During the first half of the Pleistocene, about 1.8 to 0.8 million years ,
a desert climate prevailed in Egypt, which made the river dry out. When the
climate became more humid again 800,000 to 400,000 years , the Prenile
was formed. It was the first fluvial system in this region that had a connec-
tion to East Africa. It was made possible because the Ethiopian highland was
tilted northward, so that the main drainage in Ethiopia was to the north and
no longer to the Red Sea in the east, as had been the case previously. Thus
the waters of the rivers that were the forerunners of the Blue Nile and the
Atbara could break through the Nubian Shield and feed the main river. During
the course of these events the six Nile cataracts existing today were formed.
This Prenile had a much greater discharge than the later Neonile and its
sediments were coarser. The discharge of the Neonile changed greatly during
the 400,000 years of its existence. Repeatedly the connection to its southern
catchment areas was interrupted when the Nubian Shield was uplifted. Also
the alternation between dry phases and wet phases played an important role
in both discharge and sedimentation.