The illustration describe the process of forming the sand in the deserts or on the coast.
Overall, there are three ways, begins with the sand particles are blown by the wind into the wet sand, following by the movement of those sand particles from a wet area of sand to another one and ends with a huge amount of small sand particles is blown by the wind into the humid sand, creats some large sand dunes on the ground.
At the first step, the horizontal flow of wind delivers the sand particles from dry ground into the wet sand. After that, the wind continues to transform those little things on the dry surface to the humid sand. At this step, a small sand dune starts performing and those small particular of sand keep moving to another wet surface because of the wind, a one more tiny sand dune will be created.
At the final stage, a large quantity of small sand particles moving by the wind gathers into humid sand, goes on blowing with the wind flow to the other humid sand. A huge sand dune appears, bring with itself a small sand dune more, thanks for the movement of the wind.