A dune is a hillock of sand built up by a prevailing wind, found mostly in desert regions. There are several forms, the commonest being barchans, crescent-shaped with the horns pointing downwind; and transverse, elongate at right angles to the wind direction.

 

Seif

A seif is an elongated type of sand dune. Seifs may be several kilometers long and often occur in groups. The narrow ridge of the dune runs parallel to the direction of the prevailing wind, and its height and width can be increased by the action of cross-winds. Seifs are found mainly in the sand deserts of North Africa.

 

Seif dunes are long ridges of sand with bare rock between. Each dune may be up to 40 meters (130 feet) high, 600 meters (1,960 feet) wide, and 400 kilometers (250 miles) long. The prevailing wind blows parallel to the ridges.

Barchans are isolated and crescent-shaped dunes that slowly migrate downwind, horns forward. They occur only in areas such as Turkestan where thewind always blows from the same direction..