The Black Hills, situated between the Belle Fourche and Cheyenne Rivers, are featured in this low-oblique, south-southwest-looking photograph. The Black Hills cover approximately 6000 square miles (15 500 square kilometers), rising abruptly 3000 feet (900 meters) above the surrounding plains. A major recreational area, the hills are the site of Mount Rushmore and the new Chief Crazy Horse Monument, which is being carved. This small group of mountains on the South Dakota-Wyoming border lies just south of the geographic center of the United States. Geologically the Black Hills are an elliptical dome dissected by streams, comprising high ridges, deep valleys, and a plateau. The Black Hills received their name because of the dark pine-covered slopes of the Black Hills National Forest, which appears black from a distance. Gold was discovered in the hills in 1874 during an expedition led by General George Custer; the resulting gold rush drove out the Lakota Sioux Indians. Gold is still mined in the area; Homestake Mine is the largest producing gold mine in the United States. In the eastern foothills lies Rapid City (barely discernible), the major commercial, trade, and transportation center for the region.