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The Liwa area is located in the south-central parr. of the UAE along the northeastern upwind margin of the Ar Rub’ Al Khali sand sea and covers about 3694 km(2). It has one type of dunes: a mega-barchan that is compound and complex in form characterized by the superimposition of smaller dunes. Due to the giant size of barchans, interdune areas appear as closed depressions that vary in shape from irregular, elongate, triangular, to circular or semi-circular. These depressions are mostly covered with small barchans or sabkha deposits with patches of gypcrete in their upper parts. Scattered flat-top mounds are also observed in interdune areas. The size of dunes in Liwa is directly controlled by Quaternary climatic changes: multiple generations are superimposed on one another separated by calcretic horizons. The climate during dune formation alternated between relatively arid to humid. The dune sands are unimodal with a modal class of fine sand, moderately to very well sorted, and near symmetrical and mesokurtic. Quartz grains are mainly subrounded to subangular. Based on the heavy-mineral suites, the sand came mostly from the ultrabasic rocks that form the ophiolitic sequence of the Oman Mountains, the acidic rocks of Iran. and from the Arabian Shield. (C) 2000 Elsevier Science B.V. All rights reserved.