The Black Hills were formed by an uplift that occurred near the end of the Cretaceous Period or the beginning of the Paleogene Period, 65–70 million years ago. The uplift created an elliptical dome, at the center of which is a crystalline core, comprised of the oldest rocks in the hills. This core is Pre-Cambrian, dating back over two billion years.
The granite peaks near Mount Rushmore, dominated by the 7,242-foot Harney Peak, are at the center of the uplift. At the time of the uplift, it is estimated that the hills reached an elevation of over 15,000 feet and that the work of wind and water over the last 65 million years has created the hills we see today.
Encircling the Pre-Cambrian core of the Black Hills are progressively younger rock layers. Time in the Black Hills looks much like a topographic map, each band a different geological period, marching downward and outward and around the hills, from oldest to youngest through the Precambrian, Paleozoic, Mesozoic, and Cenozoic Eras.
Unlike the Black Hills, in the Badlands, the youngest formations are on the top and time marches straight down. Formed primarily by two geologic processes, deposition and erosion, the oldest exposed (and lowest) layers of the area were created between 69–75 million years when the surface area was covered by a warm inland sea. When the hills uplifted, the sea drained away and was replaced by a river floodplain that deposited a new layer every time a flood occurred. A drier period followed, bringing sediments deposited by the wind, the colors varying with time and volcanic activity. As the uplift to the west continued, the water current increased and began to carve into the very deposits earlier streams had left behind. Some 65 million years later, we have the eroded spires and valleys of the Badlands today.
© Laural A. Bidwell from Moon Mount Rushmore & the Black Hills, 1st Edition