Mostly sand is composed of quartz particles. This does not mean that the desert regions or the source regions of sand are composed of such rocks, which are predominantly, or markedly of quartz like sandstones, quartzites, gneisses, etc. Yet quartz particles are so conspicuous or predominant in the desert sand. The reason is that quartz is more resistant to weathering and erosion than most minerals found abundantly in rocks. The other minerals are, therefore, worn down to particles of finer size whereas quartz grains endure. The finer particles are blown out or remain mixed with sand mass as subordinate inconspicuous companions.
Moreover, because of the paucity of water there is limited sifting action in the desert. Suppose we have a quantity of sand partly mixed with clay and other finer particles. We may put part of such sediment in a dry bucket. We put an equal quantity in a bucket containing water. In the dry bucket, sand will look predominant. In the wet one sand, which is of larger particles will settle down and a clay layer will appear at the top. For the desert which has scarcity of water the position is represented by the dry bucket where there is limited sifting of particles.
While the movement of water may be unidirectional along slopes and streams, the movement of sand is in diverse directions with the complex movement of winds. Even the unidirectional winds have eddies and multidirectional flow. Sometimes and in some places winds are convergent or divergent or opposite. All these facts bear upon the depositional forms particularly sand dunes. Topography and vegetation play important part in dune morphology. The irregularities of the rock floor or those of the already existing depositional landforms, the smallest plants and the scantiest vegetation contribute to the morphology of the depositional features.