Dune, or Sand Dune, a hill or ridge of sand deposited by the wind. Dunes form in areas where the wind blows most frequently in one direction at speeds sufficient to move large amounts of sand. Some dunes reach heights of 600 feet (180 m) or more, but they are usually much lower. Unless anchored by vegetation or artificial means, dunes are usually moving formations, pushed ever onward by the wind. Some dunes travel as much as 150 feet (45 m) a year. Quartz is generally the chief constituent of the sand.

Dunes have different shapes, caused mainly by the direction and strength of the wind, the amount of sand available, and the nature of the terrain. Crescent-shaped dunes, known as barchans, are probably the most numerous. Dunes also commonly form ridges, stars, and U's.

Dunes are found mostly in deserts, especially those in the subtropics and the middle latitudes. They also occur along sandy lake and ocean shores and in other areas with sufficient sand and favorable winds. The largest areas of dunes are found in the Sahara in Africa and in the deserts of the Middle East and Central Asia. In the Sahara the vast expanses of dunes are known as sand seas and ergs.