Mount Rushmore National Memorial: Geologic Resource Evaluation Report

Geologic Resources Division, Natural Resource Program Center

The Natural Resource Publication series addresses natural resource topics that are of interest and applicability to a broad readership in the National Park Service and to others in the management of natural resources, including the scientific community, the public, and the NPS conservation and environmental constituencies. Manuscripts are peer- reviewed to ensure that the information is scientifically credible, technically accurate, appropriately written for the intended audience, and is designed and published in a professional manner. The iconic presidential faces of Mount Rushmore National Memorial are 18 meters (60 ft) high and carved into the Harney Peak Granite (fig. I). The Harney Peak Granite is the primary component of the Precambrian pluton exposed in the core of the Black Hills in South Dakota. The Black Hills, a prominent 200- by 105- kilometer (125- by 65- mi) elliptical dome, arched upward during the late Cretaceous to early Tertiary Laramide Orogeny. Precambrian mica schists, quartzites, and granite form the core of the Black Hills uplift while younger Paleozoic and Mesozoic sedimentary rocks rim the dome. The Harney Peak Granite was selected as the medium for the memorial sculptures because it is a massive, uniform granite, relatively free of joints and fractures. The location of the sculptures allows a southeast- facing exposure with optimal lighting. Work began on the sculptures in 1927 and was finished in 1941. The four presidents represented in the memorial are George Washington, Thomas Jefferson, Abraham Lincoln, and Theodore Roosevelt. These IDen served as president during significant junctures in the development of our nation and remain an inspiration to those who visit Mount Rushmore National Memorial. Geologic processes have played a significant role in developing the landscape of the Black Hills and in sustaining regional population growth. Tectonic upheaval deformed the rocks into a distinctive dome that drew Native Americans, trappers, and miners to the area from the surrounding plains. The discovery of gold led to a population boom in the late nineteenth century.