USGS1961
Jewel Cave National Monument occupies 2 square miles of a broad plateau of sedimentary rocks in western Custer County, S. Dak., and is at an altitude of about 5,400 feet above mean sea level. The sedimentary rocks that constitute the plateau range in age from Cambrian to Pennsylvanian. Rocks of Silurian and Devonian age are absent. The presence of rocks of Ordovician age has not been established definitely but they may be represented by 10 feet of sandstone directly beneath the Englewood limestone of Mississippian age. The sedimentary formations are underlain by schist of Precambrian age. Study of outcrops in the vicinity of the monument confirms the existence of a fault about 1,500 feet north of the entrance to Jewel Cave. The fault trends generally east-west across the monument and has a displacement of about 120 feet about 1 mile west of the entrance to the cave. The effect of the fault on the occurrence of ground water near the cave is not known. In addition to the spring that furnishes the present (1959) water supply for the facilities at Jewel Cave, three springs outside the monument were visited during the study. Combined yield of the 3 springs is less than 2 gpm (gallons per minute). A single test well indicates that the monument at the well site is underlain by 665 feet of limestone, dolomite, and sandstone of Paleozoic age and an undetermined thickness of quartzbiotite schist of Precambrian age. Pumping tests using a cylinder pump indicate that the test well is capable of producing 15 to 18 gpm for a short time from 2 zones of sandstone below the Englewood limestone. These sandstones are believed to present the best possibilities for development of a permanent water supply at the monument.