Imperial Sand Dunes – the largest mass of sand dunes in California. This dune system extends for more than 40 miles along the eastern edge of the Imperial Valley agricultural region in a band averaging five miles 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.