The Ouachita Mountains are the southernmost subdivision of the Interior Highlands. It is a lens-shaped area having a width of 60 miles and a length of 225 miles extending from southeastern Okalahoma to the west central portion of Arkansas. This mountainous and geologically complex region is surrounded on the east and south by the coastal plain, and on the west by the dry, slightly elevated central lowlands. On the north, the low Arkansas River Valley separates the Ouachita Mountains from the mountainous Ozark region. The relief of the region is 2,381 feet with elevations ranging from 300 feet at Fourch-LeFave River to 2,681 feet at Rich Mountain. It is predominately made up of sandstone, limestone, and conglomerate along with metamorphic rocks such as shale and chert.