Immense areas of sand dunes called areg (sing., erg) occupy parts of the desert. The largest such region is the Great Eastern Erg, where enormous dunes two to five metres high are spaced about forty metres apart. Sand dunes are the most prominent features of this area's topography, but between the desert areas of the Great Eastern and the Grand Western Erg and extending north to the Atlas Saharien are plateaux, including a complex limestone structure called the Mzab.

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