Pediments and Pediplains
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.
FAQs on Pediplain
1. What is a pediplain in physical geography?
A pediplain is an extensive, low-relief plain that forms in arid or semi-arid climates. It is considered an erosional surface, created by the joining of multiple individual pediments. This landform represents a late stage in the cycle of erosion, characterised by a gently sloping bedrock surface covered by a thin layer of sediment.
2. What are the main characteristics of a pediplain?
The main characteristics of a pediplain are:
- Low Relief: It is a vast, nearly flat plain with a very gentle slope.
- Erosional Origin: Formed primarily through the erosion and backward retreat of mountain slopes, a process known as pediplanation.
- Bedrock Surface: Beneath a thin veneer of alluvium, the pediplain is a solid rock surface.
- Climate: Typically develops in arid, semi-arid, and savanna regions where mechanical weathering is dominant and vegetation is sparse.
- Associated Landforms: Often features residual hills or isolated rock masses called inselbergs rising abruptly from its surface.
3. How is a pediplain formed through the process of pediplanation?
A pediplain is formed through pediplanation, which involves the parallel retreat of slopes. In arid regions, weathering breaks down the rock on mountain fronts. Runoff, primarily from infrequent but intense rainfall, transports this weathered material away. This causes the steep mountain front to retreat backward while maintaining its angle, leaving behind a gently sloping bedrock surface called a pediment at its base. Over millions of years, as multiple mountain fronts retreat and their pediments expand and merge, they form a vast, continuous erosional plain known as a pediplain.
4. What is the key difference between a pediplain and a peneplain?
The key difference lies in their formation process and the climate in which they develop. A pediplain forms in arid climates through the parallel retreat of slopes (back-wearing), resulting in a sharp angle between the plain and any remaining hills (inselbergs). In contrast, a peneplain forms in humid climates through down-wearing of the landscape by rivers, resulting in a gently undulating surface with smooth, convex-concave slopes.
5. Is a pediplain an erosional or a depositional landform?
A pediplain is fundamentally an erosional landform. Its creation is dominated by the removal of rock material and the backward retreat of escarpments. While it is often covered by a thin layer of sediments (alluvium) transported from the highlands, this deposition is secondary to the primary large-scale erosional processes that sculpt the bedrock surface underneath.
6. How do landforms like inselbergs and pediments relate to a pediplain?
These landforms are all integral parts of the same landscape evolution. A pediment is the foundational block—a gently sloping erosional surface at the foot of a receding mountain. A pediplain is the large-scale result of several pediments growing and merging together. An inselberg (meaning 'island mountain') is a steep-sided, isolated residual hill or mountain that remains on the pediplain, representing the last remnants of the original highland that has not yet been fully eroded.
7. Where can examples of pediplains be found in the world?
Examples of pediplains and their associated features are commonly found in arid and semi-arid regions across the globe. Notable examples can be observed in the Basin and Range Province of the Western United States, the landscapes of South Africa, and along the forearc of the Andes in South America. These areas showcase the characteristic vast plains with occasional inselbergs.
8. Who is credited with introducing the concept of pediplanation?
The concept of pediplains and the process of pediplanation were introduced by geologist Lester Charles King in his 1942 book, South African Scenery. His theory was developed as a direct alternative to the widely accepted concept of peneplanation, offering a different model for the final stage of landscape evolution, particularly suited for non-humid climates.
