulphur, which is equal to 2.59 per cent. of pyrites. The Rock Creek coal is situated about 6 miles southeast from the LARAMIE PLAINS. 87 Rock Creek Station of the old Overland Stage Road. Owing to some misunderstanding in regard to the directions for finding the deposit, it was not visited by our parties, but through the kindness of ranchmen living on Rock Creek, we were able to procure specimens of the coal, together with the overlying sandstone. It would appear from descriptions, and general structure of the country, that the coal beds oceur in the Fox Hill formation in a somewhat similar manner to those on Cooper Creek, and in nearly the same geological horizon. The coal is said to oceur in a rich carbonaceous clay, with a dip slightly to the eastward, and has been known for many years, having been used by the Overland Stage Company for blacksmithing purposes. Neither of these coal banks have been thor- oughly opened, nor their true value and extent for commercial purposes made known, although they are situated within 15 miles of the railroad, with a gentle down grade for the entire distance. Rock Creek, which has a broad open valley, is the most important stream north of the Little Laramie River, and. drains, as has already been mentioned, into the Medicine Bow River north of the limits of the map. The upper valley runs through Cretaceous sandstones, stretching from the Medicine Bow Range, far out to the eastward. A short distance above Rock Creek Station, on the railroad, the soft clays and marls of the Fort Benton group reach the surface, the character of the country changing very greatly, while the broad valley narrows and cuts a deep channel through the easily-eroded clays. West of Miser Station, the sandstones give place to the argillaceous beds, and from there westward, nearly to Como Station, the Fort Benton and Niobrara Cretaceous occur as the surface formations. At Rock Creek Station, the Fort Benton beds are well shown on the high banks, and in the numerous narrow ravines and gullies, cutting through the clays at right angles to the main stream, which expose between 350 and 400 feet of dark plastie clays, with interstratified beds of sandstone and avenaceous clays. The Fort Benton formation is in many localities char- acterized by beds of ferruginous clays, which, when the iron is present in only small quantities, gives them either a black earthy appearance, or is indicated by a nodular and concretionary structure, with a considerable variety in color of the different layers, especially on exposed surfaces. R8 DESCRIPTIVE GEOLOGY. The presence of iron in the clays frequently affects its physical appearance to such an extent as to suggest, when in doubt, to which group the clays may belong. In many localities, the lower clays are quite rich in iron, which forms marked layers or deposits of clay ironstone, cropping out for long distances. At Rock Creek, these clay ironstones are well developed, presenting a characteristic feature of the bluffs, which, owing to the great variety of color derived from the different degrees of oxidation of the iron, and its admixture with clay, gives, on the surface, the appearance of a vol- canic formation of ash and scoria. The iron deposits le with the strata, and are found approximately horizontal, but with a thickness varying from a few inches up to 2 or 3 feet. In composition, they are argillaceous carbonates of a brownish-black color and compact texture, which, upon being treated with acid, effervesce briskly, leaving a residue of sand and particles of fine clay. They fre- quently appear considerably fissured, with the seams and cracks filled with crystallized spathic iron. They bear a close resemblance to other beds found in the same horizon on the Wyoming Plains, but especially to the deposits at Elk Mountain, where they are largely developed, and but for the fact that at the latter locality they are turned up at a high angle, show the same mode of occurrence. An analysis of the clay ironstone from Elk Mountain, which will be found in the description of the Cretaceous forma- tion of that locality, probably represents the general character of the Rock Creek deposits. In the dark-colored stratified clays, which overlie the carbonate of iron deposits, were found numerous small fish-scales, so characteristic of the Fort Benton division. Beyond the railroad-station, Rock Creek runs through a narrow open- ing in the Colorado clays, which offers steep, almost impassable walls from the stream-bed to the summit, on the top of which the country presents a dreary parched plain, with scarcely any vegetation to relieve the clay barrens or dull monotony of surface features. The Cretaceous strata, which have been dipping eastward, here begin to show the influence of the dying- out of the Medicine Bow Range, and are seen encircling the northern Archean flanks, gradually trending off to the westward, with east and west LARAMIE PLAINS. 