The most outstanding characteristic of desert topography is the prevalence of closed basins, where the general slopes, instead of inclining toward the sea, converge toward the center of a bowl. When the desert wadi terminates in an alluvial region, the sediment which it carries remains in the basin; it comes to rest there, heaps up and becomes stratified. In a desert as arid as the present Sahara a great number of the rivers are short and intermittent torrents, with an alluvial region beginning immediately on their emergence from their native mountains, so that their erosive power is lost almost at once. The French officers of the meharists, or camel corps, have been in intimate contact with the ergs of the Algerian Sahara for more than half a century, and they have noticed no change in them.