Origin of the Rocks. There are two main types of rock at Mount Rushmore, very old granite and much older metamorphic rocks. The metamorphic rocks we now see began as sediments at the bottom of a shallow sea during the Precambrian period - sometime between 1.6 and 2.5 billion years ago. Dates this old are very difficult to determine precisely, mainly because they pre-date all life on the earth. What is now western South Dakota was then near the edge of a sea. Because there was no plant life to hold the forces of erosion in check, erosive forces acted much faster during the Precambrian. Vast quantities of sand and clay were washed down into the sea.
[{"rock_types": "granite", "era_formed": "n/a"}]
[{"rock_types": "granite", "era_formed": "n/a"}, {"rock_types": "metamorphic ", "era_formed": "Precambrian"}] [{"rock_types": "granite", "era_formed": "n/a"}] [{"rock_types": "metamorphic", "era_formed": "n/a"}, {"rock_types": "sediments ", "era_formed": "Precambrian period"}] [{"rock_types": "granite", "era_formed": "n/a"}, {"rock_types": "metamorphic", "era_formed": "Precambrian "}] [{"rock_types": "metamorphic ", "era_formed": "Precambrian"}, {"rock_types": "granite ", "era_formed": "n/a"}] [{"rock_types": "old granite", "era_formed": "Precambrian period"}]