During the Pliocene an uplift in the Strait of Gibraltar separated the Medi-
terranean Sea from the ocean, so that it dried out. Its bottom, lying 3,000
to 4,000 m below the surface of the surrounding land, functioned as a new
base level for the erosion, which was much lower than it had been before,
so that the river valleys cut in much more deeply. The first Nile, the Eonile,
was formed in a structural lineament between the old Basement Complex and
the younger plateau in the west. It was mainly fed from the mountains along
the Red Sea to the east, which were much higher at the time than they are
today. The valleys formed during this more humid phase still exist as wadis
which are periodically activated today. The incision caused by the Eonile,
which reached 2,500 m in northern Egypt, equalled in dimension the Grand
Canyon of the Rocky Mountains of today. When the passage between the
Mediterranean and the ocean was opened up again, the Nile canyon turned
into a gulf which was gradually filled up by sedimentation.