When the speed of the wind increases, the particles of the sand and dust blow with the wind. As the wind slows down, the suspended particles drop down as the wind can no longer support them. The process is known as deposition. The heavier particles drop down close by but the lighter particles are carried away to a great distance. The deposition made by wind is also called aeolian deposition. The sand deposits gives rise to many types of land forms described here.
(1) Ripples. The sand deposits in a desert appear in the form of waves. The spacing between them measures a few cm. This type of deposit is called ripples.
Ripples are formed due to some irregularity of the surface. In this way, many waves of the sand deposition go on forming. Ripples develop at right angles to the direction of wind. The spacing between the two ripples depends upon the speed of the wind and the size of the particles.
The size of the particles and the wind speed at a place determine the height and the spacing of the ripples. As soon as the wind speed changes, the height and the spacing of the ripples also change.
(2) Sand Dunes. Sand dunes are those dunes which are formed by the deposition of sand by winds. Sand dunes are formed usually in those places where loose sand is amply available and the direction of wind remain almost unchanged.
The slope of the windward direction of a sand dune is gentle (5o to 15o) but that of leeward direction is steep (20 o-30 o). The sand particles roll up the leeward slope and stay at the crest or roll down the other side.
Air eddies are produced on the leeward side. The eddies obstruct the downward movement of sand particles but help the particles to go up. The wind, here, is stagnant. It results into the formation of a steep slope on the leeward side. It is, therefore, possible to know the direction of wind by observing the slope of the dunes.
Dunes are of various types depending upon their location constituents and shape and position e.g. coastal dunes, longitudinal and barkhan or crecent.
(3) Loess. Pine particles of sand are suspended in winds. They are deposited at places much away from their sources. A deposition of these suspended particles in an area is called loess. It contains silt, clay and some sand. This deposition is brought by winds from glacial out washes and the flood-plains of rivers on the desert areas.
Usually a loess does not have layers because the particles are so fine that they are incapable of forming layers. The insects and roots of plants go on turning over and churning up the sediments, while it is deposited.
(4) Dust Devil. Due to turbulent winds the fine dust particles are continuously raised up. The whirl winds convert these particles into a devil. It is known as dust devil. These dust devils wander away to very far off places starting from their places of origin.
(5) Bajada. A sudden cloud-burst gives a heavy rainfall. Floods are caused for a short time. Temporary rivers cut valleys on steep slopes. A sediment on the slope is eroded very fast and alluvial fans are formed. These fans are not large in humid regions because water flows almost throughout the year and the erosion of sediment is a continuous process. In arid regions, the sediment eroded on the slopes is all used up in the formation of alluvial fans. These fans expand laterally and coalesce into one another. When many fans coalesce in a certain basin, a piedmont slope area is formed. It is known as bajada.
(6) Playa. A part of rain water received at the time of cloud-burst percolates in a porous area composed of pebbles and cobbles and seeps underground. A pan of this seeping water rises in the lower portion of the fans. A lake is formed wherever a depression exists. Such lakes are called playa lakes. The water of these lakes dries up very fast but some water continues to rise up through fine pores. This water is evaporated the moment it reaches the surface but salt present in the water is left on the soil. At many places salt mixed clay areas are very extensive and are known as Playa. If the amount of salt is much the hard surface layer composed of salt is called Salina.
(7) Bolson. If the rivers of an extensive desert surrounded by a mountain bring sediment in times of floods towards the lower portion of the desert, the floor of the basin is filled up with alluvium. Such basins are called bolsons.
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