The stratigraphical position of the Surface-water Sandstone will be seen by reference to the geological sequence given in a preceding chapter, and to the section drawn across the oasis. The beds forming this division have an average thickness of 45 metres, and consist almost entirely of fine sandstones or coarser grits, often containing an abundance of oxide of iron, and occasionally beds of alum and Epsom salts, or, to be chemically exact, hydrous sulphates of alumina and magnesia. Thin bands of shale are frequently met with, intercalated in the sandstones, generally near the top or base of the series. On the level or gently undulating floor of the oasis, to the east of the line of disturbance passing through Jebel Têr, the Surface-water Sandstone has a wide outcrop, while to the west, where the general level of all the formations is higher, the same stage forms the foot-hills of Jebel Tarif, of Jebel Têr, and of the high cliffs which rise to the north of Um el Dabâdib and Ain Lebekha. It is probable, moreover, that the sandstones which form the surface of the desert between Kharga and Dakhla, and which cover immense areas to the south of the oases, also belong to this group. Over a portion of the floor in the north of the depression these sandstones have been entirely removed by denudation, so that in this area the underlying grey shales form the desert surface. At one time the sandstone was continuous across the depression, and entirely covered by an impervious mantle consisting of the Purple Shales. It was then, in all probability, fully charged with water under pressure, as is the true Artesian-water Sandstone at the present day; and it is possible, as we have already suggested, that when the sandstones in question were first laid bare by the active agents of erosion, innumerable springs broke forth, and gave rise to a lake of considerable magnitude, which may, indeed, have been that which persisted into the historic period. In the district round Headquarters several pits have been sunk in this sandstone, from which the water is lifted by power for purposes of irrigation. In six pits the maximum level of the water varies from 55·38 to 56·52, the average being 55·74 metres. From Headquarters the general surface of the country falls to the north, south, and east, and curiously the water-level in the sandstone falls in the same directions. In a pit near Bore No. 43, about 3½ kilometres to the south-west, the water-level is 52·4 metres; at points 640 metres and 2 kilometres to the west it was found to be 52·16 and 50·21 metres respectively; while near Bore No. 33, 4 kilometres to the north, the surface-water stands at 52·71 metres above sea-level. The ground-levels at these points are 54·54, 57·90, 53·61, and 57·61 respectively, while that at Headquarters averages 59·57 metres above sea-level.