Materials that have been eroded and transported by winds must come to rest somewhere. The finest dust travels enormous distances in the air, and may be moved completely out of the desert. It has been estimated that some dust grains travel as far as 3700 km (2300 miles) before they are finally deposited on land or sea. The dust from the Sahara Desert is sometimes blown across the Mediterranean to fall as ‘blood rains’ in Italy or on the glaciers of Switzerland. Dust that settles in the Hwan Ho basin from the Gobi Desert has accumulated over past centuries to a depth of several hundred metres. As windborne materials are sifted according to their coarseness, it can be expected that the coarser sands will be too heavy to be blown out of the desert limits. They remain as dunes or other depositional landforms within the deserts themselves. Since they are rarely static, their migration pattern depends on a number of factors; the size of the particles they contain, the direction and velocity of the winds, the location and nature of the surfaces over which the particles are transported and the presence or absence of water and natural vegetation.
The following are some of the major features of wind deposition.
1) Dunes: Dunes are in fact bills of sand formed by the accumulation of sand and shaped by the movement of winds. They may be active or live dunes, constantly on the move, or inactive fixed dunes, rooted with vegetation. Dunes are well represented in the erg desert where a sea of sand is being continuously moved, reshaped and redeposited into a variety of features. Because of their great contrast in shape, size and alignment, they have been given a long list of fanciful names, such as attached dune or head dune, tail dune, advanced dune, lateral dune, wake dune, star dune, pyramidal dune, sword dune, parabolic blowout dune, hairpin dune, smoking dune and transverse dune. However, the following two types of common dunes, barchans and seifs, will be described in more detail.
(a) Barchan: These are crescentic or moonshaped dunes which occur individually or in groups. They are live dunes which advance steadily before winds that come from a particular prevailing direction. They are most prevalent in the desert of Turkestan and in the Sahara. Barchans are initiated probably by a chance accumulation of sand at an obstacle, such as a patch of grass or a heap of rocks. They occur transversely to the wind, so that their horns thin out and become lower in the direction of the wind due to the reduced frictional retardation of the winds around the edges. The windward side is convex and gently-sloping while the leeward side, being sheltered, is concave and steep (the slip-face). The crest of the sand dune moves forward as more sand is accumulated by the prevailing wind. The sand is accumulated by the prevailing wind. The sand is driven up the windward side and, on reaching the crest, slips down the leeward side so that the dune advances. The rate of advancement varies from 8m (25 feet) a year for the high dunes measuring up to 30 m (100 feet) high to 15 m (50 feet) a year for the lower dunes which may be only a dozen metres high.
The migration of the barchans may be a threat to desert life for they may encroach on an oasis, burying palm trees or houses. Long-rooted sand-holding trees and grasses are therefore planted to halt the advance of the dunes , thus preventing an area of fertile land from being devastated. Under the action of winds, barchans take a chaotic changing pattern. Several barchans may coalesce into a line of irregular ridges, ever-changing with the direction of the winds. Ergs or sandy deserts are thus most difficult to cross.
(b) Seifs or longitudinal dunes: Seif is an Arabic word meaning ‘sword’. They are long, narrow ridges of sand, often over a hundred kilometres long, lying parallel to the direction of the prevailing winds. The high serrated ridges may attain a height of over 60 m (200 feet). The crestline of the seif rises and falls in alternate peaks and saddles in regular succession like the teeth of a monstrous saw. The dominant winds blow straight along the corridor between the lines of dunes so that they are swept clear of the sand and remain smooth. The eddies that are set up blow towards the sides of the corridor, and, having less power drop the sand to form the dunes. In this manner, the prevailing winds increase the length of the dunes into tapering linear ridges while the occasional cross winds tend to increase their height and width. Extensive seif dunes are found in the Sahara Desert, south of the Qattara Depression; the Thar Desert and the West Australian Desert.
2) Loess: The fine dust blown beyond the desert limits is deposited on neighbouring lands as loess. It is a yellow friable material and is usually very fertile. Loess is in fact, fine loam, rich in lime, very coherent and extremely porous. Water sinks in readily so that the surface is always dry. Streams have cut deep valleys through the thick mantle of soft loess and badland topography may develop. It is so soft that roads which are constructed through a loess region soon sink and their walls rise steeply.
The most extensive deposit of loess is found in north west China in the loess plateau of the Hwang-Ho basin. It is estimated to cover an area of 650 000 sq km (250 000 square miles), and the deposits have accumulated to a depth of 60 to 150 m (200 to 500 feet)! In China, such yellowish wind-borne dust from the Gobi Desert is called ‘Hwangtu’ – the yellow earth. But the original term loess actually comes from a village in Alsace, France bearing that name, where such deposits occurred. Similar deposits also occur in some parts of Germany, France and Belgium, and are locally called limon. They are also wind-borne but were blown from material deposited at the edge of ice sheets during the Ice Age. In parts of the Mid-West, U.S.A ., loess was derived from the ice sheets which covered northern North America and is termed adobe.
3) Ripples: The large-scale depositional forms-dunes (barchans and seifs) and loess – have been seen above. In addition to these large-scale forms, one can recognize the small-scale sand ripples. Ripples are wavy surfaces on sand sheets or on the slopes of sand dunes. The ripples are usually a few centimetres high.