Dunes are mounds of sand that have been piled by the action of winds. The sand is usually composed of bits of minerals that have been eroded from rocks, picked up by water or winds, and then re-deposited somewhere else. Typically, the sand is deposited behind some object that is a barrier to the movement of air currents, which causes the windspeed to slow suddenly so that the load of sand particles can no longer be contained against the force of gravity, and it falls to the ground. If there is a suitable source of sand, dunes may deposit along the edges of oceans, large lakes, rivers, and even in inland locations.

The mineralogical composition of sand in dunes varies greatly from place to place. The most common minerals are quartzitic (this is a silica sand), but other minerals may also be mixed in...