The rocky material that is transported and DEPOSITED by rivers, seas, glaciers, and the wind is called sediment. Clay, sand, and gravel are all types of sediment. Sediments build up to form features such as mud banks along rivers or dunes in deserts. Sediments deposited on the seabed often build up over millions of years to form sedimentary rocks.
The laying down of sediments in water or on the ground is called deposition. Sediments are picked up by fast-flowing water, by strong, swirling winds, or by the ice in glaciers. Sediments are deposited when flowing water, wind, or glaciers cannot carry it any further – for example, when the water or wind slows down or stops, or when the glacier’s ice melts.
The shape of a sand dune depends on the strength and direction of the wind, how much sand there is, and whether plants grow on the dune or on the ground. Barchan (crescent-shaped) dunes form when the wind blows from one direction most of the time. They move forwards at up to 30 m (100 ft) a year as the wind blows sand over the crest.
A dune is a heap or hill made of sand or other small particles. Dunes are formed in sandy deserts and on coasts when loose, dry sand is picked by the wind and then deposited. In the Sahara Desert, North Africa, dunes grow up to 100 km (60 miles) long and 200 m (650 ft) high, forming an ever-changing landscape like giant ocean waves.