Paul E. Lovejoy

"The presence of salt in the southern Sahara and sahel is the result of geological change over the past ten millennia. As the more humid conditions of the quaternary gave way to drier eras, the great lakes of the Sahara gradually receded, depositing salt in a few locations, most notably at Amadror, Kawar, and Fachi in the central Sahara.....
"The evidence establishes clearly that Kawar was once the basin of a great lake which probably stretched 120 km north of Bilma and 20 km westward from the cliff. The lake may have extended further south than Bilma, perhaps having an outlet in that direction. The great lake was at its fullest more than 10,000 years ago: the desiccation of the region gradually resulted in its contraction, a process which probably took several hundred years before the water was confined to the basins at Bilma and elsewhere in Kawar. The contraction resulted in the concentration of salt through precipitation, which accounts for the salt layers in the cliff at Bilma and elsewhere in the Kawar region....
"The salt at the bottom of the depressions - the salt which has been actually exploited for commercial purposes for at least the past millennium - traces its origin to a later development. After the disappearance of the great lake, smaller lakes continued to occupy the depressions at Bilma, Dirku, Seguidine and elsewhere in Kawar. ...The drainage of the surrounding countryside reconcentrated salt in these smaller lakes. The salt inflow included some of the eroded deposits from the great lake but other salt too. Faure estimates that these later deposits, occurring over many thousands of years, could have resulted in accumulation of 500 tonnes of salt per year. The total deposit was certainly of the order of another million tonnes and perhaps as much as 3 million tonnes.
"These reserves provide the basis of the salt industry." pp. 39-40… (meer)