Badlands National Park consists of 244,000 acres of sharply eroded buttes, pinnacles and spires surrounded by a mixed-grass prairie ecosystem. The landscape of Badlands National Park is roughly half badlands geologic formations and half mixed-grass prairie ecosystem. The mixed grass prairie is a transitional zone between the tall-grass prairie to the east and the short-grass prairie to the west. This land was formed by the geologic forces of deposition and erosion. Deposition of sediments began 69 million years ago when an ancient sea stretched across what is now the Great Plains.