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
Grand Erg Oriental the language of the nomads, who habitually reside in such places." For these and similar reasons concerning the desert ecology of water, the geographer and historian of the Sahara E.-F. Gautier once wrote referring to the Grand Erg Oriental as "the great Igharghar Erg". Accordingly, surface moisture, seasonal pastures, and wells may reflect unseen conditions beneath the sands. What appears as an entirely inhospitable erg can elsewhere, in stark contrast, offer the fruits of "some buried wadi". To the north of the erg, the Aurès mountains provide abundant runoff. These waters feed the artesian aquifer of the Jerid, despite its