Alabama Landscapes
The Cumberland Plateau
Physiographic Districts
A abbreviated version of this article occurs in The Encyclopedia of Alabama (http://eoa.auburn.edu/face/Article.jsp?id=h-1301)
The distinction between plateaus and valleys leads to several districts
in the Cumberland Plateau (Figure CD1):
Figure CD1: Physiographic districts of the Cumberland Plateau. BM: Blount Mountains, JCM: Jackson County Mountains, LM: Lookout Mountain, MV: Murphrees Valley, SM: Sand Mountain, SV: Sequatchie Valley, WB: Warrior Basin, WV: Wills Valley. Black circles N to S: Huntsville, Birmingham. (National Map Viewer, USGS and Geological Survey of Alabama)
Relationship of Physiographic Districts to Geology
In the Cumberland Plateau the relationship between geology (rock types and the distribution) and the districts is striking. The most resistant rocks in this section are sandstones of various ages and they are the ridge formers. In particular, Pennsylvanian sandstone (belonging to the Pottsville Formation) form the major plateaus (Figure CD2). The valleys are cut in shale, limestone and dolomite. Of these three, limestone is most easily weathered and eroded; thus the deepest valleys are floored in this rock.
Figure CD2. The bluffs on Smith Lake (Winston County) are composed of Pottsville sandstone. (Image © Mike Neilson)
As
shown by Figure CD3, the valleys (Wills and Sequatchie Valleys) developed
on anticlines (up arches in rocks) and along the
thrust faults. The
plateaus (Lookout Mountain and Sand Mountain) developed on synclines
(down arches in rocks). 
Figure CD3. Topographic profile 1 (see Figure CG3 for location) showing the major geologic structures in the Cumberland Plateau. The blue line shows the base of the Pottsville Formation, the most resistant rock in the area. The blue and red broken lines show the approximate location of the Pottsville and the thrust faults before erosion has reduced the surface to the black solid line.
Weathering and erosion were much more effective in the areas of fractured rocks. Thus, valleys developed along the thrust faults and large fractures that developed parallel to the crests of the anticlines.