Mounds of sand that have been blown and shaped by wind are called dunes. A dune is a wind-built landform that may appear as a mound, ridge, or hill.
Dunes form when loose sediment is available, wind can move it, and something causes the grains to settle. Individual grains commonly travel by saltation, hopping along the surface, or by creep, rolling and pushing other grains. As sand accumulates, wind drives it up the gentler windward slope; once it reaches the crest, it can avalanche down the steeper slip face.
Dunes are strongly associated with deserts, but they also occur on beaches, beside rivers, in semi-arid regions, and on other worlds. Their shapes reveal wind patterns. Crescent-shaped barchan dunes usually develop where winds blow mainly from one direction, while star dunes form where winds arrive from several directions.
An erg, or sand sea, is not a single dune. It is a broad desert region covered by windblown sand and many dunes. Dune movement can also threaten roads, settlements, and farmland when vegetation no longer stabilizes the sand.