Dune types
A barchan (pronounced bar-KAN) dune, sometimes known as a crescentic dune, is a crescent or U-shaped dune that has its "horns" or tips pointing downwind or away from the wind. Barchans arise where sand supply is limited, where the ground is hard, and where wind direction is fairly constant. They form around shrubs or larger rocks, which act as anchors to hold the main part of the dune in place while the tips migrate with the wind. Barchan dunes occur widely in deserts around Earth.
A parabolic dune is similar in shape to a barchan, but its tips point into the wind. Its formation is also influenced by the presence of some type of obstruction, such as a plant or a rock. Just the opposite of a barchan, a parabolic is anchored at its tips by the obstruction, which acts to block the wind, while its main body migrates with the wind, forming a depression between the tips. Because of this formation, parabolic dunes are also known as blowout dunes.
A linear, or longitudinal, dune is one that forms where sand is abundant and cross winds converge, often along seacoasts where the winds from the sea and winds from the land meet and push the sand into long lines. These high, parallel dunes can be quite large: Scientists have recorded linear dunes reaching 655 feet (200 meters) in height and 62 miles (103 kilometers) in length. The crests or summits of linear dunes are often straight or slightly wavy.
A transverse dune also forms where sand supply is great. This dune is a ridge of sand that forms perpendicular to the direction of the wind. The slip face of a transverse dune is often very steep. A group of transverse dunes resembles sand ripples on a large scale.
Deserts in Egypt encompass all these types of Dunes, especially in the Western Sahara.
