There are five physiographic provinces in Alabama (figure from Ebersole and others, 2019; click to expand), which are differentiated by topographic relief and patterns, rock types, and geologic structures. These provinces include the Central Basin, Appalachian Plateaus, Interior Low Plateaus, Valley and Ridge, Piedmont, and East Gulf Coastal Plain provinces (figure below). Geologic information below includes revised excerpts from the Stratigraphy of Alabama, Circular 140, by Raymond and others (1988) and Physiographic Regions of Alabama, Special Map 168, by Sapp and Emplaincourt (1975). See the GSA Publications webpage for more information and references

Physiographic regions of Alabama (Ebersole and others, 2019). Click to expand.

Central Basin Geology

The smallest physiographic province in Alabama, the Central Basin, lies along the Tennessee-Alabama line and is primarily underlain by Ordovician limestones. The small knobby hills are capped by Devonian Chattanooga Shale and cherty Mississippian age carbonate units. These characteristic knob hills are remnants of maturely dissected low plateaus and include the drainage area closest to the Elk River in Limestone County and small outliers in other counties.

Appalachian and Interior Low Plateaus Geology

Most of north Alabama is divided on the basis of physiography into two provinces, the Appalachian and Interior Low Plateaus. The Interior Low Plateau to the north is primarily a limestone plateau of moderate relief. To the south and east is the Appalachian Plateaus, comprised of submaturely to maturely dissected sandstone and shale synclinal plateaus with moderate relief. In the eastern part of the Appalachian Plateaus are three linear anticlinal limestone valleys (Murphrees Valley, Wills Valley, and Sequatchie Valley) characterized by resistant sandstone ridges with moderate relief.

Valley and Ridge Geology

The Valley and Ridge province in central and northeastern Alabama consists of a series of subparallel ridges and valleys trending generally northeast-southwest. This characteristic topography is developed on folded and thrust-faulted sedimentary rocks. The ridges are formed by sandstone and chert beds that are resistant to erosion; valleys are underlain by less resistant shale and carbonate rocks. The northwestern half of the province has well developed Valley and Ridge topography. The southeastern part of the province is characterized by a wide plain of varied relief containing irregularly spaced parallel ridges and valleys. In the extreme northeastern part of the province, mountainous terrain is developed on faulted and folded sandstone and quartzite.

Piedmont Geology

The crystalline rocks of the Alabama Piedmont are subdivided into two districts that contain crystalline (igneous and metamorphic) rocks: the Northern Piedmont and Southern Piedmont. Each is bounded by a major regional fault. Metamorphic grade increases across the Piedmont from low-grade greenschist facies on the northwest to high-grade migmatite facies on the southeast. Elevations in the Northern Piedmont range from approximately 1,000 feet on the northwest to 500 feet in the south, and sets of prominent ridges subdividing the district's topography. The highest ridges include Cheaha Mountain (2,407 feet in elevation), the highest point in Alabama. Elevations in the Southern Piedmont district are more subdued, from approximately 800 feet in the north to approximately 500 feet in the south. The boundary between the two districts coincides with the approximate location of the Brevard fault zone.

East Gulf Coastal Plain Geology

The East Gulf Coastal Plain physiographic section of the Coastal Plain province in Alabama is an area of Mesozoic and Cenozoic sediments in the southern and western parts of the state, where mostly unconsolidated sediments of the Coastal Plain overlap consolidated rocks of the Plateaus, Valley and Ridge, and Piedmont provinces. In northwestern Alabama, Coastal Plain sediments are up to 1,000 feet thick and cap hills and plateau remnants of Paleozoic rock. The sediments thicken rapidly to the south and are more than 24,000 feet thick near the coast. Most of the Coastal Plain is low to moderate relief, and the topography is gentler than that of the other regions, although stream valleys may have greater relief. Resistant beds in some areas form broadly arcuate cuestas 50 to 200 feet above the surrounding prairie in parts of central Alabama. Further south, in parts of Choctaw, Clarke, and Monroe Counties, are hills underlain by resistant parts of Eocene formations.

Selected References

Ebersole, S. E., Guthrie, G. M., and VanDervoort, D. S. 2019, An update to the physiographic districts of Alabama: Alabama Geological Survey Open-File Report 1901, 20p + plate.

Raymond, D. E., Osborne, W. E., Copeland, C. W., and Neathery, T. L., 1988, Stratigraphy of Alabama, Geological Survey of Alabama Circular 140, 97 p.

Sapp, C. D., and Emplaincourt, Jacques, 1975, Physiographic regions of Alabama: Alabama Geological Survey Special Map 168.