The whole process indicates how sand dune is formed.
Given is a diagram illustrating the forming process of sand dune. As we can see from the diagram, the main part forming sand dune is sand particle. Its movement is driven by the power of the wind, the shape of the sand dune is decided by the wet sand on drying ground. The wind is denoted by arrows, while the sand particle is marked as black spots.
At the first stage, we can see the the movement of sand particles are blocked by wet sand. Sand particles are sent by strong wind, once the sand particles are blown onto the wet sand, they meet resistance from it. Most of the sand particles are stuck together with the wet sand. At the second stage, when the sand dune has more than enough sand particles, the extra sand particles will be blown away by the wind, and they will form a new sand dune. When the sand dune is formed, more sand particles arrive at the sand dune, driven by the power of wind, after which the extra particles would be blocked by another wet sand spot. At the final stage, when strong wind send particles on the first sand dune, the sand dune would be loosened, and some particles would fall off, and then they are blown by the wind to the next sand dune.
Thus, it is reasonable to surmise that the forming process of sand dune is primarily determined by the effect of wind and wet sand.