
The Coastal Plain developed on geologically young Mesozoic to Recent (from about 140 million years ago to the present) sedimentary rocks and sediment. The geologic units, composed mainly of sediments, are described variously as gravels, sands, silts, and clays. The rocks are mainly composed of chalk, sandstone, limestone, and claystone. The beds slope gently southward at about 40 feet per mile and are progressively younger from the fall line to the coast. Locally, higher elevations are underlain by more resistant material (in some areas it is sediment, in others sedimentary rock), and the lowlands are underlain by softer material. The type of resistant material varies from one physiographic district to another.

Districts
Eight districts [See Figure 4] occur in this section: Fall Line Hills, Black Prairie, Chunnenuggee Hills, Southern Red Hills, Lime Hills, Southern Pine Hills, Dougherty Plain, and Coastal Lowlands.
The most northerly district of the Coastal Plain is the Fall Line Hills. Its rounded hills are cut by valleys with local relief of between 200 and 250 feet and are underlain by sands and gravels, clays, beds of fossil oyster shells, and occasionally sandstone. For most of its southern boundary, the Fall Line Hills contacts the Black Prairie at elevations that range from 200 to 250 feet above sea level. East of Hurtsboro, in Russell County, however, it contacts the Chunnenuggee Hills along the Sand Fort Cuesta, whose north-facing slope rises 130 feet.


The Chunnenuggee Hills district, which includes the Sand Fork, Enon, Lapine, High Ridge and Ripley cuestas, formed on sands and sandstones. Along the 175-foot-high Lapine cuesta, in the central region, the hills reach elevations of more than 570 feet above sea level. In the western regions, the Chunnenuggee Hills are very narrow and are bounded by the Ripley and Troy cuestas.
The Southern Red Hills district formed on sands, limestone, marls, clay, and silt, and elevations in these hills commonly reach more than 400 feet above sea level with local relief of as much as 200 feet. The northern boundary is the Troy cuesta (between 80 and 140 feet high), which developed on sand. The southern boundary occurs where the hills give way to the flatlands of the Dougherty Plain district in the east and central areas.
The Chunnenuggee and Southern Red Hills districts become narrower to the west. Collectively, they are about 60 miles along the Alabama-Georgia border and about 20 miles at the Alabama-Mississippi border.
The Lime Hills district occurs in the southwestern part of the Coastal Plain. Although generally lower than the surrounding districts, the hills are rugged, with very narrow and steep ridges and valleys (with relief from 150 feet to 170 feet) cut into sands, silts, and clays. The Lime Hills grade into the Dougherty Plain.
The Dougherty Plain district is a flatland slightly tilted to the south and underlain by residual material from the weathering of limestone, sand, and clay. It runs from just east of Monroeville, Conecuh County, east-southeast for about 100 miles to the Alabama-Florida-Georgia border. It widens to about 25 miles in Covington and Houston Counties. Elevations are between 300 and 350 feet along its boundary with the Southern Pine Hills district and as low as 160 feet in Houston County. The plain includes much of the Wiregrass region of Alabama. A characteristic of this area is the absence of streams, because the rocks are so soluble that most water flows beneath the surface.
The Southern Pine Hills district occurs in the extreme southern portions of the state, from western Covington County to the Mississippi border. The boundary between these hills and the Dougherty Plain in the east and the Lime Hills to the north and west is marked by a noticeable increase in elevation. The Southern Pine Hills district is generally around 300 feet in elevation, but some hills reach elevations of slightly more than 500 feet above sea level near Grove Hill, in Clarke County. At the coast, elevations decrease to less than 50 feet.
The Coastal Lowlands district developed primarily on sand and mud. The landforms are highly variable, being continually modified by wind, tides, currents, and waves.
River Systems
Five main rivers flow through the Coastal Plain: the Chattahoochee, Choctahatchee, Conecuh, Alabama, and Tombigbee-Black Warrior [See Figure 5]. With the exception of the Tombigbee River, the rivers generally flow south-southwest across the physiographic districts and all cut though the prominent cuestas and hills. In many places, the tributaries flow at right angles to the main streams in the valleys and flatwoods. This pattern can be explained by the varying erosive forces of the rivers. The streams with higher discharge had sufficient erosive power to cut through both resistant and soft materials and formed major southwesterly trending rivers. Smaller tributaries tend to erode only the softer materials, thus forming the cuesta-flatwoods landscape.

The Tombigbee and Black Warrior rivers join at Demopolis, in Marengo County, and are part of the $2 billion Tennessee-Tombigbee Waterway. Approved by Congress in 1946, the aim of the waterway was to shorten the distance that freight had to be carried to the Gulf Coast. The TVA excavated a 27-mile-long, 12-foot-deep navigation canal to join the headwaters of the Tombigbee at Pickwick Lake on the Tennessee River and constructed 205 miles of canals, five dams, and 10 locks on the Tombigbee River. The Heflin and Bevill lock-and-dam systems are located in Alabama, whereas the rest are in Mississippi.
Natural Resources

Additional Resources
Adams, G.I., et al. Geology of Alabama. Geological Survey of Alabama Special Report 14. Tuscaloosa, Ala.: Geological Survey of Alabama, 1926.
Jones D. E. Geology of the Coastal Plain of Alabama. Guidebook of the 80th Meeting of the Geological Society of America, New Orleans, 1967.
Lacefield, J. Lost Worlds in Alabama Rocks. Tuscaloosa: Alabama Geological Society, 2000.