Sand Dunes

A dune is a hill of sand built either by the flow of wind or water. Dunes come in different forms and sizes. Most dunes are longer on the windward side where sand is pushed up to form the dune. However dunes are shorter on the'slip face' in the lee of the wind. A 'slack' is the valley between two dunes. A "dune field" is an area covered by widespread sand dunes. Large dune fields are called "ergs".
Dunes protect the land against likely ravages by storm waves from the sea, thus, dunes are very important. Though the most widely distributed dunes are linked with coastal areas, the largest groups of dunes are found inland in dry regions and are linked to ancient lake or sea beds.
Dunes are also formed on sand or gravel beds of rivers, waterways and the sea-bed.

Sources: Wikipedia, Google Images
en.wikipedia.org/wiki/Dune‎