An Egyptian oasis

by H. J. L. Beadnell

We have seen that a flowing well can be obtained (in a pit) a few metres below ground-level, and under certain conditions it may be possible to obtain flowing wells at the surface from the sandstone under consideration; the occurrence of artesian water at El Dêr el Ghennîma, on the east side of the depression, may, indeed, be a case in point. To the east of Headquarters, as may be gathered from the section, the Surface-water Sandstone dips gently under the Purple Shales, the thickness of the latter increasing as the eastern escarpment is approached. Running north and south through El Dêr is an anticlinal fold, and flowing water is obtainable from wells sunk on its crest through the shales to the underlying sandstone. The original depths of the ancient wells in this district cannot be determined with certainty, but when one of those to the north of the ruined fort was cleaned out and cased, flowing water was obtained when the drill reached a depth of 41 metres. Below this the ground appeared to be untouched, the bore being carried a few metres deeper into fresh sandstone, which yielded an increased supply. This well has given a steady discharge of about 40 gallons a minute for over eighteen months. Quite recently the flow has been augmented to just over 50 gallons per minute by its outlet being lowered about 1½ metres, to 92½ metres above sea-level.

The water here seems to be derived from the Surface-water Sandstone, though, considering that the well is situated on a line of folding, it would not be advisable to entirely disregard the possibility of the presence of fissures, through which the water might rise directly from the Artesian-water Sandstone below. If, as appears to be the case from the depth and position of the bore, the water does have its origin in the Surface-water Sandstone, the explanation may be that the necessary working head or pressure is furnished by the difference of level of the sandstone here and in adjacent districts. In the extreme north of the oasis the same sandstone lies at a considerably higher level, and, at the same time, holds large volumes of water, and it may be that the pressure of this water, acting on that contained in the beds at lower levels, gives rise to flowing wells when bores are sunk in favourable localities.

A very great drawback to the water of these upper sandstones is its variable quality. In the Headquarters district it is usually more or less ferruginous, and in some pits may contain as much as 63 grains of dissolved solids per gallon, the salts consisting of iron, potash, and soda, with traces of lime and magnesia, mostly in the form of sulphates and chlorides. Mr. William Garsed[9] has calculated that this water would, if used for irrigation at the rate of 3 gallons a minute per acre, effect an annual deposition of over 3 tons of sulphate of potash and common salt on each acre of land. Salts of this nature in such quantities would, of course, have a very deleterious effect on the crops, not to mention the mechanical binding action of the iron on the soil.

It appears that the want of uniformity in the quality of the water is due to local causes, chief of which may be the relative abundance of fissures, the purity of the sandstone, and the presence or absence of mineralized shales. In those areas where the water is moving only slowly—where, in fact, it tends towards stagnation in the strata—it is probably liable to become more or less highly charged with mineral salts; certainly the best qualities seem to coincide with relative abundance, and the largest supplies are undoubtedly found where the beds are traversed by fissures.

In many parts of the oasis perfectly sweet water is obtainable from the sandstones of this series, and this source, as an auxiliary to the artesian supplies obtained from deep borings, was taken full advantage of in olden times. The ancients constructed the most marvellous systems of subterranean aqueducts to tap these sources, more especially in the neighbourhood of Um el Dabâdib, Qasr Lebekha, and Qasr Gyb, where such works were especially applicable, owing to the fact that the sandstones in those districts form extensive hills above the general level of the surrounding cultivable ground. These underground works are, in many respects, of far greater interest than the ancient monuments, and they will therefore be fully described in a later chapter.

The Artesian-Water Sandstone.

The source of the great majority of the flowing wells of the oasis is the group of sandstones underlying the Impermeable Grey Shales. Although the beds of the series are nowhere visible to the eye, their general characters can be judged by an examination of the material brought to the surface during drilling operations. The samples prove that, in general lithological characters, the Artesian-water Sandstones do not essentially differ from those just described. Throughout the area over which boring operations have recently been carried out no well-defined, continuous, argillaceous bands have been met with, though lenticular intercalations of clayey strata are not uncommon. Up to the present time the base of the Artesian-water Sandstone has not been reached, although the deepest borings have been carried down to a depth of 122 metres below its junction with the confining shales above. The different bands vary considerably in coarseness and porosity, in hardness, and in the amount of cementing material between the individual grains of the rock, all of which characters have a marked influence on their capacity as water-carriers. Judging from their unfossiliferous nature, from the presence of thin seams of lignite associated with bands and nodules of iron pyrites, and from certain other considerations, we are led to infer that these sandstones were originally laid down on the bed of an immense inland fresh-water lake.

During the past half-century the natives have put down a considerable number of deep bores in this and the neighbouring oasis. No written records concerning these are, however, available, so that in seeking information one has to rely on the memory of the men who sunk the wells. By careful cross-questioning I have collected a large amount of interesting and valuable information, but still, in describing the artesian wells themselves, it will be more satisfactory to confine our attention to those which have been drilled on the Headquarters area during the last two or three years, and of which accurate and reliable records have been preserved.

The Headquarters area, occupying the central part of the depression between Kharga village and Jebel el Ghennîma, is one of the few large districts entirely devoid of old wells and traces of former cultivation. A combination of unfavourable circumstances appears to have led the ancient well-borers to avoid this district. Firstly, the general elevation is comparatively high, meaning small flows from wells of ordinary depth; secondly, the superficial alluvial deposit is clayey and heavy, necessitating a considerable expenditure of time and labour to bring it into satisfactory condition for cultivation; and, thirdly, and probably most important of all, the presence of a copious supply of sub-surface water, which would have greatly hampered, if not made impossible, the sinking of wells by the ancient system.

It may therefore be assumed that, owing to the entire absence of both ancient and modern wells, the sandstones of this district were practically fully charged with water at the time the first bore was sunk.