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.
[Illustration: _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
[Illustration: _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.