A number of sand-dunes have been recognized, on the basis of their shapes, as follows:
(i) Barchans
These are asymmetrical, crescent shaped dunes in which the points or wings of the crescent are directed down wind. These dunes are formed in strong winds and are situated perpendicularly to the main direction of the moving wind.
They develop, generally, on a flat floor, with wind blowing from a constant direction and where the sand supply is limited.
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Barchans are usually separated from one another, and their heights vary between 1-2 metres to about 20-30 metres. The formation of a barchan begins with the emergence of an obstacle, such as a boulder or even a patch of pebbles, which obstructs the free flow of the wind thereby reducing its speed and transporting power.
Deposition occurs on the windward side of the obstacle and the sand then piles up until the dune attains a shape which offers the least resistance to the flow of the wind. Gradually wind is deflected around the sides of the growing dune, where its frictional contact with the sand slows down the wind to a little extent.
Sand is deposited there in the form of’ long, pointed horns or wings, following the direction of the wind and thus giving the dune its characteristic crescent- shape. These dunes, generally, have a gentle windward slope and a much steeper leeward slope.
Brachans tend to arrange themselves in chains extending in the directin of the prevailing wind and they also gradually move forward. Speeds of their movement as great as 47 metres per year have been recorded in Peru.
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(ii) Longitudinal or Seif dunes
These are large symmetrical ridges of sand parallel to the wind direction. While some authors have distinguished seifs from longitudinal dunes, others use them as synonymous terms.
According to E.W. Spencer (Basic Concepts of Physical Geology, Oxford & IBH Publishing Co. New Delhi, 1970) the seifs are dunes similar to the barchans except one wing is missing, which is caused by an occasional shift in wind direction but not in the direction from which sand is being supplied; whereas the longitudinal dunes form when sand is in short supply and the direction of the wind is constant.
According to Aurthur Holmes (Principles of Physical Geology. The English Language Book Society and Nelson 1975) Linear ridges or longitudinal dunes (known as seifs in the Sahara), which commonly occur in long parallel ranges, each diversified by peak after peak in regular succession like the teeth of a monstrous saw.