The Mount Rushmore or the Mount Rushmore National Memorial, near Keystone, South Dakota, is a monumental granite sculpture by Gutzon Borglum (1867–1941), located within the United States Presidential Memorial that represents the first 150 years of the history of the United States of America with 60-foot (18 m) sculptures of the heads of former United States presidents (left to right): George Washington (1732–1799), Thomas Jefferson (1743–1826), Theodore Roosevelt (1858–1919), and Abraham Lincoln (1809–1865). The entire memorial covers 1,278.45 acres (5.17 km2) and is 5,725 feet (1,745 m) above sea level. It is managed by the National Park Service, a bureau of the United States Department of the Interior. The memorial attracts approximately two million people annually.
Mount Rushmore is largely composed of granite. The memorial is carved on the northwest margin of the Harney Peak granite batholith in the Black Hills of South Dakota, so the geologic formations of the heart of the Black Hills region are also evident at Mount Rushmore. The batholith magma intruded into the pre-existing mica schist rocks during the Precambrian period about 1.6 billion years ago. However, the uneven cooling of the molten rock caused the formation of both fine and coarse-grained minerals, including quartz, feldspar, muscovite, and biotite. Fractures in the granite were sealed by pegmatite dikes. The light-colored streaks in the presidents' foreheads are due to these dikes.
The Black Hills granites were exposed to erosion during the late Precambrian, but were buried by sandstones and other sediments during the Cambrian Period. The area remained buried throughout the Paleozoic Era, but was exposed again to erosion during the tectonic uplift about 70 million years ago. The Black Hills area was uplifted as an elongated geologic dome which towered some 20,000 feet (6,100 m) above sea level, but erosion wore the area down to only 4,000 feet (1,200 m). The subsequent natural erosion of this mountain range allowed the carvings by stripping the granite of the overlying sediments and the softer adjacent schists. The contact between the granite and darker schist is viewable just below the sculpture of Washington.
Borglum selected Mount Rushmore as the site for several reasons. The rock of the mountain is composed of smooth, fine-grained granite. The durable granite erodes only 1 inch (25 mm) every 10,000 years, indicating that it was sturdy enough to support sculpting. In addition, it was the tallest mountain in the region, looming to a height of 5,725 feet (1,745 m) above sea level. Because the mountain faces the southeast, the workers also had the advantage of sunlight for most of the day.