The implements occur chiefly along the margin of the highest level of the old lake, and have probably in many cases been buried in the lake clays until the present time, which would account for their beautiful state of preservation. We have not, however, yet actually detected them enclosed within the clays, although commonly found lying on the clean wind-worn surface. From the fine degree of workmanship we may undoubtedly refer these flint implements to the Neolithic or later stone-age, although the exact date is doubtful. That they were made and used while the lake still stood at its highest level seems certain, but as we have shown above, the lake, as a sheet of water up to 23 metres above sea-level or thereabouts, probably existed far anterior to the Egyptian historic period. They might, on the other hand, as far as the evidence from the position of the lake goes, have been used by the inhabitants of the lake-margin down to the great reclamation which took place in Ptolemaic times. As it seems impossible to date them by comparison with flints of known age from any Egyptian period, we may perhaps conclude that they are at least of older date than the earliest Egyptian records.[95] _P._—=Historic.= In historical times, under conditions almost identical with those of the Nile of to-day, there would have been an annual inflow during the flood and outflow back to the Nile when the latter subsided; during the inflow a constant supply of Nile mud was brought into the lake and deposited on the surface of the earlier alluvium, continually augmenting the thickness of the latter and raising its surface, until in the central area marshy land began to appear. In the XII Dynasty this natural backwater of the Nile, which acted as a more or less efficient regulator of high and low floods, was brought under human control by Amenemhat I, and a considerable area of land reclaimed from the shallowest part of the lake, or that part of the country now lying near Edwa, Medinet el Fayûm, etc. The new artificially controlled lake was called Moeris, and its wonders are mentioned by Herodotus, Diodorus Siculus, Strabo and Pliny.