89 strikes and northerly dips. Six miles east of Como Station, the country presents a somewhat strange appearance; the arenaceous clays on the south side of the railroad being traversed by numerous cracks or fissures, which suggest earthquake-openings or crevasses in glaciers. They are, however, the courses of old stream-beds, now dry, except in seasons of unusual flood and freshet: some of them are but two or three feet wide, and at least 200 feet deep, with nearly precipitous walls of clay. On the north side of the railroad, a broader drainage-channel affords a good exposure of these beds, which, without doubt, belong to the Fort Benton division of dark blue clays, with layers of fine-grained, hard ferruginous clays and marls, carrying large numbers of fossils of the genera Inoceramus and Ostrea; while in the more fissile slaty beds imperfect remains of fish appear abundant at certain horizons. On the top of the clay wall, close by the railroad, occurs a layer of brown medium-grained sandstone, which derives some interest from its having been quarried at one time quite extensively by the railroad company for building as well as for construction purposes, and transported for long distances. The formation belongs to the sandstone strata, which characterize the Fort Benton division, and, although if im no wise appears superior as a building stone to other similar beds, it may, from its position and occurrence, be worked to great advantage. The lines of bedding are distinctly marked, the blocks quarrying from one to two feet in thickness, with the formation lying inclined at an angle of from 25° to 34°. Embedded in the sandstone are found numerous fragments of stems and deciduous leaves, most of them ina very imperfect state of preservation. It is possible, however, that a collection might be made sufficiently well preserved to allow of specific determination, which would be of considera- ble importance, as the sandstone, which is about 60 feet in thickness, lies between well-defined beds of the Colorado group. The sandstones are overlaid by bluish-gray marls and clays, which probably belong to the Niobrara division; at least, the latter formation occurs not far distant, with the well-characterized yellowish-white chalky beds, and the sandstones cannot be far below the junction of the two groups. 90 DESCRIPTIVE GEOLOGY. These yellow marls stretch off to the north with a low dip of 3° or 4°; the Colorado group extending northward beyond the limits of our map. Still farther westward, the Colorado group forms the surface formation, inclined at a gentle angle, and presenting but little of special interest. Between one and two miles east of Como Lake, they begin to show a higher dip, and to indicate the structural features produced by the Como Ridge, a somewhat singular uplift of the lower Mesozoic strata. Como AnvTicLINAL.—Como Ridge is situated in the extreme northwestern corner of the Laramie Plains, just west of the 106th meridian, and along the northern border of the map, the boundary-line cutting through Como Lake. The ridge, which takes its name from the lake, derives its importance solely from the geological interest which centres in the region, where it forms the southern side of a local anticlinal axis. The upper Cretaceous beds form the overlying strata of the plains over wide areas; but here, at Como, the lower rocks down to the Red Beds are exposed in a gentle uplift, the later beds dipping away in all directions. The main elevating force has acted approximately in an east and west direction, producing an anticlinal ridge, with its southern side again lifted up above the main axis of elevation. Direetly along the axis of this fold lie the Jurassic marls and soft friable sandstones, which have undergone considerable erosion, leaving a shallow basin now occupied by the lake and a narrow anticlinal valley. On the south side of the lake, through the valley, runs the railroad, beyond which stands the main ridge, forming the south side of the fold. This ridge rises between 200 and 300 feet above the level of the lake, with a steep mural face toward the north, but falling off gently on the opposite side. On the summit, the ridge has a strike of north 60° east, with a uniform dip of 20° to z5° to the southward. Southeast of the lake occur the lowest beds exposed, which consist of hard yellowish-red sandstones, and may belong to the uppermost members of the Red Beds, or may simply be a compact reddish stratum in the Jurassic. On account of this doubt, the Triassic formation is not represented on the geological map, although it is not at all improbable that the lower red sandstones should be so referred. Directly overlying these red sandstones occur well-defined Jurassic strata, which form the northern face of the ridge, and are in turn overlaid COMO ANTICLINAL. 