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Action of Wind

In the desert or arid regions, wind is the most active agent of erosion. Wind can lift and carry with it loosed, fine and light rock waste which lies upon the surface. Three processes are involved in wind erosion:

Deflation
This refers to the reduction of the land surface through the blowing away of unconsolidated rock materials. The very fine materials are carried away as dust storm.

Heavier ones in the form of sand grains are moved along in sandstorms while coarser ones are dragged along the surface through a series of hops.

Abrasion
This is a process whereby a load of hard quartz grains and other rock particles are blasted on rock surfaces. Its effect is most noticeable at the foot of rock outcrops.

Attrition
Rock materials that are being moved by wind do not only wear away the land surface and rock outcrops they meet on the way, but also rub against each other thereby wearing themselves away. This produces rounded sand grains popularly termed millet seed.

Features of wind erosion
Several features are produced by wind erosion and these include:

1. Ventifacts: The rocks that are too heavy to be moved by the wind are worn on the side facing the wind. Such features are referred to as ventifacts. A special type that has been blasted on three faces is known as Dreikanter.

2. Rock pedestals: If projecting rock masses of varying degrees of hardness are attacked by differential abrasion, groves and hollows are created on the rock surfaces following lines of weakness such as joints. This produces grotesque rock pillars known as rock pedestals. Often these rock pedestals have a mushroom shape. As a result, they are called mushroom rocks or gour (singular-gara)in the Sahara.

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Rock pedestals or gour

3. Zeugen: Zeugens are formed when hard and soft rock strata lie horizontally upon one another, and through weathering and breaking by wind abrasion, distinctive tabular masses are left standing upon the softer rock stratum. These tabular masses are termed zeugens. Azeugen measures up to 30m above the surrounding level.

img 24. Yardang: This looks very much like the zeugen except that the hard and soft rock  bands are vertical. Such bands are roughly parallel to the direction of prevailing wind. Through wind abrasion, a ridge is furrowed and relief is produced. Examples exist in the Atacama desert.

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5. Mesas and buttes: Mesas occur where a harder rock stratum lies above a softer one. With weathering and erosion, the harder stratum is left as a tabular mass capping the softer one. Over time, mesas are reduced to buttes. Good examples are found on the Jos Plateau of Nigeria, in Ginalimaka and the sandstone region of eastern Mauritania.

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6. Inselbergs: These are steep-sided isolated residual hills with rounded tops. They stand out prominently above the surrounding level. Inselbergs are not only found in core desert areas, but also in semi-arid regions. Examples are found in northern Nigeria, the Kalahari desert and western Australia.

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7. Deflation hollows: Wind deflation is a very important cause of the formation of depressions or hollows in desert and semi-desert regions. Minor fault displacements may trigger off a hollow, especially when a resistant surface stratum is fractured. Within, wind eddies erode the less resistant strata. This may continue until the water table is reached thereby reaching 134m below sea level. Another example is the Faiyum Depression in Egypt lying 40m below sea level.

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Features of Wind Deposition
Whenever erosion takes place, the detached materials are transported to other locations and deposited there. These are deposited in different forms. So also, all the materials eroded by wind are deposited in different forms. Here we are going to study the various features of wind deposition.

1. Dunes: These are the most popular features of wind deposition found. In a general sense they are low hills possessing varying shapes and extent formed by the accumulation of sand. Several types of dunes exist. They include barkhan, amers, self dunes, star dunes, hairpin dunes, attached dunes and head dunes, pyramid  dunes, sword dunes, smoking dunes and oblique dunes. Some of them are always on the move and thus termed active or live dunes. Barkhans and self dunes are quite distinct and so are the ones studied here.

a. Barkhan: It is also spelt as barchan or barchane. Barkhan dunes or simply barkhans are live dunes with a moon or crescent-shape. Such dunes are traverse to the wind with their horns following the direction towards which it is blowing. The windward s!de is gentle, but with a steeper leeward side and a height of up to 30cm. Barkhans occur mostly in groups and occasionally as isolated hills. Being always on the move, they have constituted a threat to oases. Shelterbelts of acacias and eucalyptus are often planted to stabilise these dunes. Also the ground is sprayed with specially formulated oil and sometimes with plastic film to help the surface sand to cohere.

b. Seif: This is also spelt as seif and equally called longitudinal dune. Seif dune or simply seif exists as a long ridge, sometimes several kilometres in length, and lying parallel to the direction of the prevailing wind. It may be caused by the joining of a series of barkhans and their tails are swept off by prevailing winds. The sides of seifs are built up by eddies. Examples are found in the Sahara south of the quattara depression, in southern Iran, and in the west Australian.

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Crescentic sand dune – barchan

c. Loess: It refers to the accumulation of the wind-borne materials usually found far beyond the limits of the deserts. Where similar deposit occurs in Germany and France it is known as limon, while in the American states of Mississippi and Missouri, as adobe. Loess appears as a yellow, friable, porous material covering several thousands of kilometres and its prevailing wind grains are very coherent. Its surface is usually dry because of its porosity since water easily sinks through it. Examples are commonly found in north-western China, and in central Asia.

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