Winds

Wind is one of the two dominant agents shaping the hot deserts, the other being water in the form of sheet floods. To understand what wind does here, you first need to see why desert floors behave the way they do. The desert floor is dry and barren, so it heats up too much and too quickly. This heated ground warms the air directly above it, and that hot, lighter air rises with turbulence. Any obstruction in its path sets up eddies, whirlwinds, updrafts and downdrafts. Winds also sweep along the desert floors at great speed, and obstructions in their path create further turbulence. This constant, vigorous movement of air is the engine behind a whole family of erosional and depositional landforms.

Erosional Landforms

Deflation Hollows and Caves. When wind currents blow persistently in one direction, they blow out the weathered mantle from over rocks or bare soil. This process can create shallow depressions called deflation hollows. Deflation also produces numerous small pits or cavities over rock surfaces. The rock faces suffer the impact and abrasion of wind-borne sand, and the first shallow depressions created this way are called blow outs. Some blow outs become deeper and wider, and when they grow large enough, they are called caves.

Mushroom, Table and Pedestal Rocks. Many rock outcrops in deserts are easily susceptible to wind deflation and abrasion and are worn out quickly. What remains are the resistant parts of the rock, polished beautifully by the wind. These remnants often take the shape of a mushroom — a slender stalk with a broad, rounded, pear-shaped cap above. Sometimes the top surface is broad like a table top, and quite often the remnants stand out like pedestals.

Depositional Landforms

Wind is a good sorting agent. Depending on its velocity, different sizes of grains are moved along the floors by rolling or saltation, and carried in suspension. In this process of transportation itself, the materials get sorted. When the wind slows or begins to die down, the grains begin to settle, depending on their sizes and their critical velocities. So in depositional landforms made by wind, you find good sorting of grains. Since wind is everywhere, depositional features in arid regions can develop anywhere — wherever there is a good source of sand and constant wind directions.

Sand Dunes. Dry hot deserts are good places for sand dune formation, and obstacles to initiate dune formation are equally important. There can be a great variety of dune forms, and the book describes six distinct types.