The Great Plains cover most of the western two-thirds of South Dakota. West of the Missouri River the landscape becomes more arid and rugged, consisting of rolling hills, plains, ravines, and steep flat-topped hills called buttes. In the south, east of the Black Hills, lie the South Dakota Badlands. Erosion from the Black Hills, marine skeletons which fell to the bottom of a large shallow sea that once covered the area, and volcanic material all contribute to the geology of this area.