The most extensive sandstone formations are at Silsilis (fig.

49).

Here the cliffs were quarried from above, and under the open sky.

Clean cut and absolutely vertical, they rise to a height of from forty to fifty feet, sometimes presenting a smooth surface from top to bottom, and sometimes cut in stages accessible by means of steps scarcely large enough for one man at a time.

The walls of these cuttings are covered with parallel striae, sometimes horizontal, sometimes slanting to the left, and sometimes to the right, so forming lines of serried chevrons framed, as it were, between grooves an inch, or an inch and a half, in width, by nine or ten feet in length.