Finally, from the way in which the gypsum and gypseous deposits are laid on the terrace gravels, and from their extension and thickness, we may presume that they were deposited on the bottom of just such a lake on evaporation of its water, when the sulphate of lime in solution, becoming more and more concentrated, may have been finally precipitated.[89] SECTION XIII.—PLEISTOCENE. The course of events in Pleistocene times is at present obscure. As far as can be judged it was during this period that a freshwater lake, the precursor of the great Mœris, came into existence. It might be thought that the early Pleistocene prehistoric lake was a relic of the still older body of water of Pliocene times, in which the gravel terraces and gypseous deposits were laid down. But such a remnant would have been of a high degree of salinity and could not have given rise to the fresh water Mœris. Most probably at the close of the Pliocene period, after the formation of the gypseous deposits, the area became elevated and cut off from the sea and from the Nile Valley marine fiord; probably an extensive body of water remained as an isolated lake, but this, cut off from external supply, would have gradually evaporated, its salt being left as a superficial deposit on the dried up bed. In early Pleistocene times we may presume the area became dry and was gradually eroded to its present shape and depth. The superficial deposits of salt and gypsum were for the most part removed as the depression was deepened, while the continuous terrace of gravel laid round the greater part of the rim was broken through, except where protected in favourable localities, the constituents being washed down and spread out over the lower ground. In course of time the region was moulded to its present form and dimensions, or rather to what it would be if the local alluvial deposits were stripped off and the water of the lake baled out. The area was an inland depression, probably sparsely vegetated like the rest of the higher country and separated from the Nile Valley by a low rocky ridge surmounted by a more of less continuous terrace of gravel of considerable height and thickness. In our opinion desert conditions had already set in before the early Nile broke down the ridge and formed a lake in the Fayûm; the date when this important event first took place is a matter of considerable doubt, as has already been mentioned (pp. 24, 25). We know that at the close of the Pliocene period the Nile Valley was a marine fiord (connected with the Fayûm and the Mediterranean) which was replaced in Pleistocene times, probably in consequence of slight elevation, by a series of fresh water lakes throughout the valley. These lakes were probably two or three in number and drained one into the other; the exact position of the barriers is as yet a matter of conjecture. Within these lakes thick lacustrine deposits were accumulated, so that the basins eventually became to a great