This year marks the 75th anniversary of the completion of Mount Rushmore. The monument features the six-story-tall faces of Presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln. Sculptor Gutzon Borglum’s team of about 400 workers finished carving the monument, which is located in the Black Hills of South Dakota, in 1941. Construction took 14 years. 

The monument is carved mainly into granite. This igneous rock formed from molten rock, called magma, about 1.7 billion years ago, says geologist Mark Fahrenbach of the South Dakota Geological Survey. Heat and pressure from the magma also altered the chemical makeup of surrounding rock layers. It transformed them into a metamorphic rock called schist.