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WEEK 4
TOPIC: ACTION OF WINDS
CONTENTS:
(1) Features of wind deposition and their mode of formation
There are depositional features in the arid and semi-arid region, this includes: Dunes, Barchans, loses and seifs dunes.
DUNES: Dunes are hills or ridges of sand formed by the pilling up of sand into hill-shaped by the action of wind. It is the accumulation of sand and shaped by the movement of winds. They may be active dunes constantly on the move or inactive fixed dunes rooted with vegetation. Dunes are well represented in a TRUE DESERT where sea of sand is being continuously re-deposited and reshaped into a variety of features. Because of their great contrast in shape, size and alignment, they have been given a long list of names; such as alluvial dunes, head dunes, tail dunes, advanced dune, lateral dune, pyramidal dune, sword dune, parabolic blowout dune, barchans dune, hairpin dune, smoking dune, transverse dune, seifs dune. We shall only consider two which include barchans and seifs.
(a) BARCHANS: These are crescent or moon-shaped sand dunes. They may occur in groups or singly. They have horns which project from the crescent and thin out to become lower in the direction of the wind. They are desert landform. Barchans are formed by wind deposition in deserts. It has a convex shape on windward side and a concave shape on the leeward side with horns of 15–30m long. Examples are common in Sahara, Chilean and Persian deserts.
MODE OF FORMATION
Barchans are formed when an obstacle like rock impedes or prevents the movement of winds. These obstacles may be rock outcrop, a dead animal or a patch of grass which lies in the path of a prevailing wind. The materials carried by the wind are dropped and deposited behind the obstacle. Continuous deposition leads to the growth of the Barchans. The windward side of a barchans is convex and gentle, while the leeward side of a barchans is sheltered in concave and steep.
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