
The landforms found in the Highland Rim are the result of differential erosion of the underlying rocks. These rocks consist of middle- to upper-Paleozoic sedimentary rocks (490 to 323 million years before the present), most of which formed during the Mississippian period (353 to 323 million years before the present). The main ridge-forming rock is known as the Hartselle Sandstone, and the valleys cut through the Bangor Limestone and Tuscumbia Limestone Formations [see Figure 2].
Districts
The Highland Rim is comprised of three districts: the Tennessee Valley, Little Mountain, and the Moulton Valley [see Figure 3].
The Tennessee Valley district occupies most of the Highland Rim, stretching north from the base of the northern face of Little Mountain to the Tennessee border. The floor of the valley ranges from 15 to 23 miles wide. Elevations increase moving north from the river to the Tennessee border because the underlying chert-bearing limestones north of Athens, Limestone County, are more resistant to weathering than those to the south of the city.

Moulton Valley, which formed in the Bangor Limestone, separates the Warrior Basin of the Cumberland Plateau from Little Mountain. The valley narrows from about eight miles wide in the east, in the vicinity of Hartselle, to less than three miles wide in the west, south of Russellville. The valley's southern border is a steep sandstone cliff face, or scarp, bordering the Warrior Basin that rises 400 feet above the valley floor. To the north, the valley wall tops out at about 300 feet.
The Tennessee River
The Tennessee River flows through the valley in a northwesterly direction, and its small tributaries flow from Little Mountain and the northern uplands into the main river. Throughout most of its course, the Tennessee flows wide and deep in its own alluvium, which is the loose material deposited by the river when it floods. Prior to construction of Wilson and Wheeler Dams, the river consisted of rapids, shoals, and shallow water from present-day Florence to Decatur, where it cut through the Tuscumbia Limestone and struck the more resistant Fort Payne Chert. In 1924, Wilson Dam and its locks were completed at Florence, and in 1936 Wheeler Dam and its locks were completed upstream, thus making the Tennessee River navigable. The Tennessee Valley Authority (TVA) operates three hydroelectric plants on the river, and the two largest plants in the state, the Wheeler and the Wilson plants, lie within the Highland Rim section.
Natural Resources
Numerous quarries extract limestone from both the Tuscumbia and Bangor Formations. Except for one quarry, which produces dimension stone used for architectural and decorative purposes, the limestone is used as construction aggregate.
Additional Resources
Adams, G.I., et al. Geology of Alabama. Geological Survey of Alabama Special Report 14. Tuscaloosa, Ala.: Geological Survey of Alabama, 1926.
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 Museum of Natural History, 2013.
Pashin, J. C. "New Perspectives on the Mississippian System." Geological Society of Alabama Guidebook 30. Tuscaloosa: Geological Society of Alabama, 1993.