There is good reason to believe that the depression was inhabited previous to the formation of the lake, and although we have no information as to when the first deep borings were made, there is some evidence which leads me to suspect that wells existed prior to the time when the surface of the lake began to fall, if not very much earlier. The evidence to which I refer is the discovery of a portion of an earthenware pipe embedded in situ in the lake clays at a height of 42·85 metres, of the type used by the ancients for lining their water-channels. Is it possible, therefore, that, as the result of the industry of the ancient well-borers— following their initial discovery of these deep-seated sources—the long-confined waters welled up with irresistible force, and gradually flooded the country. At the present day, when the pressure throughout the artesian basin must be very much less than formerly, it is not uncommon for new bores to get out of control and flood considerable areas of country. The same thing may possibly have happened on a very much larger scale thousands of years ago, when the first bores were sunk into a previously untapped artesian basin, fully charged with water under great pressure.
FLINT IMPLEMENTS.
There is another explanation which it is advisable to keep in mind, though it has never hitherto, as far as I am aware, been advanced as a possible cause of the formation of lakes. The very existence of artesian water depends on the presence of porous strata overlain by impermeable beds. If one or other of the porous beds, charged with water under pressure, should, through the action of denudation on the overlying beds, become exposed at the surface, the waters would escape through natural springs in very large quantities. This might, indeed, continue for a long period of time, until the bed was nearly depleted and the pressure reduced to nil. There is little doubt that the beds which we have named the ‘Surface-water Sandstone,’ and which are now exposed in places on the floor of the oasis, were originally entirely covered by impervious clays, and contained artesian water under pressure; they may, in fact, have been in the same condition as is the Artesian-water Sandstone at the present day. It is conceivable, therefore, that when those beds became exposed at the surface, owing to the removal of the overlying confining strata, their contained waters escaped in such quantities as to have given rise to a lake of considerable dimensions, if not to one equal in size to that which we have been considering.
No one can be more conscious than myself of how much there is still to be learnt regarding the topographical aspects of the oasis in early historic times, and although the theory that the lake may have been formed by waters which escaped from the underlying water-charged beds (either artificially through bore-holes or naturally as springs) may at first sight appear fanciful, it is one which, in the present imperfect state of our knowledge, is at least worth bearing in mind. A more detailed examination of the lacustrine deposits would probably throw further light on the matter; and a minute analysis of the nature, composition, and arrangement of the individual grains would almost certainly show whether the beds have in the main been formed of sediment carried into the lake by streams from the neighbouring cliffs and plateaux, or whether they represent wind-borne accumulations of sand and clay-dust from the surrounding plains.
At whatever period and in whatever manner the lake may have been formed, it is quite clear that it existed well into historic times, and that on its bed were laid down thick deposits of clay and sand, which at the present day form the bulk of the cultivated lands of the oasis.
CHAPTER IX
THE UNDERGROUND WATER-SUPPLY
The Water-bearing Strata underlying the Libyan Desert — Essential Conditions required to produce an Artesian Basin — The Surface-Water Sandstone — Collecting Pits — Fissures — Yield of Water — Flowing Wells from this Sandstone — Bores at El Dêr el Ghennîma — Variable Quality of Water — Ancient Subterranean Aqueducts — The Artesian-Water Sandstone — General Characters — The Headquarters Area avoided by the Ancients — Drilling Difficulties — Results of New Bores — Factors determining Discharges of Wells — Temperature and Chemical Composition of Artesian Waters.
When laboriously traversing the hot and arid plateaux of the Libyan Desert, our thoughts divided between the fertile plains of the Nile we have left behind and the still far distant oasis, it is difficult to realize the presence, within a distance of a few hundred yards, of an abundant supply of the purest water. Yet there is little doubt that the water-bearing beds underlie practically the whole of the Libyan Desert, though it is only on the floors of the depressions that they lie within accessible distance of the surface. On the high tablelands the cost of sinking bores to reach the sandstones would be prohibitive, and without the aid of powerful and costly pumps the water would not rise to the level of the ground. But although the subterranean waters of the great desert plateaux cannot economically be made available at the surface, it must not be forgotten that the deeply buried sandstones of these regions must act to a great extent as storage reservoirs capable of replenishing the beds underlying the oases-depressions, from which large volumes of water are continuously drawn by the numerous artesian wells.
The essential conditions required to produce an artesian basin, which, when tapped by borings, will produce self-flowing wells, are the presence of strata of sufficient porosity to carry water, enclosed above and below by beds of sufficient impermeability to prevent the escape of that water; the outcrop of the porous beds in some higher and distant region, with an adequate exposure to an abundant source of water, whether rain, river, or lake; and, finally, the absence of an easy escape at a lower level, unless at a considerable distance from the sites of the wells.