To the south-west of the limestone plateau which lies west of the Nile and extends northwards from lat. 23° 30′ there is fairly open sandstone country, and the oases of Dakhla and Kharga occupy great bays in the escarpment which marks the south-west termination of the plateau. It seems reasonable to suppose that the Eocene and Cretaceous seas had their southern shores somewhere about the latitude of Wadi Halfa, for the limestones do not extend far beyond the tropic; and to the south the hard limestones of the plateau probably passed gradually into softer shallow-water deposits, which would lend themselves easily to degradation and to removal by the agency of streams whose traces have long since disappeared. We can thus possibly invoke water-action to account for the primitive formation of the “bays” in which Kharga and Dakhla are situated. But we cannot in this way explain either the further deepening of these oases, which converted them into basins, or the excavation of the more northerly depressions such as Baharia and Qattara. The problem as to how the excavation of the northern depressions, and the deepening of the southern ones, were accomplished is not altogether an easy one. There is, of course, no doubt that wind has been the main excavating agency. All the depressions occur in areas where soft rocks are overlain by hard ones, and once the hard overlying rock was removed at any place, the action of the wind on the softer beds would soon excavate a hollow. The difficulty is only as to how the hard overlying rock was first removed. To some who have studied the question, a sufficient explanation is to be found in the general degradation, combined with the tectonic disturbances—folds and faults—which are known to have affected some of the oasis-areas, as, for instance, that of Baharia. Given an anticlinal fold, a general lowering of the surface by wind-degradation would eventually result in the exposure of the soft underlying beds at the highest point of the fold, and subsequently continued wind-erosion would proceed most rapidly on these soft underlying rocks. Others, while admitting this explanation as partly accounting for the formation of the depressions, consider that other factors must have assisted to cause penetration of the hard overlying beds, especially in the areas where folding is not very marked.