91 by Dakota sandstone, which serves as the capping-stone to the ridge, and the overlying rock on the southern and gentler slope. Nowhere in the Rocky Mountain region, within the belt of exploration, are Jurassic rocks better defined, exhibiting all the characteristic strata which have been observed in other localities, and at the same time associated with organic remains about the age of which there can be no doubt. The following section along the bluff-tace of the ridge was taken from top to base, in descending series: Dalota Cretaceous. 1. Compact yellowish-brown sandstone, forming the summit of the ridge. Jurassic. PMG TA Va Seu Cly SIAL | epee arete 5 ioielou dey eet aie Wi cre cla ete ye ee ) 3. Cream-colored marls, with sandstone layers ....--......-+-----.-. | Aye blaish-drabicherty limestonG.. se" 29. == see eee eaters = ; : : ‘ ; + 175-200 feet. 5. Fine ash-colored marls, with thin beds of light-colored limestone... { 6. Gray and Orange marls, with coarse sandy material ........-..... | | 7. Reddish-yellow sandstone... .......2-. ----+ e022 -- eee ee ee eee \ Triassic ? 8. Brick red compact sandstone. The Dakota beds are a dense, tough series of sandstones, distinctly bedded, and breaking up into rough angular blocks in a similar manner as _ observed east of the Colorado Range. In the Jurassic, the bluish-drab limestone, which is everywhere so persistent a stratum, is here well devel- oped, of a characteristic light drab color, flinty fracture, and carrying numerous small calcite crystals, scattered through the rock. Diligent search was made in this limestone for fossiliferous remains, but without success. In the marls and limestones, however, both above and below it, oceur well-marked forms, which are sufficient to prove the Jurassic age of this horizon, although no very great variety in species has been identified. The following species have been found by our parties: CRINOIDS. Segments of the column of Pentacrinus asteriscus CEPHALOPODA. Belemnites densus. LAMELLIBRANCHIATA. Tancredia Warreniana. Trigonia quadrangularis, a. sp. 92 DESCRIPTIVE GEOLOGY. About two miles east of Como Station, in the cream colored marls. were found a few small Lamellibranchiate shells, but not sufficiently well preserved to allow of specific determination. It would seem somewhat remarkable, that while organic remains should be so difficult to find over wide areas of exposed Jurassic strata, here certain species, as Pentacrinus asteriscus and Belemnites densus, should prove to be so abundant. The latter type, pencil-shaped and of a dark slate color, occurs from 2 to 6 inches in length, and as they withstand atmospheric agencies remarkably well, and the marls in which they are embedded erode so readily, large numbers of them may be picked up along the foot of the bluffs, from which they have been washed out. As shown in the section given above, the well-defined Jurassic rocks have a thickness of nearly, if not quite, 200 feet. Only a limited development of red sandstones is exposed, consisting of yellowish-red beds, and compact mud-rocks, with but little local interest. East of the lake, on a gentle slope, they exhibit in a most marked manner the peculiar cracks and ripple-marks so charac- teristic of Triassic sandstones. Here we have deep symmetrical grooves occurring at intervals of 2 or 3 inches, for nearly 100 feet, covering the entire exposed rovk-surface with the regularity of human workmanship. On the north side of the lake, the Dakota Cretaceous forms a low but abrupt wall along the shore, dipping to the northeast at an angle of 35° to 40°. At the extreme northeast corner beds of fine marl and coarse friable sandstone would indicate that the Jurassic beds occurred directly under the level of the lake. Along the anticlinal valley, on the east side of the lake, no outcrops are visible, the surface being covered with low, irregular sand- dunes; but on the opposite shore it is hemmed in by a low bank of Dakota sandstone. Como Lake, which is probably quite shallow, occupies the anticlinal valley, and measures about one mile in length by half a mile in width. It scarcely differs from the many other small lakes, which occur seattered over the Cretaceous and Tertiary plains of Wyoming, unless it be that the sur- roundings are somewhat more than ordinarily dreary and desolate. Irony certainly prompted the name for this sheet of water. The water possesses a mild brackish, alkaline taste, but deposits only a slightly saline inerustation COMO ANTICLINAL. 93 along the shore during the dry season.