Erg Chebbi is one of Morocco's several ergs – large seas of dunes formed by wind-blown sand. There are several other ergs such as Erg Chigaga near M'hamid. Technically all these ergs are within an area of semi-arid Pre-Saharan Steppes and not part of the Sahara desert which lies some distance to the south.
In places, the dunes of Erg Chebbi rise up to 150 meters from the surrounding hamada (rocky desert) and altogether it spans an area of 28 kilometers from north to south and up to 5–7 kilometers from east to west lining the Algerian border. From Wikipedia