622 SEDIMENTARY PROCESSES/Aeolian Processes migrate laterally at a very slow rate and also extend downwind Star dunes are essentially stationary forms Dune Sediments Dunes are composed of moderately to well-sorted sands (63–1000 mm), with a mean grain size in the range 160–300 mm Most dune sands are composed of quartz, but may include significant quantities of feldspar Dunes composed of volcaniclastic materials occur in some areas Carbonate-rich dune sands are found adjacent to many subtropical coastlines and gypsum dunes occur close to some playas Three primary modes of deposition occur on dunes: migration of wind ripples, fallout from temporary suspension of grains in the flow separation zone (grainfall), and avalanching of grains initially deposited by grainfall (grainflow) These deposition modes create three basic types of aeolian sedimentary structures: wind ripple laminae, grainfall laminae, and grainflow cross-strata The packages of laminae may be separated by a hierarchy of bounding surfaces representing episodes of reactivation, dune growth, and migration of dunes in conditions of bedform climbing Crescentic dunes are dominated by grainfall and grainflow cross-strata (Figure 12A) Linear dune structures imaged using ground-penetrating radar show a change in the dominant type of sedimentary structures as the dune develops Small dunes are dominated by stacked sets of cross-strata that dip in opposite directions and are formed by the migration of the elements of a sinuous crestline up the dune The structures of larger dunes are dominated by trough cross-strata created by the migration of superimposed bedforms (Figure 12B) Sedimentary structures of star dunes are not well known, but are likely complex Sand Seas Desert sand seas are major depositional landforms that comprise dunes of varying size, spacing, and morphological type; sand seas also have areas of sand sheets as well as interdune deposits (e.g., playas, lacustrine deposits) and extradune fluvial, lacustrine, and marine sediments Major areas of sand seas lie in Figure 14 Linear dune generations in western Mauritania; LANDSAT image (left) and interpreted dune trends (right)