52 U.S. NATIONAL MUSEUM BULLETIN 2 75 



in their movements by the Nile include Pachuromys and Sekeetamys 

 Ellerman. The effect of the Nile as a barrier to dispersal during modern 

 times is further indicated by the following species of North African 

 rodents which are represented by different subspecies occurring on 

 opposite sides of the river: Gerbillus henleyi; Meriones crassus; 

 Psammomys obesus; Jaculus jaculus; and Gerbillus gerbillus. 



Judging from what is known of the past distribution of North African 

 rodents, however, the barrier effect of the Nile has not been pro- 

 nounced. The manner in which rodent populations in the past managed 

 to surmount this apparent barrier is largely unknown, but it may be 

 related to the developmental history of the river. 



According to Said (1962, p. 24), an ancient pre-Miocene precursor 

 of the Nile is evidenced by fluviatile sands and gravels that overlie 

 the Eocene in Egypt. The modern Nile is therefore thought to represent 

 a "much diminutive representative" of this ancient river. If this 

 larger river was not the actual precursor of the Nile, it apparently 

 followed a similar course and must have had a similar effect on the 

 dispersal of rodents. 



Opinions differ regarding the developmental history of the Nile. 

 Said (1962) stated that the valley of the Nile in its present form was 

 established following a regional uplift near the middle Miocene. The 

 mechanics of the origin of the Nile Valley are not thoroughly under- 

 stood, but available evidence indicates that a crustal disturbance 

 probably established the course of the Nile. Some students feel that 

 the Nile developed primarily as an erosional feature following a north- 

 south syneline, while others favor a trough-fault origin. Recent 

 drilling in the Nile Valley supports the theory that the Nile was eroded 

 along a line of faulting and rifting (Said, 1962). 



Most students of African geology agree that during the period 

 between the end of the Cretaceous and middle Pleistocene important 

 tectonic events resulted in the warping and uplifting of large portions 

 of the African continent. This period was characterized by much fault- 

 ing and deformation of the land surface. Toward the end of this long 

 period of violent crustal disturbances, a Pliocene-Quaternary deforma- 

 tion produced additional movements along the large faults of the 

 Red Sea and the great Rift Valleys of East Africa (Furon, 1960). 

 These crustal disturbances presumably had pronounced effects in 

 North Africa, including the Nile Valley. 



In eastern Egypt, it is possible that these tectonic disturbances 

 caused regional and local deformation of the surface configuration 

 sufficient to alter the course of the Nile. Several large wadis, such as 

 the Wadi Araba and the Wadi Ghuweibba, originate east of the Nile 

 Valley and drain easterly into the Gulf of Suez. During a period of 

 crustal disturbance one of these wadis could have usurped the function 



