The Geologic Story of the Great Plains / A nontechnical description of the origin and evolution of the landscape of the Great Plains

by Donald E. Trimble

BLACK HILLS

The Black Hills is a huge, elliptically domed area in northwestern South Dakota and northeastern Wyoming, about 125 miles long and 65 miles wide ([fig. 10]). Rapid City, S. Dak., is on the Missouri Plateau at the east edge of the Black Hills. Uplift caused erosion to remove the overlying cover of marine sedimentary rocks and expose the granite and metamorphic rocks that form the core of the dome. The peaks of the central part of the Black Hills presently are 3,000 to 4,000 feet above the surrounding plains. Harney Peak, with an altitude of 7,242 feet, is the highest point in South Dakota. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form the core of the uplift. The heads of four of our great Presidents are sculpted from this granite at Mount Rushmore National Memorial. Joints in the rocks have controlled weathering processes that influenced the final shaping of many of these landforms. Closely spaced joints have produced the spires of the Needles area, and widely spaced joints have produced the rounded forms of granite that are seen near Sylvan Lake ([fig. 11]).

Marine sedimentary rocks surrounding the old core rocks form well-defined belts. Lying against the old core rocks and completely surrounding them are Paleozoic limestones that form the Limestone Plateau ([fig. 10]). These tilted layers have steep erosional scarps facing the central part of the Black Hills. Wind Cave and Jewel Cave were produced by ground water dissolving these limestones along joints. These caves are sufficiently impressive to be designated as a national park and a national monument, respectively. Encircling the Limestone Plateau is a continuous valley cut in soft Triassic shale. This valley has been called “the Racetrack,” because of its continuity, and the Red Valley, because of its color. Surrounding the Red Valley is an outer hogback ridge formed by the tilted layers of the Dakota Sandstone, which are quite hard and resistant to erosion. Streams that flow from the central part of the Black Hills pass through the Dakota hogback in narrow gaps.

Figure 10.—Diagram of the Black Hills uplift by A. N. Strahler (Strahler and Strahler, 1978). Used by permission.

Dakota Sandstone hogback Limestone plateau Belle Fourche River Spearfish Bear Butte Sundance Red Valley Rapid City Red Valley Hot Springs Cheyenne River Edgemont Mt. Rushmore National Monument Jewel Cave National Monument Wind Cave National Park

Figure 11.—Jointed granite rounded by weathering at Sylvan Lake, in the central part of the Black Hills, S. Dak.

The Black Hills, then, is an uplifted area that has been carved deeply but differentially by streams to produce its major outlines. Those outlines have been modified mainly by weathering of the ancient core rocks and solution of the limestone of the Limestone Plateau.