The caves of South Dakota are located in the Black Hills a hilly
landscape around Mt. Rushmore. The name of this area comes from
the dark forest of conifers. The most famous tourist attraction,
Mt. Rushmore Monument, shows the geological structure of this
area: the central granite core. The Pahasapa limestone was
created during the Mississippian period, approximately 360 to
330 million years ago. A first karst system developed in this
undisturbed layers which was later buried by younger sediments.
Into this layers the central Black Hills granite intruded from
below, 60 to 40 million years ago. The granite intrusion, lava
coming from the upper mantle broke through Pahasapa limestone,
but didin't reach the surface. Inside the rocks it took very
long to cool down. During this time the ground water in the
limestone was continually heated by the intrusion and was so
able to dissolve and transport several minerals from the granite
into other rocks. This minerals were transported into cracks
forming ore veins. In th