Erosion continued to be the dominant force as Tertiary rock history began. By Early Oligocene time, stream gradients were so reduced that the streams could no longer carry away their erosion products, and deposition started on the plains adjacent to the Black Hills. Gradually, the lower two-thirds of the Black Hills became buried by light-colored clays and sands, derived not only locally, but from mountain areas to the west. Volcanic activity, probably near Yellowstone Park, contributed large volumes of windblown volcanic ash to the sediments. By the end of Oligocene time, it is possible that the Black Hills projected less than 2,000 feet above this apron of sediments.
Uplift, or a change in climate, or both, caused a renewal of erosion. The soft unconsolidated sediments were attacked and gradually, the lower part of the Black Hills were exhumed. The Black Hills today probably looks very much like they did 40 million years ago. The sediments, eroded and carved into a very distinctive type of topography, can be viewed in the Badlands in southwestern South Dakota.