As with Yosemite and Acadia, the geologic story of Mt. Rushmore comprises two chapters: the origin of the rock and how those rocks became exposed at the Earth’s surface.
About 1.6 billion years ago magma from the mantle made its way upwards through the crust and cooled deep below the surface forming the granite that is the core of the Black Hills.
The granite lay buried until about 70 million years ago when tectonic forces within the Earth caused the Black Hills region to rise.
Erosion stripped off less resistant sedimentary rocks, and as uplift continued the harder igneous rock was exposed at the surface. In an area of the Black Hills called the Needles, vertical joints in the granite resulted in the formation of thin granite spires (see photo).
And in a large block of unjointed granite, sculptor Gutzon Borglum carved his iconic tribute to 4 U.S. presidents.
More information here.
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