14 HISTORY AND SIGNIFICANCE OF SCIENCE 



Plateau Plateau 



Possible Fjiord-Lake Surfaces 



Lake Bottom at Final Lake-Stage 



Beds of Lacustrine Deposits 

 ._.,*... washed in from Plateau 



Line of maximum 

 erosion of 

 lacustrine ' 

 deposits - 



FIG. 2. Schematic Cross-Section of the Nile Valley, at a stage when the fiord- 

 lake had reached its maximum development. The lacustrine beds are 

 shown filling a considerable portion of the rift produced by sinking of 

 fault-block a a' from its original position A A' B E' , The downward 

 displacement of the fault-block amounted to some 900 feet, as shown by 

 the present height of the valley walls. The distance across the rift, as it 

 now exists, varies from 4 to 15 miles. The extent to which the lacustrine 

 deposits have since been eroded is indicated by the profile of the present 

 valley which is outlined as in Fig. 3. The vertical dimension is greatly 

 exaggerated. Compare with Fig. 3. (Adapted from Breasted and Blanck- 

 enhorn.) 



dwindling stream became converted into the modern Nile 

 and began to deposit the present alluvium. As shown by 

 Fig. 3, the remains of the old lake-beds are exposed along the 

 sides of the great rift. Below them are the remains of two 

 so-called river terraces, marking stages during which the 

 stream maintained its volume for a time sufficient to estab- 

 lish flood planes and lay down alluvial terraces that have 

 since been washed down to rounded contours. 



The peculiar features of the foregoing geological events 

 enable us to trace the cultural record in a definite manner. 

 The numbers on the right hand side of Fig. 3 indicate the 

 location of the items which may now be enumerated in the 

 order of their antiquity. 



1. Many implements and flint workings, similar to palaeolithic 

 remains elsewhere in Northern Africa, are found scattered over 

 the plateau. 



