Although the geologic structure of both banks of the Saoura river, Algeria, is essentially the same, no ground-water aquifers occur on the right bank due to the absence of covering sands such as are present on the left bank where an extensive aquifer occurs in Pliocene limestones overlain by dunes and sands of the western Grand Erg. Formation of the aquifer is due to accumulation of rain waters which have percolated through the desert sands and not to infiltration of waters from intermittent streams descending the slopes of the Atlas mountains.