Wind Action and Desert Landscapes

By itself, wind can only remove uncemented rock debris or soil, which it can perform more effectively if the material is dry rather than wet. But once armed with particles, the wind becomes a noteworthy agent of abrasion. The size of the particles that the wind can transport depends on the strength of the wind as well as the shape and weight of the particles. The distance that the wind, given that its velocity remains constant, can carry particles depends principally on their size. 

Generally, a rough surface tends to reduce the velocity of the wind immediately above it. Consequently, particles of a certain size are not as likely to be blown away as they would on a smooth surface. Even so, Bagnold (1941) found that grains of sand less than 0.03 mm in diameter were not lifted by the wind if the surface on which they lay was smooth. On the other hand, particles of this size can easily remain suspended by the wind. The longer the surface distance over which a wind can blow without being interrupted, the more likely it is to attain optimum efficiency. 

There are three types of wind erosion, namely, deflation, attrition and abrasion. Deflation results in the lowering of land surfaces by loose unconsolidated rock waste being blown away by the wind. The effects of deflation are seen most acutely in arid and semi-arid regions. For example, basin-like depressions are formed by deflation in the Sahara and Kalahari deserts. 

However, downward lowering is almost invariably arrested when the water table is reached since the wind cannot readily remove moist rock particles. What is more, deflation of sedimentary material, particularly alluvium, creates a protective covering if the material contains pebbles. The fine particles are removed by the wind, leaving a surface formed of pebbles that are too large to be blown away. The suspended load carried by the wind is comminuted further by attrition, turbulence causing the particles to collide vigorously with one another. 

When the wind is armed with grains of sand, it possesses great erosive force, the effects of which are best displayed in rock deserts. Accordingly, any surface subjected to prolonged attack by wind-blown sand is polished, etched or fluted. Abrasion has a selective action, picking out the weaknesses in rocks. For example, discontinuities are opened and rock pinnacles developed. Since the heaviest rock particles are transported near to the ground, abrasion is there at its maximum and rock pedestals may be formed. In deserts, flat smoothed surfaces produced by wind erosion are termed desert pavements. 

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