The Geology of Mount Rushmore: Carved From Ancient Harney Peak Granite

Mount Rushmore National Memorial lies in the central Black Hills of southwestern South Dakota, about 2 miles (3.2 km) southwest of Keystone in Pennington County. Authorized on March 3, 1925, the 1,278-acre (517-hectare) memorial preserves the famous granite sculpture of Presidents George Washington, Thomas Jefferson, Theodore Roosevelt and Abraham Lincoln.

Inspired by the artist Gutzon Borglum, the carving took a crew of about 400 workers from 1927 to 1941. The 60-foot (18-meter) busts, shaped with dynamite, drills and hard labor, are an engineering marvel. Set against ponderosa pine forest and the granite walls of the Black Hills, the sculpture is an internationally recognized symbol of American democracy and freedom.

Granite outcrops in the Black Hills. Image from the National Park Service's page.

The Black Hills Uplift

The memorial sits in the Great Plains physiographic province, within the Black Hills Uplift — a lopsided dome covering about 2 million acres (0.8 million hectares) along the South Dakota–Wyoming border. The uplift formed mostly during the Paleocene, about 60 to 50 million years ago, driven by the collisional forces of the Laramide Orogeny.

As the dome rose, erosion stripped the younger Paleozoic and Mesozoic rock layers from its center, exposing a core of ancient Precambrian metamorphic and igneous rock — the rock found at Mount Rushmore.

The rocks

• Paleoproterozoic rocks: metamorphosed black shale, meta-graywacke and the Harney Peak Granite. The Harney Peak Granite matters most here: it was the stone chosen as the medium for the sculpture.
• Quaternary deposits: young alluvial and talus deposits at the surface.

Geologic map of the memorial, from the NPS Geologic Resources Inventory.

Old mines

Abandoned mine sites in national parks can be valuable cultural resources and wildlife habitat, but many are dangerous to people and animals and can harm water quality and the landscape. The National Park Service's advice: stay out and stay alive.

Part of a larger region

Mount Rushmore shares its geologic history and several characteristic formations with the wider Great Plains province. The NPS Geologic Resources Inventory has produced digital geologic maps and a report for the memorial, available through the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Mount Rushmore National Memorial, South Dakota," citing Lisenbee (1988), Lisenbee and DeWitt (1993) and National Park Service (2016, 2017).