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Pediments and Pediplains

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A pediplain is an extensive flat land formed by the coalescence of pediments. In Geology and Geomorphology, the term pediment is derived from the Latin word ‘pes’ which means the genitive case, and ‘ped’ which means foot. On the other hand, the term pediment is a gently sloping bedrock surface formed by lateral erosion and mechanical weathering. The process through which pediplain is formed is known as pediplantation, and the concepts that help in explaining this phenomenon were first introduced by geologist Lester Charles in 1942.


What are Pediplains?

A pediplain is a relatively flat rock surface formed by joining several pediments. Pediments are generally found in arid and semi-arid areas and may have a thin veneer of sediments. It may be said that pediplain may be the last stage of landform evolution, and the final stage of the erosion process.

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What are Pediments?

A pediment, also known as concave slope or waning slope, is a gently sloping erosion surface or plane of low relief formed by running water in arid and semi-arid areas at the base of the receding mountain front.

 

Pediments are generally erosional surfaces. A pediment develops when sheets of running water wash over it in intense water. A pediplain is covered by the thinly discontinuous veneer of soil and alluvium derived from the upward areas. Much of this alluvial material is transitted across this surface, mining during episodic storm events or blowing winds.


The term pediment should not be confused with the bajada which is a merged group of alluvial fans.

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How Do Pediments and Pediplains Form?

The formation of pediplain depends on erosion, which is the force behind the formation of pediments. Pediment forming processes are much talked about, but it is seen that rocks such as granite and coarse sandstone form all the pediments virtually in the Mojave desert. The pediment formation has not been well documented and accordingly became a subject matter of study, but there are pre-existing theories that attempt to explain this process.


Erosion begins along the steep margin of the landmasses or the steep side of the tectonically controlled steep incision feature over the landmass. The pediments generally have slopes between 0.5 and 7 degrees and are concave in shape. The rock continues to degenerate grain by grain rather than fracturing and further being minimized in grain size by alluvial transport processes. As the pediments are formed by a  steep slope followed by the clive or free face above it, the steep wash slope and free face move backward.


Where Do Pediments Form?

Pediments are formed in the arid and semi-arid areas where rainfall is immense for brief periods of time. This phenomenon of erosion is termed as the retreat of slopes through backwashing. So, through parallel retreat of slopes, the pediments extend backward at the expanse of the mountain front, and gradually the mountain gets minimized leaving an inselberg which is termed as the remnant of the mountain.


Pediments that formed in humid areas are generally obscured by vegetation and may be difficult to observe. This is how the high relief in desert areas is minimized to low featureless plains known as pediplains.


Did You Know?

  • In 1877, Grove Karl Gilbert first observed the pediments in the Henry Mountains in Utah.

  • As per Gilbert, the origin of sediments was in the Henry Mountain area, due to stream plantation and active erosion of the desert.

  • A pediment, also known as concave slope or waning slope, is a very gently sloping ( 0.5 - 0.7) inclined bedrock surface.

  • The concept of pediplain was first introduced by geologist Lester Charles King in his 1942 Book South Artificial Scenery.

FAQ (Frequently Asked Questions)

1. Who Introduced the Concept of Pediplain and Pediplanation?

Ans: The concept of Pediplain and pediplanation was first introduced by the geologist Lester Charles King in his 1942 book South African Scenery. The concept gained fame as it was in contrast to peneplanation. Pediplain is generally formed in the area of arid and semi-arid climates.

2. What are the Characteristics of Pediplain?

Ans: Pediplain generally consists of thin alluvial soil that has undergone extreme erosion and hence it is not compacted. Pediplain is more commonly found in arid, semi-arid, and savannah lands where the erosion is more intense due to the lack of adequate vegetation cover. It is believed that pediplain could be the largest stage in the evolution of landforms, and the end result of the erosion process.

3. Where are Pediments Commonly Found?

Ans: Pediments are commonly found in arid and semi-arid climates and are generally found in the Western United States. However, the pediments are also found along the forearc of the Andes in South Africa and South America. Recently, it is found that pediments are formed in temperate and humid climates and in a variety of tectonic settings, and the characteristics of bedrock are not sensitive to formation.