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 they recommended using the prefix ""C.G.S. unit of ..."" and requested that the word "erg" or "ergon" be strictly limited to refer to the "C.G.S. unit of energy". In 1922, chemist William Draper Harkins proposed the name micri-erg as a convenient unit to measure the surface energy of molecules in surface chemistry. It would equate to 10 erg, the equivalent to 10 joule. The erg has not been a valid unit since 1 January 1978 when the EEC ratified a directive of 1971 which implemented the International System (SI) as agreed by the General Conference of Weights and Measures. It