Except for the mountainous section in the northeast (the southern end of the Cumberland Plateau) Alabama is a rolling plain with an average elevation of c.500 ft (150 m) in two geologic regions—the Appalachian Piedmont above the fall line and the coastal plain below. These plains, drained by the Alabama and the Tombigbee rivers and their tributaries, are primarily devoted to agriculture. Montgomery is the capital and Birmingham the largest city of Alabama. Mobile is the state's major seaport. Places of interest in Alabama include Russell Cave National Monument, near Bridgeport, the site of caves that were inhabited almost continuously from 6000 B.C. to A.D. 1650, and Mound State Monument, near Tuscaloosa, the site of numerous early Native American mounds.