The diagrams illustrate the process through which sand dunes are formed from the movement and accumulation of sand particles.
Overall, the formation of a sand dune occurs in three main stages, beginning with the transportation of loose sand grains by wind and ending with the gradual build-up of a mound that grows in height over time.
At the initial stage, individual sand particles are blown across the surface by prevailing winds. As the particles move forward, some of them fall into small natural depressions on the ground. These shallow hollows trap the grains, preventing them from being carried further.
In the next stage, more sand continues to be transported by the wind and accumulates in the same depression. As a result, the hollow gradually fills up, and the sand begins to pile up above ground level as additional particles settle on top.
In the final stage, the accumulated sand forms a small mound, which becomes increasingly elevated as more grains are deposited. The slope facing the wind remains gentle, while the opposite side becomes steeper. Over time, this continuous deposition leads to the development of a fully formed sand dune.