but everywhere forms a rim rock surrounding the basins and valleys that have been eroded out of the underlying Colorado shale. The belts of carbonaceous shale in the middle member locally contain thin seams of coal. At numerous localities throughout this field there occurs near the top of the Eagle sandstone a thin layer of small black chert pebbles. On many of the long dip slopes at the top of the Eagle sandstone there are large numbers of these pebbles. They were observed over the long dip slope in the eastern part of T. 4 N., R. 21 E., and along the main road leading up out of Hailstone Basin toward the east. These pebbles as a rule do not exceed an inch in diameter and are generally flattened in one direction and embedded in a coarse sandy matrix. Their smoothness and uniformly small size indicate perfect separation and transportation for a considerable distance. Similar pebbles were observed in the Claggett formation about 125 feet above the base in T. 4 N., R. 22 E., but the pebbles are not so well rounded as those at the top of the Eagle. The following sections of the Eagle formation measured at two widely separated localities indicate the composition of the formation in this field: Section of the Eagle sandstone along Canyon Creek, in the southern part of T. 1 N., R. 23 E. Feet. Sandstone, gray, commonly massive but locally very thin bedded.... 51 101 91 243 Section of the Eagle sandstone a quarter of a mile west of main wagon road, in sec. 4, T. 4 N., R. 19 E. Feet. Interval, not well exposed, probably shaly sandstone or sandy Sandstone, thin bedded. Sandstone, massive; forms a vertical cliff.. Shale, carbonaceous.. 21 Sandstone, in thin shaly layers; exhibits much cross-bedding; con CLAGGETT FORMATION. 1 The name Claggett was given by Stanton and Hatcher to the formation overlying the Eagle sandstone. The name was adopted because the formation is well exposed in the neighborhood of Judith (old Fort Claggett), on Missouri River, the type locality. The relation of the Claggett formation to the underlying Eagle sandstone in the Lake Basin field indicates a sudden change from conditions resulting in the deposition of coarse sand to those under which fine sands and silts were the predominant sediments. That all these sediments were laid down in comparatively shallow water seems reasonably certain. The strand origin of the upper portion of the Eagle sandstone is indicated by the coarse grain of the sandstone, the numerous impressions of the fossil seaweed Halymenites major, the presence of false bedding, as shown in Plate XIX, A, and the almost universal distribution of small flattened chert pebbles near the top of the formation; whereas the shallow-water origin of the sandy shales and thin-bedded sandstones of the Claggett formation is indicated by the presence of ripple marks at several horizons in the formation. The Claggett formation in the Lake Basin field consists of belts of sandy shale alternating with those of thin-bedded sandstone, but the proportion of sand appears to decrease rapidly from southwest to northeast across the field. The following stratigraphic section measured along the valley of Canyon Creek shows an abnormal development of the sandstone belts, but farther east along the Yellowstone these belts are scarcely recognizable. Section of the Claggett formation along the valley of Canyon Creek, in T. 1 N., R. 23 E. Shale, soft, flaky, including a few concretions. Thin layers of sand- Feet. 152 Sandstone, gray, thin bedded; includes a few thin bands of inter- Shale, gray, soft, flaky.. Sandstone, gray, thin bedded; forms a bench. Shale, soft, flaky; contains some gypsum; includes a few concretions Sandstone, gray, mostly thin bedded but with some massive layers, Shale, the upper part sandy.... Sandstone, thin bedded; contains some interbedded shale.. 567 The Claggett formation is exposed chiefly in the northern and southeastern parts of the field, where it borders the uplifts of Eagle 1 Stanton, T. W., and Hatcher, J. B., Geology and paleontology of the Judith River beds: U. S. Geol. Survey Bull. 257, p. 13, 1905. A. FALSE-BEDDED EAGLE SANDSTONE CUT OFF BY A FAULT IN THE SE. SEC. 23, B. THE JUDITH RIVER FORMATION AS IT IS EXPOSED IN THE VERY MUCH DISSECTED AREA 1 MILES SOUTH OF ACTON, MONT. sandstone. The formation, being composed largely of shale and thinbedded sandstone, is much less resistant than the underlying massive Eagle sandstone and somewhat less so than the overlying Judith River beds. Where the dip is very low the Claggett commonly underlies a comparatively low, more or less even surface extending from the Eagle sandstone escarpment back to the line of low hills formed by the lower prominent sandstones of the Judith River formation. The base of the Judith River formation was drawn at the base of these sandstones because they mark a change in lithology and because a collection of fossils from the sandstones in sec. 1, T. 6 N., R. 16 E., examined by T. W. Stanton, were said by him to suggest Judith River beds rather than anything older. Where the Claggett formation dips rather steeply the alternating belts of shale and sandstone are expressed topographically by a succession of narrow strike valleys separated by narrow, jagged ridges. 1 JUDITH RIVER FORMATION. The Judith River formation overlies the Claggett formation and underlies the Bearpaw shale. The name Judith River was first given by Hayden in 1871 to a group of sediments occurring in "Judith Basin," on Missouri River, which contain beds of lignite, fresh-water Mollusca, leaves of deciduous trees, and particularly a great number and variety of curious reptilian remains. For convenience of mapping and because of lithologic similarity, there is included at the base of the Judith River formation in the Lake Basin field, a sandstone which seems to be the approximate equivalent of the sandstone included in the top of the Claggett formation in its type area and in the eastern part of the Big Horn Basin. The Judith River differs very materially in character in the extreme western and extreme eastern part of the Lake Basin field. Near Gibson the formation is of fresh-water origin and consists of sandy shale and numerous beds of quartzitic sandstone containing abundant plant remains. The sandy shales and sandstones all contain tuffaceous material, which was poured out abundantly by the volcanoes of the Crazy Mountains. The presence of this constituent imparts to some of the softer units a rather striking yellowish-brown color. Toward the east the proportion of volcanic material decreases and the section gradually approaches that exhibited along Missouri River, in which there is an alternating series of light-colored sandstones, light-gray to white clay shales, and black carbonaceous shale, giving the exposures a very striking banded appearance, as shown in Plate XIX, B. The following stratigraphic section measured 14 miles south of Acton includes the beds shown in the view. 1 Hayden, F. V., Geology of the Missouri Valley: U. S. Geol. Survey Terr. Fourth Ann. Rept., p. 97, |