Seventy-five million years ago, an ancient sea covered this landscape, depositing layers of sediment that hardened into Pierre Shale. This forms the base of the Badlands and is its oldest layer.
On top of the Pierre Shale, 30- to 34-million-year-old rock known as the Brule formation sits just below the Sharps formation, which contains ancient volcanic ash.
About 500,000 years ago, rainwater began to seep through these layers at the relatively fast rate of one inch per year, exposing ancient rock and creating local rivers.
Wind and the effects of water seeping into cracks, freezing and then thawing also contributed to the region's erosion, most actively at the Badlands Wall, a 60-mile-long spine of buttes.