The southwestern and eastern parts of Arkansas lie within the Gulf Coastal Plain physiographic and geologic province. That part of the Gulf Coastal Plain which lies in Arkansas embraces an area of 27,370 square miles, or about 52 per cent of the area of the state.

The Gulf Coastal Plain sediments consist of sands, clays, marls, shales, and limestones but include anhydrite and volcanic rocks ranging in age from Mesozoic to Recent. They rest on a pre-Cretaceous erosional surface of folded and faulted Paleozoic and older rocks.

The oldest Coastal Plain rocks belong to the Comanche series and include the Trinity, Fredericksburg, and Washita groups. They crop out in a narrow belt in Sevier, Howard, and Pike counties. The Gulf series (Upper Cretaceous) crops out in southwestern Arkansas in a parallel, but somewhat wider belt, which extends from the Texas and Oklahoma line eastward into Clark County. The Tertiary rocks are the most widely dis-tributed, occupying, for the most part, the Coastal Plain to the south of the Cretaceous and extending eastward to a line drawn from Little Rock southward into Ashley County. The eastern part of the state, except for small outcrops of Tertiary rocks along the western margin of the Coastal Plain and narrow belts along the flanks of Crowley's Ridge, is mantled with sands, gravels, and loarns of Pleistocene and Recent age.

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