An Egyptian oasis

by H. J. L. Beadnell

In my opinion the subterranean water of the oasis is certainly of meteoric origin—that is to say, it is water which originally fell as rain, and has percolated underground from one of the possible sources above mentioned. It will, however, readily be admitted that the ordinary explanation of the flow and origin of artesian wells in regions of moderate or abundant rainfall, situated in well-defined basins, where the exact position, extent, and absorbing capacity of the water-table outcrop can be carefully determined, may be in some respects inadequate to account for the flowing wells of vast arid regions like the deserts of Africa and Australia; yet, after due consideration of the arguments for and against, I am unable to subscribe to Professor J. W. Gregory’s view[11] that a considerable portion of the waters in such regions is derived from magmatic or plutonic sources—that is to say, has its origin in the deep-seated crystalline rocks.

It may seem at first sight almost incredible that in those regions, where the outcrop of the water-bearing strata is so remote from the wells themselves and the dip over the intervening country so slight, the rise of the water in the wells could be due to direct pressure of water in the higher portions of the beds, unless on the supposition of large and continuous open fissures. That such fissures exist, and exist abundantly, is, I think, almost a matter of certainty; but it does not follow that their presence is essential to the production of flowing wells. Fissures are visible to the eye in the Surface-water Sandstone (which, as has been remarked, does not appear to differ in any important respect from the Artesian-water Sandstone), and it is through them that the bulk of the sub-surface water is obtained. The presence of fissures in the Artesian-water Sandstone is, moreover, in my opinion, almost demonstrated by the experiments on the mutual interference of wells. It seems hardly conceivable that the closing or opening of a bore could in the space of a minute or two affect the discharge of another well over ½ kilometre distant, if there did not exist a more or less open and direct connection between the two.

Rapid flow through a compact sandstone is impossible owing to friction, which increases as the size of the channels decreases; but, as pointed out by Knibbs and others, the hydrostatic pressure can never entirely disappear through friction, the rate of loss of head being dependent on the rate of flow. It therefore by no means follows that a strongly-flowing well cannot be obtained from an unfissured sandstone, for a rapid flow from the bore itself does not in any way depend on an equally rapid flow of the water through the sandstone surrounding the bore. For instance, Mr. Knibbs has calculated[12] that although in a 10-inch bore, discharging 700 gallons a minute from a 10-foot stratum, the water would have a velocity of 5½ feet a second at the bore itself, at the distance of one mile it would only be moving through the stratum at the rate of about ¹⁄₂₀₀ inch per second, or 18 inches an hour. In other words, water flowing through a 10-foot bed of sandstone from all sides towards a 10-inch bore, need, at the distance of one mile, only have a velocity of 18 inches an hour to produce a discharge from the well of 700 gallons a minute.

The importance of local pressure arising from variations in level of a fully saturated water-table in adjacent areas may be of itself quite adequate to cause flowing wells, especially if assisted by the presence of large volumes of gas under compression, such as occur in the Kharga waters. Another theory which has at times been brought forward as an adequate cause of flowing wells is rock-pressure— i.e., pressure due to the weight of the overlying strata. The objections to this theory seem to me, however, so cogent that we may at once dismiss it from our minds. There is indeed little doubt that the waters of the oasis are of meteoric origin, have travelled immense distances underground, and rise through bores placed in favourable localities by means of hydrostatic or hydraulic pressure, acting both through the pores of the rock and through open fissures. Although the rate and direction of flow through the sandstones as a whole may remain more or less matters of conjecture, it seems probable that the water of any one bore is derived from all sides, rising as the result of the pressure exerted by the water held in the same bed situated at higher levels, whether in the immediate neighbourhood or at a considerable distance.

I am conscious of having done little more than indicate the possible origin of the oasis waters and suggest the causes to which the flowing wells are due; more than this it is at present impossible to state with any confidence. The points to which attention should be directed as likely to throw further light on the subject are as follows: The area and position of the outcrops of the Impermeable Grey Shales and the underlying sandstone, and their relations to possible sources of water, whether rain, river, or lake; the nature of the bed of the swamp region of the Upper Nile; the amount and distribution of the rainfall of all surrounding regions; the volume of water lost in the different reaches of the Nile over and above that which can be directly accounted for by evaporation and by water abstracted for purposes of irrigation; and the total thickness of the water-bearing beds, the presence within them of impervious strata, and their relation to the underlying crystalline rocks.

AIN AMUR, ON THE UPPER DAKHLA ROAD.

High-Level Springs.

Before leaving the subject of water-supply we must briefly refer to two or three remarkable instances where water is found at very high elevations. The most important of these is on the upper road between Kharga and Dakhla, where a spring, known as Ain Amûr, occurs near the summit of the plateau at about 460 metres above sea-level. The water, which is quite potable, occupies the bottom of a hole about 3 or 4 metres in depth, at the base of a clump of palm-stubs, a few paces to the south of a solitary tree; it does not run, and the water-level is said to fall considerably in the summer. A small patch of green rushes lies a few paces to the west, and there is a good deal of scrub in the neighbourhood, from the position of which one is led to infer that there are several small springs thrown out along one of the bedding-planes, at the summit of the marly and clayey beds which underlie the limestones forming the uppermost portion of the cliff.

The high-level spring on the eastern wall of the oasis, in the neighbourhood of Beris, consists of a pool of clear sweet water at the base of a large fallen block of limestone, in a desolate rocky dingle. Ball determined its height as being 180 metres above Beris, or 260 metres above sea-level. The pool itself is overgrown with weeds, and, scattered about on the sides and bottom of the valley, there is a good deal of vegetation, mostly in the form of coarse grasses, prickly scrub, and tamarisk bushes. Geologically, the spring appears to be similarly situated to Ain Amûr—that is to say, it emerges near, or at the summit of, the argillaceous Exogyra Series, and below the overlying Danian limestones.