Pediment (geology)

A pediment is a very gently sloping (.5°-7°) inclined bedrock surface.[1] It typically slopes down from the base of a steeper retreating desert cliff, or escarpment,[2] but may continue to exist after the mountain has eroded away.[1] It is caused by erosion.[2] It develops when sheets of running water (laminar sheet flows) wash over it in intense rainfall events.[1][2] It may be thinly covered with fluvial gravel that has washed over it from the foot of mountains produced by cliff retreat erosion. It is typically a concave surface gently sloping away from mountainous desert areas.[3]

It is not to be confused with merged groups of alluvial fans (bajadas), which also may appear to gently slope from an escarpment, but are composed of material eroded from canyons, not bedrock.[1]

Three formational zones are recognized for pediments:[3]

  • An inner most zone of mountainous uplands that have near vertical erosion
  • An intermediate zone or degradation zone which is the pediment beyond the mountain front.
  • An outer zone or aggradation zone which extends beyond the pediment and is a zone of deposition.

Coalescence of pediments over a large area results in a pediplain.[4]

Processes responsible for carving pediments

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.[6]

  • Rillwash or rill erosion:

The development of 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.[6]

  • Mountain-front retreating by weathering

History

In 1877 Grove Karl 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.[3]

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References
  1. Encyclopædia Britannica, Pediment
  2. Essentials of Geology, 3rd Edition, Stephen Marshak, p464, pG-15
  3. Easterbrook, Don J. (1999) Surface Processes and Landforms, New Jersey, Prentice Hall
  4. Jones, David K.C. (2004). "Denudation chronology". In Goudie, A.S. (ed.). Encyclopedia of Geomorphology. pp. 244–248.
  5. Johnson, Douglas (1932) Rock Planes of Arid Regions, Geographical Review, Vol. 22, No. 4 (Oct., 1932), pp. 656–665
  6. Wilson, William E. (editor) (1998) Glossary of Hydrology, American Geological Institute
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