Mount Rushmore - Geology

Geology

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 Proterozoic, roughly 1.6 billion years ago. Coarse grained pegmatite dikes are associated with the granite intrusion of Harney Peak and are visibly lighter in color, thus explaining the light-colored streaks on the foreheads of the presidents.

The Black Hills granites were exposed to erosion during the Neoproterozoic, but were later buried by sandstone and other sediments during the Cambrian. Remaining buried throughout the Paleozoic, they were re-exposed again during the Laramide orogeny around 70 million years ago. The Black Hills area was uplifted as an elongated geologic dome. Subsequent erosion stripped the granite of the overlying sediments and the softer adjacent schist. Some schist does remain and can be seen as the darker material 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, thus was more than sturdy enough to support the sculpture and its long term exposure. The mountain's height of 5,725 feet (1,745 m) above sea level made it suitable, and because it faces the southeast, the workers also had the advantage of sunlight for most of the day.