The following two types of sand dunes are seen
1. Barchan : These are moon-shaped dunes. They are live dunes which advance steadily before winds that **** from a particular direction. They are found in the deserts of the Sahara. These are initially formed by an accumulation of sand at an obstacle, such as a heap of rocks. These occurs transversely to the wind, so that their horns thin out and become lower in the direction of the wind.
2. Longitudinal Dunes or Seifs : These are long and narrow sand-ridges which grow parallel to the direction of the prevailing wind. They are found in the interior parts of deserts. In the Libyan desert, they are known as Seifs (means ‘sword’ in Arabic). In the Thar Desert they are confined to the western margin, where the force of south-west monsoon influences their formation. An important feature of such a dune is that in its crest line there are rise and fall patterns. Seifs are found in the Sahara, Iran, Thar Desert (India) and West Australia. These sand dunes are stationary and they remain in a fixed position.
