Owing to the cover of Tertiary deposits on the plains adjoining the Republican Valley, this arch could not be traced north or south, but it appeared probable that it may extend across western Nebraska to the Chadron dome, as shown on the map (Pl. I). To the south there was some evidence of its extension in the valleys of Beaver Sappa, and Prairie Dog creeks, its axis passing along the west side of Norton County and the east side of Sheridan County and dying out near Gove. Few deep borings have been made in southern Nebraska. A 1,800-foot hole was recently put down near Cambridge, a boring 912 feet deep was made at Arapahoe, one 1.575 feet deep was made 1 mila east of Stockville, and there is a 405-foot hole at McCook. these borings are deep enough to test all the strata. WESTERN NEBRASKA. None of Most of western Nebraska is covered by so thick a sheet of Tertiary sands and clays that no knowledge of the underlying rocks could be obtained. To judge from evidence in adjoining areas, the region is underlain by Cretaceous rocks, probably in general dipping southwestward toward the Denver Basin. A small amount of late Cretaceous sandstone appearing in the western part of Scotts Bluff County gives strength to this view. It is probable that western Nebraska is underlain also by several hundred feet of Paleozoic limestones and sandstones, but no facts are available as to their structure. The Cambridge anticline may pass through Sheridan County to connect with the Chadron anticline, as shown on Plate I, but there are no exposures to throw light on this assumption. The relations of the anticline are, however, well exhibited in the northeast corner of Dawes County, northeast of Chadron, and in the adjacent portion of South Dakota. Several old holes sunk near Chadron were not sufficiently deep to test its possibilities as to oil or gas. One of these holes, a short distance north of Chadron, was reported to be 1,800 feet deep in 1904, but was discontinued in shale and sandstone of Benton age. SOUTH DAKOTA. CHADRON ANTICLINE AND SOUTHWESTERN SOUTH DAKOTA. A well-defined arch of the strata in the region northeast of Chadron, Nebr., extends for some distance into South Dakota. As stated above, it is possible that this anticline extends southeastward across Nebraska, but owing to the continuous cover of Tertiary deposits the relations in that direction could not be determined. The struc Schist ture is best shown on White River at the State line, where the chalk rock of the Niobrara formation shows an arch and the underlying Carlile shale is revealed in the river valley. A view of part of the chalk-rock exposure is shown in Plate II. The Niobrara exposure extends south of Slim Butte to Slim Butte Creek and some distance east of White River. At the Pine Ridge Agency, where the Pierre shale is exposed, the Niobrara is not far below the surface. The amount of uplift indicated by the presence of this formation is about 1,100 feet. Some features of its configuration are shown in Plate III. To the west there is a deep basin, on the farther side of which the strata rise on the flank of the Black Hills uplift, as shown in figure 7. Cheyenne River Vertical scale FIGURE -Sketch section from Pine Ridge Agency, S. Dak., to Black Hills. Vertical scale exaggerated one-third. Some holes, 1,000 and 1,800 feet deep, have been bored on or near this anticline in the vicinity of Chadron, but no records or other facts have been reported. in The structure of the portion of southwestern South Dakota west of the Chadron anticline is shown by contours in Plate I and by the cross section, figure 7. The salient features are the downward-pitching extensions of crenulations of the Black Hills dome. There are three principal anticlines strongly marked by moderately steep dips many formations from the upper members of the Carboniferous to the Pierre shale. A detailed description of the geology of the area is given in the Edgemont1 and Oelrichs folios. The rocks that underlie the region are shown in the left-hand column in figure 8. The artesian wells at Edgemont penetrated these rocks from the lower part of the Graneros shale to the Deadwood formation, as shown in figure 9. Slight indications of oil and gas were reported from one of the holes, but no facts were obtained as to depth or quantity. Evidently no useful supply was indicated. Two borings 2 Darton, N. H., and Smith, W. S. T., U. S. Geol. Survey Geol. Atlas, Edgemont follo (No. 108), 1904. Darton, N. H., U. S. Geol. Survey Geol. Atlas, Oelrichs folio (No. 85), 1902. |