Essay on The cycle of erosion in arid lands

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There is a cycle of erosion in arid lands which is similar in many ways to the cycle in humid region. But there is one difference. In humid region the formation of peneplains as the end- product of the cycle has been emphasized, whereas in arid cycle the emphasis lies on the formation of pediments.

The cycle in humid regions begins with a continuous lowering of the surface by the progressive flattering of slopes. In arid regions the removal of upland is made possible by the horizontal retreat of cliffs or escarpments.

However, like humid regions, the landscape evolution in arid lands also passes through the stages of youth, maturity and old age.

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The stage of youth:

The stage of youth in arid regions is characterized by decrease of the original relief. In the early stages of erosion, short, consequent, intermittent streams cut ravines and V-shaped valleys in the mountain slopes.

In other words, the rare and short-lived flash-floods cut ravines and gullies in the uplands. These envelop into wadis and canyons from which debris is washed down the slopes and spread over the depressions. Alluvial fans are formed at the mouths of the valleys forming playas or salinas.

If streams are able to reach the lakes, their load is deposited on the playas. Most of the ephemeral streams disappear in the sands. Lake water becomes salty due to the minerals carried in solution by the streams and by seepage waters.

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If the whole lake dries up, salts are deposited around the margins of the lake. Deflation is active. The dust and sand are blown away by the wind.

Dunes are formed here and there on the valley floors from the sand washed down by the intermittent streams. Between the uplands and the playas, the alluvial fans unite to form bajadas which increase in thickness.

The relief of the region is reduced because highlands are being eroded at the same time that the adjacent depressions are being filled with debris. Still each valley is the local base level for all the streams associated with it.

The stream-cut valleys expose bare rocks which are subjected to mechanical and chemi­cal weathering. Thus, the weathered materials are prepared for removal by slope wash and streams dur­ing the following next rainfall, if any.

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Stage of maturity:

Due to small amount of rainfall the progress from youth to maturity is very slow. The stage of maturity begins when the moun­tains become like islands half submerged in their own debris, because mountains are being cut back by erosion. The divides become narrower, and inter­vening basins or depressions become wider and higher due to valley filling.

The pediments separate the mountain fronts from the alluvium in the valley. Large alluvial fans from the two sides coalesce and divide the main valley into smaller basins. Because of head-ward erosion by consequent streams, an integra­ted drainage system develops.

As the highlands are Old Stage reduced in height, the rainfall tends to decrease. Due to the widening of basins and greater proportion of the fine material deposited into the valleys, the slopes of the streams are decreased.

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The stage of old age:

During the old age the higher basins contribute materials to the lower basins until they all unite. The highlands are worn away. Islands-like mountains or hills-called inselbergs-will stand above the surrounding pediments or bajadas.

The streams have reduced the land to the level of lowest basin which is their base level. Now, the number of dunes and wind- scoured hollows increase. The silt of the playas has been blown away.

If there is no interruption in the cycle, the region is reduced to an erosion surface, closely related to a pediplain. However, it is of more complex structure, being composed of desert pavement and vast wastes of sand.

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However, wind erosion continues indefinitely, eroding the land lower until it may be brought to an elevation far below sea level. Water table is the base level for the arid cycle of erosion.

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