The Coastal Plain
Fall Line Hills
The Fall Line Hills is the northern boundary of the Coastal Plain. Underlain by sands and gravels of the Tuscaloosa Group and the sands, clays, beds of oyster shells and occasional sandstone of the Eutaw Formation, rounded hills are cut by valleys with local relief of about 200-250 feet. The higher elevations are occupied by hills held up by the gravels of the Tuscaloosa Group and the Eutaw Formation.
For most of its southern boundary the Fall Line Hills contact the Black Prairie at elevations about 200 to 250 feet (Figure CPD3).
Figure CPD3. Topographic Profile through the central part of the northern Coastal Plain from Prattville to Fort Deposit. Vertical scale and dips much exaggerated. Number on the Arcola Cuesta is the relief of the northern slope. (Profile 2 on Figure CPD1)
However, east of Hurtsboro (Russell County) the Fall Line Hills is in contact with the Chunnenuggee Hills along the Sand Fort Cuesta, whose north-facing slope rises 140 feet (Figure CPD4). In this area the Eutaw Formation is overlain by sand and clay of the Blufftown Formation (which is responsible for the Sand Fort Cuesta).
The Black Prairie is a unique landscape in Alabama. Bordered by the Fall Line Hills to the north and the Chunnenuggee Hills to the south, this belt stretches from Gainesville (Sumter County) through historic towns such as Demopolis, Selma, and Montgomery and ends where Bullock, Macon and Russell counties meet southwest of Tuskegee. It is about 18 miles at its widest and about 145 miles long. Elevations range from about 250 feet along the Fall Line Hills to over 400 feet in the south.
The Black Prairie owes its character to the underlying chalks of the Mooreville and Demopolis Chalks (often also called the “Lower Selma Group”). The land overlying the Mooreville Chalk is low and rolling, characterized by a thick black soil and a prairie (grass and trees) vegetation. A resistant limestone at the top of the Mooreville forms the Arcola Cuesta (50 feet high) where the Mooreville gives way to the Demopolis (Figure CPD3). In contrast, the land overlying the Demopolis Chalk tends to be bare, with a very thin soil horizon, and little vegetation.
The boundary with the Chunnenuggee Hills changes from east to west. In the east (Russell County), the boundary is the Enon Cuesta, where the Mooreville Chalk meets the more resistant Blufftown Formation.
In the central and western areas the boundary is a steepening of slope, where the Demopolis Chalk gives way to the basal units of the Ripley Formation (Figure CPD5), forming the Ripley Cuesta. The north slope of the cuesta varies in height from about 50 to125 feet.
Figure CPD5 . Topographic Profile through the western Coastal Plain from Tuscaloosa to Grove Hill. (Profile 3 on Figure CPD1) Number on each cuesta is the relief of the northern slope. Vertical scale and dips much exaggerated