Imperial Sand Dunes – the largest mass of sand dunes in California. This dune system extends for more than 40 mi along the eastern edge of the Imperial Valley agricultural region in a band averaging 5 mi in width. Credit: BLM.
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.
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 m (130 ft) high, 600 m (1,960 ft) wide, and 400 km (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..|