Geologic Pediment


Geologic Pediment

A pediment is a gently inclined erosional surface carved into bedrock. It is thinly covered with Fluvial gravel that has developed at the foot of mountains. It develops when running water erodes most of the mass of the mountain. It is typically a concave surface gently sloping away from mountainous desert areas. (Easterbrook)

Processes Responsible for Carving Pediments

1) Lateral Planation or Erosion by a stream (Johnson)2) Sheetwash or Sheet Erosion-

"The removal of thin layers of surface material more or less evenly from an extensive area of gently sloping land, by broad continuous sheets of running water rather than by stream flowing in well defined channels" (Wilson)
3) Rillwash or Rill Erosion-
"The development fo numerous minute closely spaced channels resulting from the uneven removal of surface soil by running water that is concentrate in streamlets of sufficient discharge and velocity to generate cutting power" (Wilson)
4) Mountain-front Retreating by Weathering

History

In 1877 Gilbert first observed pediments in the Henry Mountains in Utah. He described the formation as "hills of planation cut across the upturned edges of tilted beds". Gilbert believed the origin of pediments in the Henry Mountains are due to stream planation and active erosion of deserts. This theory was advocated by Paige (1912), Blackwelder (1931), and Johnson 1932.Johnson came up with three zones of pediments.(Easterbrook)

Zones of Formation

1. This zone is the inner most zone. Which consists mountainous uplands that have vertical erosion

2. This zone is the middle or intermediate zone. It is known as the degradation zone. It is the pediment beyond the mountain front.

3. This zone is the outer zone known as the aggradation zone. It lays past the pediment in where there is strong deposition.(Easterbrook)

Each of these layers encroach upon each other covering each layer with alluvium beyond their normal capacity.

References

[1] "Surface Processes and Landforms" 1999 Don J. Easterbrook. Prentice Hall, Upper Saddly River, New Jersey. [2] "Rock Planes of Arid Regions" Douglas Johnson. Geographical Review, Vol. 22, No. 4 (Oct., 1932), pp. 656-665. [3] "Glossary of Hydrology" 1998 Ed. William E. Wilson. American Geological Institute.


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