When an obstacle causes wind to slow down and deposit materials, the materials pile up and eventually form a

Dune

Explanation:

Dunes are mounds of loosed sand piled up by the action of wind. The sand that makes up dune are usually well sorted .The reason for it well sorted nature is because of the wind ability to select the sizes of the loose sand it can carry. Fine grain silt and clay can easily be carried by wind and can be carry farther away than other sizes of grains. Well rounded sand grains can be carried(roll) more easily than angular grains of sand. The selective nature of the wind influence the kind and sizes of sand grains that forms the Dune.

Dunes are likely to develop in area that posses strong wind blowing in a specific direction. Dunes are mostly found in desert area. Such as the Sahara desert.

The picture illustrate the way dunes are formed