The diagram shows how sand dunes are formed by wind. Overall, the process is gradual and happens in several stages when wind moves sand across dry ground and deposits it on wet sand.
At the beginning, wind blows over dry ground and carries small sand particles with it. These particles move by short jumps along the surface. When the sand reaches an area of wet sand, the movement becomes slower. Because wet sand is heavier and sticky, the sand particles stop moving and start to collect in one place. At this stage, a small pile of sand is formed.
In the next stage, the wind continues to blow more sand from the dry ground. New sand particles land on the first small pile. As a result, the pile grows bigger and higher. Some sand may also move further and collect on another wet area nearby. This creates more than one small sand pile at the same time.
In the final stage, the sand piles become much larger. More and more sand is added to the top and sides of the piles. Over time, these piles turn into clear sand dunes. The dunes are higher and wider than before and are more stable because they contain a large amount of sand.
In comparison with the first stage, the final stage shows a big difference in size. At first, there are only a few sand particles, but at the end, there are large dunes. In conclusion, sand dunes are formed slowly by the continuous action of wind and the presence of wet sand.