Linear dunes in the southwestern Kalahari sand sea exhibit characteristic grain size and sorting patterns in which sands become coarser, better sorted, and more finely skewed from interdune to crest. In addition, at a regional scale, sands tend to become finer and better sorted in the direction of transport. These patterns can best be explained in terms of the differential movement of sand in creep and saltation populations, whereby the saltating population moves more rapidly and is concentrated downwind and in dunes. Slower-moving creep populations remain close to source areas and in interdunes. The fines will tend to be concentrated in interdunes because they can penetrate between coarse grains, where they are protected from further movement. This type of model probably explains most grain size and sorting relationships in desert sand seas.