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
Human geography human geography. Well-known geographers from this period are Fred K. Schaefer, Waldo Tobler, William Garrison, Peter Haggett, Richard J. Chorley, William Bunge, and Torsten Hägerstrand. From the 1970s, a number of critiques of the positivism now associated with geography emerged. Known under the term 'critical geography,' these critiques signaled another turning point in the discipline. Behavioral geography emerged for some time as a means to understand how people made perceived spaces and places, and made locational decisions. The more influential 'radical geography' emerged in the 1970s and 1980s. It draws heavily on Marxist's theory and techniques, and is associated with