Photographer's Note

An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word ʿarq” meaning "dune field".
Ergs are concentrated in two broad belts between 20° to 40°N and 20° to 40°S latitudes, which include regions crossed by the dry, subsiding air of subtropical high presuure. Active ergs are limited to regions that receive, on the average, no more than 150 mm of annual precipitation.
As the wind transports sand from the windward slope (the erosional side) to the crest, sand is piled up and a threshold condition is reached, whereby further accumulation sends the sand avalanching down the slipface (the depositional side) as the angle of repose for dry sand is exceeded. This pattern is repeated and causes the dune to migrate downwind, maintaining its aerodynamic shape. Typical dune heights range from 3 to 100 meters, although dunes of greater height are known.
The largest hot desert in the world, the Sahara, covers 9 million square kilometres and contains several ergs, such as the Chech Erg, the Issaouane Erg in Algeria and erg Chebbi in Morrocco (from Cooke, R., A. Warren, and A. Goudie. 1993. Desert Geomorphology. London: University College London Press)

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Additional Photos by Ecmel Erlat (ecmel) Gold Star Critiquer/Gold Note Writer [C: 125 W: 0 N: 223] (1642)
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