47).
It answered two purposes, firstly, as a means of storing the water of the inundation for the use of the workmen in the neighbouring quarries; and, secondly, as a barrier to break the force of the torrents which rush down from the desert after the heavy rains of springtime and winter.

The ravine measures about 240 feet in width, the sides being on an average from 40 to 50 feet in height.

The dam, which is 143 feet in thickness, consists of three layers of material; at the bottom, a bed of clay and rubble; next, a piled mass of limestone blocks (A); lastly, a wall of cut stone built in retreating stages, like an enormous flight of steps (B).

Thirty-two of the original thirty-five stages are yet _in situ_, and about one-fourth part of the dam remains piled up against the sides of the ravine to right and left; but the middle part has been swept away by the force of the torrent (fig.
48).

A similar dike transformed the end of Wady Genneh into a little lake which supplied the Sinaitic miners with water.
Most of the localities from which the Egyptians derived their metals and choicest materials in hard stone, were difficult of access, and would have been useless had roads not been made, and works of this kind carried out, so as to make life somewhat less insupportable there.
[Illustration: Fig.