A dune is a landform and is a large mass of wind-blown sand. Dunes are most common in deserted environments, such as the Sahara, and also near beaches. An area with dunes is called a dune system. The Sahara is mainly rocky hamada, ergs form only a minor part, but many of the dunes are over 180 meters high. Wind or rare rainfall 0, and salt flats. Unusual landforms include the Richat Structure in Mauritania.