Dunes

Dunes are a type of terrain.

Description
In physical geography, a dune is a hill of sand built by aeolian processes. Dunes occur in different form and sizes, formed by interaction with the wind.

Most kinds of dunes are longer on the windward side, where the sand is pushed up the dune, and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as "ergs".