measured in the Electric coal field in Park County. Southward from Billings to Bridger and Elk Basin and thence to Cody, on the west side of the Big Horn Basin, the sandy portion of the Colorado becomes much thicker, and the upper shale portion rises very rapidly in the stratigraphic column. From Cody eastward and southward to the central Wyoming area the thickness of the lower sandy portion of the Colorado remains very constant, as indicated by the sections, although there is a great variation in the number and character of the sands from place to place. The Musselshell River section shows two prominent sandstones in the lower portion of the Colorado shale, the upper one of which has a thickness of approximately 200 feet. The few deep well records in the Lake Basin field, however, indicate no such prominent sandstone but rather a series of thin beds considerably lower in the section. It is quite possible that these well records do not represent the true condition in the Lake Basin field, for the sands in the lower part of the Colorado shale throughout north-central Wyoming and central Montana are inclined to be rather lenticular. For example, at Greybull, Wyo., according to Hewett and Lupton,' the Frontier formation of the Colorado group contains only two beds of sandstone, but in the southern and western parts of the Big Horn Basin five or six sandstones are present. This fact is clearly brought out in the stratigraphic section measured along Shoshone River near Cody. According to Wegemann,2 in the interval between the Mowry shale and the Wall Creek sandstone at Salt Creek there are several thin sandstones, which change considerably from place to place. At certain places one is conspicuous and at others another, indicating that the sands are very lenticular. According to Wegemann's report on the Powder River field the shale in that locality, which overlies the Mowry, contains several sandstone beds in its upper part. Wegemann says that these sandstone beds are most variable and that although in the southern part of the Powder River dome four distinct sandstone strata occur below the outcrop of the Wall Creek sandstone, in the northern part only two of these sandstone beds are present, the others apparently having been replaced by shale. In a recent report on central Wyoming Hares says: The Mowry shale is overlain by shale and sandstone that are referred to the Frontier formation, of Upper Cretaceous age. The sandstones, of which there are three distinct divisions corresponding in ascending order to the Peay, an intermediate sand, and the Wall Creek are of medium to fine grain, gray, somewhat massively bedded, and from 20 to 200 feet thick. 1 Hewett, D. F., and Lupton, C. T., Anticlines in the southern part of the Big Horn Basin, Wyo.: U.S. Geol. Survey Bull. 656, 1917. a Wegemann, C. H., The Salt Creek oil field, Natrona County, Wyo.: U. S. Geol. Survey Bull. 452, 1911. 3 Wegemann, C. H., The Powder River oil field, Wyo.: U. S. Geol. Survey Bull. 471, p. 63, 1912. Hares, C. J., Anticlines in central Wyoming: U. S. Geol. Survey Bull. 641, p. 246, 1916. According to Hares these sandstones can generally be recognized, although they vary a great deal from one place to another. As stated in a preceding paragraph, only a few hundred feet of the Colorado shale is exposed anywhere in the Lake Basin field. Underlying the lower massive sandstone of the Eagle formation is a belt consisting of sandstones and sandy shales, which merge downward into pure shale. Approximately 90 feet below the base of the Eagle there ordinarily occurs a light-gray sandstone from 15 to 25 feet thick. This sandstone is massive, free from bedding planes, and separated by minor joints into an infinite number of small irregular-shaped masses. This sandstone is in many places capped by a thin slab of reddish-brown sandstone, commonly separated into a series of rectangular blocks by two systems of parallel joints. This resistant cap protects the underlying soft sandstone and gives rise to a well-defined terrace. Here and there a very pronounced checkered appearance is presented for some distance back from the escarpment, owing to the luxuriant growth of vegetation along the joint planes. In the course of the field work the writer collected from this sandstone an undetermined species of Baculites, together with fish scales, bones, and teeth, which, according to T. W. Stanton, are probably of Colorado age. The following stratigraphic sections measured on opposite sides of the Lake Basin field show the nature of the upper 300 or 400 feet of the Colorado shale: Section measured down the steep north slope of Locomotive Butte, in sec. 2, Ț. 4 N., R. 19 E. Feet. Sandstone, yellowish brown, massive (base of Eagle formation).... 80 6 65 Sandstone, yellow, soft.. 12 Shale, gray to brownish. 8 Sandstone, shaly, containing large concretions of yellowish-brown 17 Shale, dark gray, almost entirely free from sandstone layers.... 225 413 Section of the upper beds of the Colorado shale along Canyon Creek, in T. 1 N., R. 23 E. Feet. Shale, gray, with faint bluish tint, soft and flaky; contains much 81 gypsum. Sandstone, gray, coarse grained, soft and somewhat shaly. 22 66 Sandstone, gray, thin bedded; ripple marked at top and forms a prom- 11 Shale, light gray with slight bluish tints, very sandy; contains con- 40 1 Personal communication. MONTANA GROUP. SUBDIVISIONS. The rocks of the Montana group, of the late Upper Creta occupy about nine-tenths of the total area included in the Basin field. The five formational subdivisions of the Mo which are present are, in ascending order, the Eagle sand Claggett formation, Judith River formation, Bearpaw shale Lennep sandstone. Before entering upon the detailed descriptions the writer d to call attention briefly to the nonpersistence of the lithologic The relationship of these units is more easily understood by ref to the diagrammatic representation on Plate XVIII. Cond here and elsewhere indicate that the Eagle sandstone represe wedge of littoral and continental sediments built seaward afte end of Colorado time. The Judith River formation resemble Eagle in mode of origin, but it is much thicker and exhibits water and estuarine phases much farther east than the Eagle. Lennep sandstone represents a third littoral and continental w characterized by an abundance of volcanic material among component sediments. Separating these coastal-plain deposit the marine shales of the Claggett and Bearpaw formations, ind ing oscillatory movements of the shore and a corresponding adv of marine waters. The nonpersistence of some of these format is brought out in the following detailed descriptions. EAGLE SANDSTONE. The term Eagle sandstone was used by Weed to apply to formation overlying the Colorado shale in north-central Mont and typically exposed on Missouri River at the mouth of E Creek, 40 miles below Fort Benton. In the type locality the mation consists of three more or less distinct units-an upper mem of thin-bedded sandstone, a middle member of shale, and a lov member of massive ledge-making sandstone. The lower mem has been found to be very persistent over a large area in nort central Montana, even where the other members of the formati can not be recognized, and for that reason a name has been adopt by the United States Geological Survey to apply to this member the Virgelle sandstone member of the Eagle sandstone. tt f mari The Eagle sandstone as exhibited in the Lake Basin field r sembles that at the type locality in that all three members are get erally recognizable. The middle member, however, does not, as rule, contain very much shale. It is ordinarily composed of belts o thin-bedded shaly sandstone and very sandy and carbonaceou shale, but locally it is made up almost entirely of sandstone. Th lower massive sandstone member varies considerably in thickness 1 Weed, W. H., U. S. Geol. Survey Geol. Atlas, Fort Benton folio (No. 55), 1899. |