Keystone is a town in the Black Hills region of Pennington County , South Dakota , United States . The population was 337 at the 2010 census . It had its origins in 1883 as a mining town , and has since transformed itself into a resort town , serving the needs of the millions of visitors to the Mount Rushmore National Memorial , which is located just beyond city limits . Keystone was heavily damaged in the 1972 Black Hills flood
1972 Black Hills flood
1972 Black Hills flood
Keystone, South Dakota