Sand dunes are hills of sand that are formed by wind as it blows across deserts, beaches, and other areas with loose sand. The more powerful the wind, the farther it can carry particles of sand, which eventually accumulate to form dunes. Dunes can take the form of mounds, ridges, or hills, and they can cover areas ranging from a few square meters to several hundred square kilometers.
Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash, and in other areas where poorly cemented sandstone bedrock disintegrates to produce an ample supply of loose sand.
Sand dunes can have a negative impact on humans when they encroach on human habitats. Preventing sand dunes from overwhelming towns, villages, and agricultural areas has become a priority for the United Nations Environment Programme. Planting dunes with vegetation also helps to stabilize them.
Dunes are almost invariably built of particles of sand size, with common dune sands having median grain diameters between 0.02 and 0.04 cm (0.008 and 0.016 inch) . The sand dunes in Great Sand Dunes National Park in the US are the highest in the country, with some rising over 750 feet.
