Introduction
Wind action is conspicuous in semi-arid and arid regions, but it is particularly strong in deserts. Aeolian topography is created by the geological faction of wind, which can conveniently be divided into the following three stages :
(a) Erosion
(b) Transportation
(c) Deposition
(a) Erosion. The wind accompolishes erosion by three means :
(i) Deflation. (ii) Abrasion. (iii) Attrition.
(i) Deflation. Deflation is the process of removal of loose soil or rock particles, along the course of the blowing wind. This process operates well in dry regions with little or no rainfall.
(ii) Abrasion. Abrasion is the sand blast action of wind with sand against the rocks. The loose particles that are blown away by the wind serve as tools of destruction and when they move on some rock-surface they bring about a scraping of the surface.
(iii) Attrition. Attrition is the grinding action. While on transit wind born particles often collide with one another. Such mutual collision brings about a further grinding of the particles.
Important Erosional Features and Associated Landforms
(i) Hamada. Due to deflation, when the loose particles are swept away, only the hard mantle is left behind which is known as Hamada.
(ii) Yardang. A grooved or furrowed topographic form produced by wind abrasion, which is elongated in the direction of prevailing winds and is usually strongly under cut, is known as Yardang.
(iii) Pedestal rock. A wide rock-cap standing on a slender rock column, produced because of the wind-abrasion, is known as a pedestal rock.
(iv) Ventifacts. These are pebbles faceted by the abrasive effects of wind-blown sand. Ventifacts with one smooth surface is called Einkanters and with three smooth faces as Dreikanter ; when only two abraded faces are left, it is called zweikanter.
(v) Mushrom-table. Itis a tabular mass of more resistant rock resting on under-cut pillars of softer material. They are very often elongated in the direction of the prevailing wind and are also known as ‘Zeugen’.
(vi) Honey-comb structure. Rocks consisting of hard and soft parts get differential abrasion and the resulting feature is known as honey comb structure.
(vii) Blow-outs. These are broad-shallow caves in bills, broad shallow depression in deserts.
(viii) Desert pavement. It is made up of a layer of residual pebbles and cobbles strewn upon the surface while intervening finer particles have been removed as a result of deflation.
(xi) Millet-seed sands. These are rounded desert sand grains, resulted through the process of attrition and have resemblance with millet seed grains.
(b) Transportation. Wind-transportation is totally dependent on wind-velocity. There are three methods of wind transportation :
(i) Traction. Where particles are removed through rolling and creeping.
(ii) Saltation. Here the particles, which are too heavy to remain in suspension and lighter to be transported in traction, are transported through a series of bounces.
(iii) Suspension. Very light particles like dust and cloud, smoke etc. move with the wind quickly but settle very slowly, remain in suspension in the air.
(c) Deposition. Wind formed deposits are called aeolian deposits. Wind is an excellent agent for sorting of materials according to their size, shape or weight. Pebbles and boulders cannot be carried away and are left back to form lag deposits. The clayey and silty fractions are deposited as loess, which does not show any stratification. Wind deposits take two general forms as :
(i) Sheets, (ii) Piles.
'Sheet deposits’ are the dust deposits laid down on large ares. ‘Piles deposits’ include the various types of dunes which accumulate from sand and silt carried in saltation.
Depositional Features
(i) Sand hill. Mounds of sand whose surface is irregular is called sand hill.
(ii) Sand dune. When the mound is in the form of a round hillock or a ridge with a crest, it is called a sand dune. In structure a dune has a gentle slope towards the wind ward side and a steep face towards the lee side. The shape of a dune is controlled by
(a) amount of sand supply
(b) wind-velocity
(c) constancy of wind direction and
(d) amount and distribution of vegetative cover
Types of Sand Dune
(i) Barchan. Barchan are the ctrescentic shaped dunes with the points or wings directed downwind.
(ii) Seif. It is similar to barchan except one wing is missing, caused by an occasional shift in wind direction.
(iii) Transverse dune. Elongeted dunes form at right angles to the prevailing wind.
(iv) Fore dune. Ridge-like deposits of wind borne sand formed along the coast of sea or lakes.
(v) Longitudinal dune. Elongated ridges of sand found to lie parallel to the direction of blowing wind.
(vi) Parabolic dune. These are of parabolic shape, their horns point towards the direction opposite to that of the blowing wind.
(vii) Whale back dune. It is a very large longitudinal dune with flat tops, on which barchans or seifs may occur.
The space between the dunes is called ‘Gassis’ and the water which is available in shallow wells and support vegetation in desert areas, form what is known as ‘Oasis’.
Source
A Text Book Of Geology, By- G.B Mohapatra