Alabama Landscapes

Geology, Geologic Processes and Geologic History

External Geologic Processes

External Processes get their energy from the Sun. Sedimentary rocks are one product of external processes.

The external processes are:

Weathering: in-place disintegration (mechanical weathering) or decomposition (chemical weathering)  of material on or near the Earth’s surface. The main weathering agents are gravity and water (Figures EP1 and 2).

Figure EP1. Disintegration of sandstone. Little River Canyon Cherokee County. Fine material carried by the water has rounded these sandstone boulders. (Image © Mike Neilson)

Figure EP2. Decomposition of dolomite. Logan Martin Dam, St. Clair County. The cracks (called joints) in the dolomite have been opened as the dolomite is dissolved by water. (Image © Mike Neilson)

Erosion: breaking down, loosening and movement of material. Water, wind and ice are the main agents of erosion (Figures EP 3 and 4).

Figure EP3. Stream erosion of river bed. Below Thurlow Dam, Tallapoosa County. The cylindrical holes (called potholes) were drilled by rock fragments rotated by the turbulent water. (Image © Mike Neilson)

Figure EP 4.  Shades Creek. Birmingham, Jefferson County. The brown color is due to all of the eroded material being carried by the creek. Birmingham had 6" of rain that day, resulting in much erosion. (Image © Mike Neilson)

Deposition: return of eroded material to the Earth’s surface (Figure EP5). The deposited material is called sediment.

Figure EP5.  Stream deposition. Sougahatchee Creek, Tallapoosa County. The sand deposit (called a point bar) forms on the inside curve of the river where water slows down ad drops its largest fragments. (Image © Mike Neilson)

Biological activity: aids weathering, erosion and deposition (Figure EP6)

Figure EP6. Trees growing in joints. Bald Rock Mountain, St. Clair County. The roots of these trees aid in the weathering and erosion of the sandstone. (Image © Mike Neilson)

Click here to view the internal processes

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