The primary purpose of this report is to show the geologic structure and the altitude of the top of the Minnelusa Formation in the southern Black Hills. It can be used in conjunction with topographic maps to estimate the depth to the Minnelusa Formation at a specific site. The Minnelusa Formation was mapped because it is the deepest aquifer for which there is extensive data. The secondary purpose of this report is to describe the possible effects of geologic structures and hydrologic conditions on cave formation in the Madison Limestone, which underlies the Minnelusa Formation. Two hypotheses of cave formation were tested by investigating five caves.
Caves are large-scale examples of secondary permeability in the Madison Limestone.
The orientation and length of the passages in the five caves were measured and qualitatively compared to mapped structural features in the vicinity to identify any relation.