Grand Erg Oriental follow the wind's direction. There are other types of dunes, as well as "complex" dunes. Saharan winds are also known to clear an area of sand altogether, leaving bare rock (hamada) or gravel (reg). The "Grand Erg Oriental" is accessible overland by an Algerian road, which goes south from Constantine. It passes through the Aurès mountains, then salt lakes by the city of Biskra. Next the road skirts the western edge of the erg. After about 500 km. in all, it reaches the ancient oasis of Ourgla [Wargla]. Since late antiquity Wargla was a stop for the Saharan trade, being
Erg of Bilma been stabilized by the growth of vegetation, allowing human cultivation of crops like millet and sorghum on the slopes. The ancient Bornu Empire to Fezzan caravan routes had to cross the dunes of the erg south of Bilma as the last major obstacle before reaching the sahel. While that traffic largely ceased after 1820, trade though the erg of Bilma continues from the Lake Chad region and the Termit Massif on a small scale. Erg of Bilma The Erg of Bilma is a dune sea (Arabic: "erg") in the Ténéré desert region of the south central Sahara Desert. The Erg