South Dakota can generally be divided into three regions: eastern South Dakota, western South Dakota, and the Black Hills. The Missouri River serves as a boundary in terms of geographic, social, and political differences between eastern and western South Dakota. The geography of the Black Hills, long considered sacred by Native Americans, differs from its surroundings to such an extent it can be considered separate from the rest of western South Dakota. At times the Black Hills are combined with the rest of western South Dakota, and people often refer to the resulting two regions divided by the Missouri River as West River and East River.
Eastern South Dakota generally features higher precipitation and lower topography than the western part of the state. Smaller geographic regions of this area include the Coteau des Prairies, the Dissected Till Plains, and the James River Valley. The Coteau des Prairies is a plateau bordered on the east by the Minnesota River Valley and on the west by the James River Basin. Further west, the James River Basin is mostly low, flat, highly eroded land, following the flow of the James River through South Dakota from north to south.The Dissected Till Plains, an area of rolling hills and fertile soil that covers much of Iowa and Nebraska, extends into the southeastern corner of South Dakota. Layers deposited during the Pleistocene epoch, starting around two million years ago, cover most of eastern South Dakota. These are the youngest rock and sediment layers in the state, the product of several successive periods of glaciation which deposited a large amount of rocks and soil, known as till, over the area.
Due to a higher elevation and level of precipitation, the Black Hills ecology differs significantly from the plains. The mountains are thickly blanketed by various types of pines, including ponderosa and lodgepole pines, as well as spruces. Black Hills mammals include deer, elk (wapiti), bighorn sheep, mountain goats, pine marten, and mountain lions, while the streams and lakes contain several species of trout.