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Wind is by far the best sorting agent. Only the smaller sized particles (clay and silt) are ever transported in suspension. The coarser silt, sand and gravel, which form the dune deposits, are usually transported by saltation (bounding-bouncing movement) and surface creep (rolling and sliding) of which saltation is the much more important mechanism. The material transported in the manner rarely is swept higher than 2 m. Sands of interior desert are usually medium grained and moderately to well sorted. Coastal dunes tend to be fine grained and well sorted. Commonly occurring structural features along the windward face of a dune, ripples usually contain the coarsest sand or fine gravel with the largest grains and heavy minerals concentrated on the crests. The stratification of the dune is normally along the slip face, which are the steeper beds along the leeward side. These beds are called foreset beds. Backset beds develop on the more gently sloping windward side. The topset beds are nearly horizontal and lie on top of the inclined foreset and backset beds. Dunes are often characterized by numerous cut-and-fill structures; the bedding is often not well preserved because the laminae are destroyed as the dune migrates.
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