Sand dunes form part of a hierarchical self-organized system of aeolian bedforms which comprises: (i) wind ripples (spacing 0.1-1 m), (ii) individual simple dunes or superimposed dunes on mega dunes (also called draa or compound and complex dunes) (spacing 50-500 m), and (iii) mega dunes (spacing 500 m). Most dunes occur in contiguous areas of aeolian deposits called ergs or sand seas (with an area of 100 km2). Smaller areas of dunes are called dune fields. Major sand seas occur in the old world deserts of the Sahara, Arabia, central Asia, Australia, and southern Africa, where sand seas cover between 20 and 45% of the area classified as arid (Fig. 18.1). In North and South America there are no large sand seas, and dunes cover less than 1% of the arid zone. The majority of dunes are composed of quartz and feldspar grains of sand size, although dunes composed of gypsum, carbonate, and volcaniclastic sand, as well as clay pellets, also occur.