A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as grains of sand accumulate.

Every dune has a windward side and a slipface. A dune's windward side is the side where the wind is blowing and pushing material up. A dunes slip face is simply the side without wind. A slipface is usually smoother than a dune's windward side.

A collection of dunes is called a dune belt or dune field. A large dune field is called an erg. The Skeleton Coast Erg in Namibia extends two to five kilometers (one to three miles) in length and across a width of 20 kilometers (12.7 miles).

Dunes can also be formed by strong currents beneath the water. Underwater dunes, called subaqueous dunes, are common in the ocean, rivers, and canals.

Shapes of Dunes
Dunes can be very large geographic features or just small bumps. Most sand dunes are classified by shape. There are five major dune shapes: crescentic, linear, star, dome, and parabolic.

Crescentic dunes are shaped like crescents, or the shape of a wide letter C. The wide side of a crescentic dune is its windward side, with a small, semi-circular slipface on the other side. Crescentic dunes are the fastest-moving type of dune, and also the most common.

Linear dunes form straight or nearly straight lines. Some linear dunes are shaped like a wiggling snake, with regular curves. Linear dunes develop where wind pressures are nearly equal on both sides of a dune.

Star dunes have pointed ridges and slipfaces on at least three sides. Star dunes develop where winds come from many different directions. The sand dunes of the Sahara Desert ergs are star dunes.

Dome dunes are the rarest type of dune. They are circular and do not have a slipface. The wind can blow material onto the dune from any side.

Parabolic dunes are similar to crescentic dunes. Their shapes are roughly the same, but the slipface of a parabolic dune is on its inward side. Parabolic dunes are also called blowouts, because winds blow out the center of the dune, leaving just a rim on the outside.

Life In Sand Dunes
Few species can live in the shifting world of sand dunes.

There is little soil in a sand dune, so plants usually cannot take root. Often, sand dunes are located next to oceans, so plants must be tolerant of a very salty atmosphere. Some grasses with shallow root systems, such as beachgrass, are common to sand dune ecosystems.

Animals cannot take shelter in the unstable sand of a dune and must search for fresh water. Still, a few species thrive. In the Sahara Desert, sandfish live beneath the dunes. A sandfish is not fish at all, but a type of lizard that can retract its legs and swim through the smooth sand.

Larger animals can find a way to live among sand dunes, too. Rig-e Jenn is a vast, desolate dune belt in Irans Dasht-e Kavir desert. Rig-e Jenn is home to rare species such as the yuz, or Asiatic cheetah, and onager, a relative of the horse.