NE.

M-N

14,000'

E.

beds and sandstones. These wells are given in the following list. Some of the holes are shown in figures 12 and 13.

On the western slope of the Black Hills uplift

there are several anticlines and domes that may contain oil or gas. They have been described in reports and folios, and some of their features are

1Darton, N. H., Preliminary description of the geology and water resources of the Southern part of the Black Hills and adjoining regions in South Dakota and Wyoming : U. S. Geol. Survey Twenty-first Ann. Rept., pt. 4, pp. 489-599, pls., maps, 1901; Geology and water resources of the northern portion of the Black Hills and adjoining regions in South Dakota and Wyoming: U. S. Geol. Survey Prof. Paper 65, 105 pp., 24 pls., 1909. Barnett, V. H., The Moorcroft oil field, Crook County, Wyo. U. S. Geol. Survey Bull. 581, pp. 105-112, 1915.

Darton, N. II., U. S. Geol. Survey Geol. Atlas, Newcastle folio (No. 107), 1904; Sundance folio (No. 127), 1905.

shown by the contour lines on Plate I. The occurrence of oil at Newcastle and northwest of Moorcroft, although so far not proved to be in commercial amounts, gives some encouragement for believing that reservoirs may yet be found at some localities in this general region.

FIGURE 15.-Map of the anticline on Old Woman Creek, Converse County, Wyo.

An anticline of considerable prominence on Old Woman Creek, in Converse County, was described in 1901.1 It is a prolongation of an uplift or series of uplifts extending from Rawhide Butte to Lusk but

1 Darton, N. H., Preliminary description of the geology and water resources of the southern half of the Black Hills and adjoining regions in South Dakota and Wyoming: U. S. Geol. Survey Twenty-first Ann, Rept., pt. 4, pp. 552-554, 1901,

largely covered by Tertiary deposits. The anticline brings to the surface the Dakota to Lakota sandstones and associated formations, and a canyon near its crest reveals fossil-bearing shales in the upper part of the Sundance formation. The principal features are shown in the map (fig. 15) and cross section (fig. 16).

The central ridge of Dakota and Lakota sandstones is about 7 miles long. The dips on the west side are steep and nearly vertical in places. but the dip on the east slope is at a low angle, spreading the formations out in wide zones. The axial pitch to the south is slight, but the relations in that direction are concealed by sand and clay of the White River group. The pitch to the north is gradual, and the flexure is traceable in greatly reduced form to and beyond Cheyenne River at the mouth of Alkali Creek.

The formations that may possibly contain oil or gas in this uplift are the Minnelusa, Pahasapa, and Deadwood, which lie between 1,000 and 2,500 feet below the surface in the center of the anticline. A

FIGURE 16.-Section across the anticline on Old Woman Creek, Converse County, Wyo. a, Fox Hills sandstone; b, Pierre shale; c, Niobrara formation; d, Carlile shale; e, Greerhorn limestone; f, Graneros shale with included sandstone member; g, Dakota sandstone, Fuson shale, and Lakota sandstone; h, Morrison shale and Sundance formation; i, Spearfish formation; j, Minnekahta limestone on Opeche formation; k, Minnelusa sandstone; 1, Pahasapa limestone; m, Deadwood formation.

1,730-foot boring in this uplift has found considerable oil, doubtless in the Minnelusa sandstone, which is entered at 920 feet. It found "Red Beds" at 355 to 770 feet, underlain by 70 feet of Minnekahta limestone and 80 feet of red Opeche sandstone. These formations were penetrated by the two deep artesian wells at Edgemont, 25 miles northeast of Old Woman Creek, and at that place showed no indications of much oil.

In the vicinity of Newcastle there are two low-pitching anticlines on the west slope of the Black Hills uplift, details of which are given in the Newcastle folio and the report on the southern Black Hills above cited. In the Red Valley north of Cambria and southwest of Sundance there are low domes, the relations of which are shown in the Sundance folio. Two low elongated domes 18 miles north of Aladdin are shown in the Aladdin folio.

In the region northwest of Moorcroft there are two domes on a long, low anticline on the west slope of the Black Hills uplift. The summit of one dome is in sec. 30, T. 51 N., R. 66 W., and that of the other in the south side of sec. 11, T. 52 N., R. 67 W. Both of these

21544°-19-Bull. 691-3