The given illustration depicts a general process that details gradual formation of sand dunes. Overall, the natural mechanism of formation comprises three sequential main stages: initial displacement of sand particles from dry ground to wet regions, dune elevation, and stablized phase of dunes respectively.
In the first stage, wind serves as the major force that drives displacement of tiny sand particles normally situated on dry ground of desserts or coast. These particles are trapped into previously formed wet sand of either small or big sizes, which penetrates not only on the superficial layer but also down towards the underground.
Gradually, upon influence of wind that is diagonal relative to the ground, accumulation of silica-derived particles leads to noticeable elevation of these two dunes. In addition, sand could travel from one dune to another, resulting in re-distribution of these paricles across two sites.
Finally, once each dune reaches a certain height, stablization that enables resistance to external force from the wind is established and no cross-site traveling is observed. Throughout the entire process, the underlying cross-sectional area of wet sand remains unchanged.