Sand Dunes are created when the speed of wind decreases, huge amount of sand gets deposited and create mounds or hills of sand.
Based on the shape, Sand dunes are classified into Crescent dunes and longitudinal dunes.
Crescent dunes, also known as barchans are moon shaped sand dune having two horns.
Longitudinal dunes are formed parallel to the wind direction. These sand dunes are in the form of extended elongated hill.
As there are no or very less plants in the desert, the sand is unprotected. As a result strong wind carries sand particles forming huge depressions in the desert forming deflation hallows.