Paleontology and geology
Sea level fluctuated during the Permian, leaving behind evidence of both terrestrial and marine conditions. This evidence occurs as layers of red beds and mudstones covered by a thin sheet of fine-grained limestone, which indicates the presence of a shallow sea. This limestone, the Minnekahta Limestone, is almost devoid of fossils. The only exceptions are the fish fossils found in quarries at Rapid City on the eastern edge of the Black Hills. Preservation of these fossils is so remarkable that whole skeletons of Platysomus have been found and are easily recognizable as fish. The Minnekahta Limestone is locally mined for the manufacture of Portland cement, as ornamental flagstone, or as aggregate.
Although this map only indicates the presence of undifferentiated rocks, there are exposures of Permian rocks in the Black Hills region of western South Dakota.