Although the evidence met with in the field is altogether against the idea that portions of the plateau have been bodily let down by subsidence, there are good reasons for believing that tectonic movements have played an important part in deciding the general shape of the oases-depressions. For instance, there is considerable parallelism between the general trend of the Baharia depression and the folds which pass through that region. As a result of those folds, it is not improbable that the hard limestone beds were to some extent broken up, and the soft underlying clays and sandstones raised as compared with their position on either side of the folded belt. In Kharga, similarly, the main axis of the depression is, as we shall show, distinctly parallel to the great north and south line of flexure, and there is little doubt that a close connection exists between the two. On the other hand, I know of no folding in the case of Farafra, which appears to owe its existence solely to the fact that there was in that region an unusual development of shales at the base of the Eocene nummulitic limestones. Of the four Egyptian oases, perhaps Dakhla is the one most easily accounted for, as this depression may be regarded as simply due to the general northerly dip of the sedimentary formations, and the gradual weathering back (northwards) of the great argillaceous series (Exogyra Beds) capped by the White Chalk. The original limits of the latter may, indeed, never have been very far to the south.[3]

While all sedimentary strata—such as the limestones, sandstones, clays, and shales with which we are now dealing—were originally deposited either quite horizontally or inclined at only a very low angle, it by no means follows that this horizontality is maintained when the strata are elevated into dry land. Over the Libyan Desert as a whole the successive sedimentary formations dip steadily northwards, but at a very small inclination. This results in every stage having a wide outcrop, so much so that, if it were not for the cliff-sections of the Nile Valley and the oases-depressions, we should have to travel immense distances to obtain any idea of the true succession of rocks. This general horizontality of strata appears at first sight to be well maintained in the oasis of Kharga, as whether we stand on the summit of the eastern escarpment, on top of the great central hill-massif of Jebel Tarif, or on the plateau above Ain Amûr, we see everywhere horizontally-disposed beds of limestone forming the plateaux and upper portions of the cliffs, with parallel bands of sandstone, shale, and chalk outcropping on the slopes below.

A closer examination, however, will show that there is in reality a difference in level of more than 200 metres between the same beds on either side of the oasis; for the beds capping Jebel Tarif belong to the White Chalk of the Cretaceous system, and are therefore very much older than those of Eocene age forming the eastern plateau (see map and section). This great difference in vertical position is due partly to a steady dip from west to east, partly to a remarkable longitudinal flexure running north and south through the centre of the depression, and partly to a gentle fold near the base of the eastern escarpment. Along the actual line of flexure, which passes through Jebel Têr, Jebel Tarwan, Nadûra, Gorn el Gennâh, and Gertuma (S.S.E. of Bulaq), the different rock-stages are folded and fractured to a remarkable degree. Throughout the greater part of its course the flexure approximates to the type of disturbance known as a simple monocline, but in places, as in Jebel Têr, it passes into a syncline bounded by nearly vertical faults; while in others the beds are bent into almost symmetrical basins or centroclinal folds, typical examples of these structures being met with at points 6 kilometres south of Kharga village and 2 kilometres south-west of Qasr Zaiyan.

The importance of this line of folding and faulting must not be lost sight of, as although the dislocations produced are only actually visible in the case of the exposed upper beds of the oasis sequence, the earth-movements to which it owes its origin have had similar disturbing effects on the underlying and hidden water-bearing strata. Ball reported that the most striking evidence of faulting was between Jebel Têr and Jebel Tarif, and showed the fault as running for a short distance in a N.N.E. and S.S.W. direction, but, as already mentioned, the line of disturbance is coincident with the longer axis of the former range, so that the majority of the wells are on the west or upthrow side of the fault. The effects of this faulting and folding on the underground water-supply will be further alluded to in a later chapter.

In the early summer of this year (1908) I followed the line of flexure southwards in order to determine whether it continued throughout the oasis. As far as the small eminence of Gala, about 10 kilometres south of Bulaq, it ran in an almost straight line, but south of that point its course took a distinct bend to the west, so that the fold was very soon lost in the great belt of sand-dunes. Beyond this point its continuation could, however, be inferred by occasional exposures of steeply inclined sandstones, the most southerly point to which it was actually traced being in latitude 24° 55′ N., about 15 kilometres S.S.W. of Ain Girm Meshîm.

[(Large-size)]

GEOLOGICAL SECTION ACROSS THE OASIS FROM JEBEL TARIF TO THE EASTERN ESCARPMENT.

Before concluding our remarks on the geology of the oasis we must not omit to call attention to the beautiful and varied fossil remains which are almost everywhere to be met with in the calcareous beds of the hills and escarpments. It is, of course, by the study and comparison of these organic remains that geologists are enabled to determine the relative ages of the beds in which they occur, and thus to correlate them with the rocks of other countries. The lower argillaceous and arenaceous deposits of the oasis are comparatively unfossiliferous.

From any of the points of vantage, such as are afforded by the higher hills within the depression, the general level of the floor of the oasis does not appear to vary to any great extent, but actual levelling shows that this is not in reality the case; and it is this variation of absolute level which is the primary cause of the very varying volumes of water yielded by the artesian wells in the different districts.