Formation of the Topography
The geologic history of the region between 1.6 billion and 500 million years ago is unknown. Either no rocks formed during that time or none have survived. Several miles of rocks must have been eroded from above the granite during that time, creating marine sediments which have been found to the east of the region.
The flanks of the Black Hills are covered with rocks which formed between 500 and 100 million years ago. These rocks have been completely eroded away in the area of the memorial but they can be seen on any of the roads heading away from the central Black Hills. Many of these rocks are limestone. They were deposited in shallow seas as millions of marine organisms died, settled to the bottom, were buried and converted to sedimentary rock. These limestone formations include the rocks of both Wind Cave National Park and Jewel Cave National Monument.
About 70 million years ago the area of the Black Hills began to uplift. This uplift is believed to be related to the general uplift of the central Rocky Mountains. As the Black Hills began to rise, the sedimentary rocks above the crystalline core began to crack, and then erode away. This process of erosion is complete in the area of the memorial-only the granites, pegmatites and metamorphic rocks of the crystalline core are still visible within the boundaries of the memorial. Much of the material eroded from above the crystalline core was transported up to 150 miles to the east and forms the rock layers now being eroded again at Badlands National Park.
Mount Rushmore is here today because the granite erodes very slowly compared to the surrounding rocks. In the area of the memorial the hard granite creates mountains or hills and the equally tough quartzite forms knife ridges. The less resistant mica schist tends to be eroded into canyons and gullies.
The primary erosive forces in action today are wind, rain, snow and frost wedging. The forces of wind and weather slowly eat away at the rocks of the memorial. However, the granite is extremely resistant. The rate of erosion on the granite faces has been estimated at only 1/10 inch per one thousand years.
A more significant erosional force in the Black Hills is frost wedging. Granite has natural cracks in it. If water gets into the cracks it expands as temperatures dip below freezing - exerting great pressure on the rock and gradually expanding the cracks. This is prevented by the caulking of cracks on the memorial with a silicon solution that prevents water from entering the cracks.