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
Issaouane Erg which can take hundreds of thousands of years to form; mesoscale dunes (including the stardunes), which form on the mega dunes; and yet smaller ones which migrate over the bigger ones. In the NASA images, the light-blue spots in the valleys between the dunes (which are red from iron oxide) are "sabkhah", or salt flats, left after the evaporation of accumulated water. Neolithic artifacts, sculptures and paintings have been found in the Issouane Erg. from the Kiffian culture. The Issaouane Erg contains oil; the Tifernine oilfield is explored by the Algerian state oil company Sonatrach, which began developing the oilfield