The Coastal Plain
Chunnenuggee Hills and Southern Red Hills
South of the Black
Prairie, The Chunnenuggee and Southern Red Hills occupy an area that
diminishes in width from about 60 miles along the Alabama/Georgia border
to about 20 miles at the Alabama/Mississippi border. This is the belt
of prominent cuestas and flatwoods. (Figure CPD6)
Figure CPD6. Location of prominent cuestas in the Black Prairie, Chunnenuggee Hills and Southern Red Hills. Black circles: D: Demopolis, M: Montgomery, MB: Mobile, T: Troy. From Self and Emplaincourt (1975).
The Chunnenuggee Hills are underlain by the upper portion of the Selma Group (above the Demopolis Chalk). The Ripley Formation, consisting of sand, clay and sandstone, occurs throughout, but the nature of the other materials changes from east to west. In the east, chalky materials are absent and they are replaced by sand (Blufftown Formation, Cuesseta and Providence sands). Here the Chunnenuggee Hills abuts the Fall Line Hills along the 140 foot high Sand Fork Cuesta (Figure CPD4). The Enon Cuesta is developed on the Cusseta sand. Further to the south, the Ripley Cuesta occurs on a resistant layer within the Ripley Formation. The Troy Cuesta occurs on Providence Sand and is the southern boundary of the Chunnenuggee Hills. In the western regions, the Chunnenuggee Hills are very narrow and are boarded by the Ripley and Troy cuestas.
The Southern Red Hills are supported by units of the Midway and Wilcox Groups and and part of the Claiborne Group (Figures CDP4 and 5). Elevations commonly reach over 400 feet in these hills and local relief can be as much as 200 feet. The northern boundary is the Troy Cuesta (between 80-140 feet relief), developed in the Providence Sand in the east and central parts of the state (Figure CPD6) and the Ripley/Wilcox boundary in the west. The southern boundary occurs where the hills give way to the flatlands of the Dougherty Plain in the east and central areas. In the west, a well-defined ridge (the Buhrstone Hills) of resistant siliceous claystone, the basal unit of the Claiborne Group, defines the southern boundary with the Lime Hills. North of the Buhrstone Hills is a 6 mile-wide flatwoods, one of the widest in the Coastal Plain.
Figure CPD6. Cuesta, south of Troy, H-231. Looking NW. The beds dip towards the photographer. (Image © Mike Neilson)
The Lime Hills occur in the southwestern part of the Coastal Plain and are underlain by units of the Claiborne Group (Figure CPD7). Although generally lower than the surrounding districts, these hills are rugged with very narrow and steep sided ridges and valleys (with relief of 150-170 feet) cut into sands, silts and clays. To the east the boundary with the Dougherty Plain is gradational.
The Dougherty Plain is a slightly tilted to the south flatland that is underlain by residual material from the weathering of limestone, some sand and clay (Figure CPD4). It runs from just east of Monroeville (Conecuh County) ESE for about 100 miles to the Alabama/Florida/Georgia border. It widens to about 25 miles in Covington and Huston Counties. Elevations are 300-350 feet along its boundary with the Southern Pine Hills and as low as 160 feet in Houston County. It includes much of the Wiregrass region of Alabama. A characteristic of this area is the subsurface drainage, due to extensive solutioning of limestone.
Southern Pine Hills
The Southern Pine Hills occur in the extreme southern portions of the state from western Covington County to the Mississippi border and are underlain by Miocene sediments and the Citronelle Formation (Figure CPD7). The boundary between these hills and the Dougherty Plain in the east or the Lime Hills to the north and west is marked by a noticeable increase in elevation. The Southern Pine Hills reach elevations of slightly over 500 feet near Grove Hill (Clarke County) and commonly are 300 feet. At the coast, elevations decrease to lower than 50 feet.