Though Illinois lies entirely in the Interior Plains, it does have some minor variation in its elevation. In extreme northwestern Illinois, the Driftless Area, a region of unglaciated and therefore higher and more rugged topography, occupies a small part of the state. Charles Mound, located in this region, has the state's highest elevation above sea level at 1,235 feet (376 m). Other highlands include the Shawnee Hills in the south, and there is varying topography along its rivers; the Illinois River bisects the state northeast to southwest. The floodplain on the Mississippi River from Alton to the Kaskaskia River is known as the American Bottom.