Grand Erg Occidental

(Great Western Erg), a sandy desert in the northwestern Sahara of Algeria. The Grand Erg Occidental covers an area of approximately 80,000 sq km. The topography is characterized by polygonal ground and long corridors between short dunes, which are cut off by transverse sand necks. The dunes, up to 300 m high in the west, are held in place by grasses and brush. After rainfall the dense clays of the corridors (gassi) hold the water, and ephemeral plants appear. Groundwater is closest to the surface in the south, where there are many oases. Highways to the central Sahara run along the western and eastern edges of the erg.