Vol. 50.] PHYSIOGRAPHY OP THE LIBYAN DESERT. 541 



VI. The Erosion of the Nile Valley, etc. 



Altogether it would seem that the east-and-west folding took place 

 at a time when the Nile, as we know it, had not begun to exist, 

 probably in very late Eocene or early Miocene times ; while a later 

 north-and-south folding, 1 culminating in the Nile Valley fault at 

 Cairo and the great Araba fault in Palestine, with a large downthrow 

 to the west, finally determined the line of drainage at a later period. 

 The date of this last folding is not at present very easy to determine ; 

 but, as it seems to have been before the Jebel Ahmar Sandstone, it 

 must have been during Miocene times. 



As mentioned above, this Jebel Ahmar Sandstone lies on the eroded 

 surface of the Eocene strata, and some of the marine Miocene, e. g. 

 that south of Jebel Atakka, near Suez, is deposited in hollows eroded 

 out of the Eocene rocks, so that a certain amount of erosive action 

 had taken place in the earliest Miocene times. Still the main work 

 of carving out the Nile Valley seems to have been done later, after 

 the north-and-south folding had determined the direction of drainage, 

 and after the deposition of the Jebel Ahmar Sandstone as an estuarine 

 deposit, while it was earlier than the late Pliocene beds and sea- 

 beaches which rest against the Nile cliffs of to-day at Cairo and Giza. 

 Thus the early Pliocene times seem to have been the period when the 

 majority of the work was done, though a certain amount was 

 doubtless going on later in post-Pliocene times. Prof. Hull 2 

 attributes it to the heavy rainfall of a Pluvial Period, but for the 

 reasons quoted above the main part of the work seems to have been 

 done in two stages, and in this I have the support of M. Polland, 3 

 who describes heavy erosive action as taking place in Pliocene 

 times in Algeria and a second period of erosion in Quaternary 

 times. Under these conditions, when the northern line of drainage 

 had once been established, we should have the Nile of that period 

 flowing through a plateau and fed by tributary streams from 

 the east and west, by which the upper limestone- and marl-beds 

 would be rapidly eroded away along their dip-slope, leaving an 

 escarpment on the north which was continually being cut back. 

 As the Nile cut its way down through the softer overlying beds it 

 would in time reach the harder Nubian Sandstone or the crystalline 

 rocks, and on meeting them at the points where the anticlinal folds 

 crossed its line the river would tend to move east or west along 

 the obstruction till an easier point was reached. As the Cretaceous 

 limestones and marls were eroded away, the underlying Nubian 

 Sandstone was laid bare and erosion would now go on more slowly, 

 while to the north the limestone escarpment was being cut farther 

 and farther back. By the time the escarpment had been cut back 

 as far as the southern limit of the oases, their springs, bursting out 

 at the base of the cliffs, would enormously increase the rate of erosion. 



1 Rolland, ' Geologie du Sahara,' p. 258. 



2 ' Geology of Arabia Petraea, Palestine, and adjoining Districts,' pt. ir. 

 chap. ii. p. 113, London, 1889. 



3 ' Geologie du Sahara,' p. 260. 



