Mount Rushmore National Memorial is carved in the granite core of the Black Hills. There are no caves in the Memorial, however there is a ring of limestone around the Black Hills. This limestone layer was laid down at the bottom of an inland arm of the sea that covered the area around the Black Hills spreading east during the Cretaceous Era, 65 million years ago. In this limestone ring there are several caves, two of which are National Park Service sites, Wind Cave National Park and Jewel Cave National Monument. Check their websites for additional details on how the caves were formed and what their special features are.