plateau of southern Missouri and northern and central Arkansas. The southern Missouri and northern Arkansas Ozark plateau resembles in many ways the Appalachian plateau. The Ozarks and Ouachitas make up the U.S. Interior Highlands, the only major mountainous region between the Rocky Mountains and the Appalachian Mountains. As the coastal plain turns westward toward Texas, it borders the Ouachita Mountains of Arkansas and Oklahoma and the Arbuckle Mountains in southern Oklahoma. The Ouachitas and the Arbuckles may be considered an analog of and possible extension of the Appalachian fold and crystalline belts. Mississippi drainage basin In the embayment of the coastal plain some low escarpment-like belts of hills with associated strips of lowlands suggest the features of a belted coastal plain. The hilly belt or dissected escarpment determined by the Grand Gulf formation in western Mississippi is the most distinct. Important salt deposits occur in the coastal plain strata near the coast. The most striking feature of the embayment is the broad valley which the Mississippi has eroded across it. The small proportion of total water volume supplied from the great Missouri River basin is due to the light precipitation in that region. The lower Mississippi has no large tributaries from the lower east, but two important ones come from the west. The Mississippi Arkansas drainage area being a little less than the Ohio River and the basin of the Red River of Louisiana being about half as large. The Mississippi River drains an area of about one-third of the United States. The head of the coastal plain embayment is near the junction of the Ohio and the Mississippi. It flows southward for through the semi-consolidated strata of the plain. The river has eroded a valley about wide enclosed by bluffs one or two hundred feet high in the northern part. These bluffs decrease towards the south, but with local increase of height associated with a decrease in flood plain breadth on the eastern side where the Grand Gulf escarpment is traversed. This valley in the coastal plain, with the much narrower rock-walled valley of the upper river in the prairie states, is the true valley of the Mississippi River. However, in popular usage, the Mississippi valley is taken to include a large central part of the Mississippi drainage basin. The valley floor is covered with a floodplain of fine silt, having a southward slope of only half a foot