dune

The shape of a dune indicates the prevailing wind pattern. Crescent-shaped dunes form in sandy desert with winds from a constant direction. Seif dunes form on bare rocks, parallel to the wind direction. Irregular star dunes are formed by variable winds.

Dune formations in the Namib Desert, Namibia, southwest Africa.

Namib Desert, Namibia, southwest Africa. Some of the tallest sand dunes in the world are found here.

This photograph shows the enormous scale of the dunes in the Namib Desert. They are amongst the largest dunes in the world, and are situated between the Kalahari Desert and the Atlantic Ocean.

Track marks from vehicles are clearly visible on the sand dunes at Studland Beach, Dorset, England. Sand dunes are vulnerable ecosystems. They are easily disrupted by activities such as trampling, camping, and driving. The concentration of people and vehicles at particular places can cause great ecological damage.

A sign requests visitors to refrain from lighting fires or having barbecues on the sand dunes. The ecosystem of the beach can be easily disrupted by fire. Fire destroys the vegetation, which can take up to 50 years to recover. Part of the site has been fenced off, to prevent people from walking over it and thus preserve the dunes.

Mound or ridge of wind-drifted sand, common on coasts and in deserts. Loose sand is blown and bounced along by the wind, up the windward side of a dune (saltation). The sand particles then fall to rest on the lee side, while more are blown up from the windward side. In this way a dune moves gradually downwind. Dunes occur in areas where there is a large supply of sand, strong winds, low rainfall, and some vegetation or obstructions to trap the sand.

In sandy deserts, the typical crescent-shaped dune is called a barchan. Seif dunes are longitudinal and lie parallel to the wind direction, and star-shaped dunes are formed by irregular winds.