Introduction

Desert sand seas, or ergs, contain more than 95% of the world’s aeolian sand. Of this, 85% is contained in sand seas with an area exceeding 32,000 km 2 ( Wilson 1973 ). Major areas of sand sea accumulation lie 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. 1 ). In North and South America there are no large active sand seas and dunes cover less than 1% of the arid zone. There is evidence, in the form of dune systems fixed by vegetation, that sand seas in subtropical latitudes were much more extensive in the period coeval with the last Glacial maximum, some 18,000 years BP (Samthein 1978; Goudie 1983 ); whilst many north American dunefields were more extensive in the mid Holocene ( Smith 1965 ; Ahlbrandt et al. 1983 ). There are large aeolian sand accumulations on other planets and the North Polar Sand Sea of Mars is the largest known sand sea on the terrestrial planets, with an area of 7-8 x 10 5 km 2 ( Tsoar et al. 1979 ).