Dimensions of the Oasis-Depression — Jebel Têr, Jebel Tarif, and other Hills within the Depression — Aspect of the Oasis from the surrounding Escarpments — Geological Sequence — Nature and Thickness of the Strata — Geological History of the Oasis — Formation of the Depression — Difference of Level of Strata on either side of the Depression — The Great Longitudinal Flexure — Height of the Floor compared with Sea-Level — Altitudes.
Kharga, the eastern of the two southern oases, is a depression lying with its longer axis north and south, mostly bounded by steep and lofty escarpments, but open to the south and south-west, on which sides the country rises gradually from the floor of the oasis. The extreme length of the depression, from the northern wall to Jebel Abu Bayan, which for convenience may be regarded as the southern limit of the oasis proper, is 185 kilometres, or 115 miles. The general trend of the eastern escarpment is nearly due north and south, but that on the west is very irregular, while to the south and south-west there is no definite boundary. The breadth of the depression may therefore be said to vary from 20 to 80 kilometres.
The ranges of Jebel Têr and Jebel Tarif form isolated hill-massifs in the centre of the northern part of the depression, while Jebel Ghennîma and Jebel Um el Ghennaim are conspicuous outliers of the plateau on the east side. With the exception of these, the floor is destitute of anything beyond comparatively insignificant eminences, unless we include the small range of hills known as the Gorn el Gennâh, to the south-east of the village of Gennâh, which is noticeable more on account of its sharply-defined peaks than of its general elevation above the surrounding country. Referring to the two conspicuous peaks, Ghennîma and Um el Ghennaim—Jimmy and Jemima, as I have heard them dubbed—reminds me that on the Survey and on some of the older maps the names are reversed. I have questioned a number of natives regarding the names of these hills, and have invariably been informed that Ghennîma is the more northerly of the two.
The villages, wells, and cultivated lands lie within a north and south band, occupying the lowest portion of the floor, and following the general trend of the depression. They are, however, broken up by a broad area of barren desert into two distinct north and south groups, of which Kharga and Beris villages are the chief centres respectively. A description of these is reserved for a later chapter.
When a traveller, after crossing the broad monotonous plateau, at length reaches the scarp or wall of the oasis, and sees spread out before him a vast depression, stretching in some directions as far as the eye can reach, in others to the opposite bounding walls dimly discernible on the far horizon, he can hardly refrain from speculating as to the causes which have given rise to such huge hollows in the plateau. When he descends to the cultivated portions of the floor of the depression, and sees those numerous bubbling springs, which alone make life possible in the midst of this otherwise deadly wilderness, his second inquiry is as to whence comes such abundance of water in one of the most arid regions in the world. These questions are worth asking, and, so far as the present state of our knowledge permits, it will be my endeavour to answer them. I propose, therefore, to briefly place on record such information and data as I have been able to gain, but as both topography and water-supply are intimately connected with the geology of the district, it will be necessary at the outset to devote a few pages to a consideration of the latter.
THE CHRISTIAN NECROPOLIS AND JEBEL TER.
The geological deposits found in the oasis of Kharga are tabulated on the following page, commencing with those most recently formed. The succession, as shown in the table, is that which obtains in the northern part of the depression, but as far as is known the same stages occur throughout the oasis, and do not vary either in thickness or in lithological characters to any great extent. Over large areas the lower-lying parts of the oasis-floor are formed of those beds which we have designated the Surface-water Sandstone, though in
| Geological System. | Stage. | Thickness in Metres. | ||||
|---|---|---|---|---|---|---|
| Recent andPleistocene | } | ⎧ ⎪ ⎨ ⎪ ⎩ | Sand-Dunes | ⎫ ⎪ ⎬ ⎪ ⎭ | Very variable | |
| Spring Deposits (modern) | ||||||
| Lacustrine Sands and Clays | ||||||
| Calcareous Tufa | ||||||
| Lower Eocene | Lower Libyan | Plateau Limestone | 115 | |||
| Passage Beds | Esna Shales and Marls | 55 | ||||
| UpperCretaceous | Danian | ⎧ ⎨ ⎩ | White Chalk | ⎱ ⎰ | 70 | |
| ⎧ ⎪ ⎪ ⎨ ⎪ ⎪ ⎩ | Ash-grey Shales | |||||
| Exogyra Beds | 30 | |||||
| Campanian (Nubian Series) | ⎧ ⎪ ⎨ ⎪ ⎩ | Phosphate Beds | 70 | |||
| Purple Shales | 50 | |||||
| Surface-water Sandstone | 45 | |||||
| Impermeable Grey Shales | 75 | |||||
| Artesian-water Sandstone | 120 | |||||
| Total | 630 | |||||
places the still older underlying grey shales are exposed. The purple or red shales generally form the rising ground towards the escarpments, at the base of which are usually found the phosphatic beds, with hard, pronounced bands made up of fish-remains and phosphatic nodules. Above come the Exogyra Beds, with thick bands of limestone almost entirely composed of large oyster-shells. Rising up above these is the generally well-marked cliff of grey shales, capped by a snow-white chalk of much the same age geologically as the well-known chalk of the South of England. The summit of the chalk frequently forms a separate plateau, subsidiary to the high desert tableland, and separated from it by the cliffs formed of the massive Eocene limestones.