LANDFORMS MADE BY WIND ACTION

LANDFORMS MADE BY WIND ACTION
Wind action is very striking in arid and semi-arid regions. Wind erosion consists of abrasion which breaks up rocks and produces rock pedestals, zeugens, yardangs, and inselbergs, and deflation which blows away rock waste and thus lowers desert surface producing depressions. Wind deposition gives rise to dunes, made of sand, and loess, made of desert dust.

Various kinds of desert surfaces are recognised. A
sandy desert, called erg in the Sahara and koum in Turkey, is an undulating plain of sand produced by wind action. A stony desert, called reg in Algeria and serir in Libya and Egypt, has its surface covered with boulders, angular pebbles and gravel which have been produced by diurnal temperature changes. Rocky desert, called hamada in the Sahara, is characterised by bare rock surface formed by deflation. Badlands develop in semi-desert regions mainly as a result of water erosion produced by violent rain storn.J. The land is broken by extensive gullies and ravines which are separated by steep-sided ridges.

A desert area which has a surface layer of hard rock underlain by soft rock develops a 'ridge and furrow' landscape under wind action. The ridges are called zeugens. Bands of hard and soft rocks which lie parallel to the prevailing winds in a desert region develop another 'ridge and furrow' pattern. The belts of hard rock stand up as rocky ribs in fantastic shapes: they are called yardangs. They
are common in Asian deserts and the Atacama Desert. '

Some depressions produced by wind deflation reach down to the water table; a swamp or an .oasis then develops. In some desert regions erosion has removed all the original surface except for isolated pieces which stand up as round­topped masses called inselbergs. They are common in the Kalahari Desert, parts of Algeria, and Western Australia.

There are two types of sand dunes. A barchan is a crescent shaped sand dune, the horns of which point away from the direction of the dominant wind; the leeward slope is relatively steep and the windward slope gentle. This asymmetry is due to eddies being set up by the prevailing wind blowing over the crest of the dune. Barchans migrate as grains of sand are blown up the windward slope and roll down the leeward slope. The best examples are found in the Sahara and Turkey. A seif dune forms when a cross wind develops to the prevailing wind and the corridors between the dunes are swept clear of sand by this wind.

Some of the fine particles blown out of deserts by the winds are deposited on land where they accumulate to form loess. Loess is friable and easily eroded by rivers. There are extensive deposits of loess in northern China formed of the desert soil from the Gobi Desert. The loess deposits of central Europe were probably formed in the last Ice Age when the out-blowing winds carried fine glacial dust from the ice sheets of northern Europe. Loess deposits are unusually fertile. They are also used for building.

As the edges of desert and semi-desert highlands get pushed back by erosion and weathering, a gently sloping platform develops; this is called a pediment.