The formations in Badlands National Park and badlands formations around the world are the end-product of two simple processes: deposition and erosion. Deposition is the process of rocks gradually building up. Over the course of millions of years, the layered rocks of the Badlands were slowly stacked on top of each other like a layer cake. These rocks were deposited by a number of natural forces which range from shallow inland seas to rivers to wind. Deposition began about 75 million years ago with the formation of the Pierre Shale, the base of the geologic formations in the park. Deposition ended about 28 million years ago with the Sharps Formation, the uppermost unit of Badlands stratigraphy.