The Sahara is mainly rocky hamada. Ergs (large areas covered with sand dunes) form only a minor part, but many of the sand dunes are over 180 metres (590 ft) high. Wind or rare rainfall shape the desert features: sand dunes, dune fields, sand seas (erg), stone plateaus (hamada), gravel plains (reg), dry valleys (wadi), dry lakes (oued), and salt flats (shatt or chott). Unusual landforms include the Richat Structure in Mauritania.