erg

A vast sea of wind-deposited sand dunes — the sand desert of popular imagination, though sand deserts actually account for only about 20 percent of the world's desert area. Ergs can cover hundreds of thousands of square miles, with individual dunes reaching heights of 500 feet or more. The Sahara contains several of the world's largest ergs, including the Grand Erg Oriental and the Grand Erg Occidental in Algeria. An erg is a landscape in continuous slow motion — the dunes migrate, merge, split, and reform under the wind's direction.