Alabama Landscapes
The Valley and Ridge
Cahaba and Coosa Ridges, Cahaba Valley
East of the Birmingham-Big Canoe Valley is a 15 mile-wide complex of rugged ridges and deep valleys that comprise the Cahaba Ridges, Cahaba Valley and Coosa Ridges districts. Each of the two ridges districts is about 60 mile long. The Cahaba Ridges are buried beneath the East Gulf Coastal Plain near West Blocton (Bibb County) and terminates southeast of Springville (St. Clair County), where the Big Canoe and Cahaba Valleys meet. The Coosa Ridges district runs from northeast of Ragland (Calhoun County) to Saginaw (Shelby County). The Cahaba Valley occupies the lowlands between the ridges and melds with the Big Canoe Valley and Coosa Valley in the northeast and the Coosa Valley in the southwest.
The topographic profile (Figure VDR6) shows the relationship of the ridges and valley to the underlying geology. The folded nature of the rocks is shown by the trace of the base of the Pottsville sandstone.
Figure VRD6. Northwest-southeast topographic profile showing most of the physiographic districts in the Valley and Ridge and their relationship to the underlying geology. Relief and dip of rocks are exaggerated. Red lines: thrust faults, blue line: base of the Pottsville sandstone. (Location of profile: 2 on Figure VRG2.)
The Cahaba and Coosa Ridges consists of several parallel, zig-zag ridges capped with Pottsville sandstone (see Figure VR2). These zig-zag ridges are the sides (limbs) of plunging folds.
Shades Mountain is the northwestern edge of the Cahaba Ridges and is the northwestern limb of the Cahaba Syncline (Figures VRG2 and VRD2). Elevations decrease to the southeast across axis of the syncline. The southern limb of the fold ins missing, having been sheared off by the Helena thrust fault. The Cahaba River flows for most of its course in the Cahaba Ridges. (Figure VRD7)
Figure VRD7. Cahaba River (Jefferson County). The rocks in the stream are Pottsville Sandstone. (Image © Mike Neilson)
The Cahaba Valley (Figure VRD8) is cut into southeasterly-dipping limestone and shale . Elevations increase slightly towards the southeast until an abrupt increase in elevation of about 300 feet occurs along the northwestern face of Oak Mountain at the boundary with the Coosa Ridges.
Figure VRD8. Cahaba Valley along H-119 (Shelby County). The high ridge in the background is Oak Mountain, the northwest edge of the Coosa Ridges. (Image © Mike Neilson)
The Coosa Ridges consist of several closely-spaced plunging anticlines and synclines. The higher ridges are capped in Pottsville sandstone (Figure VRD9)
Figure VRD9. Pottsville sandstone capping a ridge in the Coosa Ridges, Cook Springs , St. Clair County. (Image © Mike Neilson)
The valleys, which are often 200-300 feet beneath the ridges, are cut into shale (Figure VDR10).
Figure VDR10. Coosa Ridges along I-20 near Cook Springs, showing the sandstone ridges and shale valleys. The high ridge in the background is Bald Rock Mountain. (Image © Mike Neilson)