Geological Survey Professional Paper 65 Geology and Water Resources of the Northern Portion of the Black Hills and Adjoining Regions in South Dakota and Wyoming

General features.—In western South Dakota and eastern Wyoming a small group of mountains known as the Black Hills rises several thousand feet above the plains. The abundant rainfall and the consequent vegetation and streams make the locality an oasis in the semiarid region. The hills are carved from a dome-shaped uplift of the earth's crust, and consist largely of rocks which are older than those forming the surface of the Great Plains and which contain valuable minerals. The length of the more elevated area is about 100 miles, and its greatest width is 50 miles. The hills rise abruptly from the plains, although the flanking ridges are of moderate elevation. The salient features are an encircling hogback ridge, constituting the outer rim of the hills; next a continuous depression, the Red Valley, which extends completely around the uplift inside the hogback; then a limestone plateau with infacing escarpment; and, finally, a central area of high ridges culminating in the precipitous crags of Harney Peak at an altitude of 7,216 feet. Two branches of Cheyenne River nearly surround the hills and receive many tributaries from them.

The central area.—The central area of the Black Hills comprises an elevated basin, eroded in crystalline schists and granite, in which scattered rocky ridges and groups of mountains are interspersed with park-like valleys. The wider valleys are above the heads of canyons of greater or less size, which become deeper and steeper sided as they extend outward to the northeast, east, and south.

Limestone plateau.—The limestone plateau forms an interior highland rim around the central area, rising considerably above the greater part of the region of crystalline rocks. Its western portion is much more extensive than its eastern portion and is broad and flat, sloping gently downward near its outer margin, but being level near its eastern, inner side, which presents a line of cliffs many miles long and in numerous places 800 feet above the central valleys. (See fig. 1.) It attains altitudes of slightly more than 7,000 feet, almost equaling Harney Peak in height, and forms the main divide of the Black Hills. The streams which flow down its western slope are affluents of Beaver Creek to the southwest and of the Belle Fourche to the northwest. They rise in shallow, parklike valleys in the plateau and sink into deep canyons with precipitous walls of limestone, locally many hundred feet high. The most notable of these canyons is that of Spearfish Creek.

FIGURE 1 —Limestone cliff at edge of high plateau, near Castle Creek, South Dakota.

The plateau, extending southward, swings around to the eastern side of the hills, where, owing to the greater dip of the strata, it narrows to a ridge having a steep western face. This ridge is intersected by the water gaps of all the larger streams in the southeastern and eastern portions of the hills. These streams rise in the high limestone plateau on the west, cross the region of crystalline rocks, and flow through canyons in the flanking regions of the eastern side to Cheyenne River. All around the Black Hills the limestone plateau slopes outward, but near its base there is a low ridge of Minnekahta limestone with a steep infacing escarpment from 40 to 50 feet high, surmounted by a bare rocky incline which descends several hundred feet into the Red Valley. This minor escarpment and slope are sharply notched at intervals by canyons, which on each stream form a characteristic narrows or "gate."

The Red Valley.—The Red Valley is a wide depression that extends continuously around the hills, with long, high limestone slopes on the inner side and the steep hogback ridge on the outer. It is in some places 2 miles wide, though it is much narrower where the strata dip steeply, and is one of the most conspicuous features of the region, owing in no small degree to the red color of its soil and the absence of trees, the main forests of the Black Hills ending at the margin of the limestone slopes. The larger streams flowing out of the hills generally cross it without material deflection, and their valleys are separated by divides which are as a rule so low as to give it the appearance of being continuous; in its middle eastern section, however, it is extensively choked with Oligocene deposits.

The hogback rim.—The hogback ridge constituting the outer rim of the hills is for the most part a single-crested ridge of hard sandstone, varying in prominence and in steepness of slope. At the north and south and locally along the middle western section it spreads out into long, sloping plateaus. It nearly everywhere presents a steep face toward the Red Valley, above which its crest line rises several hundred feet (see Pl. III, B), but on the outer side it slopes more or less steeply down to the plains that extend far out from the Black Hills in every direction. The hogback ridge is crossed by numerous valleys or canyons, which divide it into level-topped ridges of various lengths. At the southern point of the hills Cheyenne River has cut a tortuous valley through the ridge for several miles, and the Belle Fourche does the same at the northern end of the uplift.

PLATE III.—A (top), TYPICAL PLAINS IN PIERRE SHALE, NORTHEAST OF BLACK HILLS. B (bottom), THE RED VALLEY AND SLOPES OF MINNEKAHTA LIMESTONE, NEAR WHITEWOOD, S. DAK.

The plains.—The plains adjoining the Black Hills present great expanses of relatively level lands with low rolling hills and broad shallow valleys. To the east is a wide area of soft Pierre shale, much of which presents the aspect shown in Plate III, A. To the west is the somewhat rougher topography of the Fox Hills sandstones and overlying formations. Near the Black Hills there are usually long lines of low ridges marking the outcrop of the Greenhorn limestone and of some of the harder beds of the Carlile and Graneros shales. On the divide between the Belle Fourche drainage and that of Owl River are scattered buttes of considerable prominence, notably Castle Rock. (See Pl. XIII, B.)