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.