Grand Erg Occidental Grand Erg Occidental The Grand Erg Occidental (, al-ʿIrq al-Gharbī al-Kabīr), (also known as the Western Sand Sea) is the second largest erg in northern Algeria after the Grand Erg Oriental. It covers an area of approximately . The sand dunes in the erg are formed by the wind, and can be up to high. Certain crescent-shaped dunes, known as barchans, are actually mobile; the wind can push these dunes as much as 20 to 30 m (65-100 ft) in one year. It is a desert natural region that receives less than 50 mm (1,96 in) of rainfall per year.
Grand Erg Oriental of the Sahara is probably As-Sahra al-Libiyah, which straddles the inland border of Libya and Egypt. Erg is a Tamachek Berber word, and also a geographic term of art. The Grand Erg Oriental used to be associated with the Wadi Igharghar, a mostly dry and buried river with a sizable network of tributaries which, should it possess any water, would flow north into the erg from the Ahaggar mountains of the central Sahara. Yet such dry, anciently-made river beds, lying seemingly useless beneath the desert sands, can preserve the infrequent rain water, by carrying it off underground and so rescue