50 WAVES OF SAND AND SNOW 



tide on sandbanks, in which the characteristic steep 

 and gentle slopes were also the lee and weather 

 side of succeeding waves of sand. 



The higher part of the sandbank near to the 

 permanent bank of the river was of course the first 

 to be uticovered during the subsidence of the Nile, 

 and had therefore been longer exposed to the action 

 of wind. This part of the sandbank was dotted 

 over with mounds of loose sand separated from 

 each other by bare earth. Most of them were of 

 a definite and remarkable crescentic shape. The 

 profile — that is to say, the section taken in the 

 direction of the wind, which I call the longitudinal 

 section — is very nearly the same as that of the 

 sand-waves which are in regular ridges. It con- 

 sists of a gently sloping weather face and a cliff 

 on the lee standing at the steepest angle allowed 

 by the looseness of the material. The cliff is 

 highest in the centre, and from that position extends 

 forwards on either side, the mound having two arms 

 or horns similar in shape to the cusps of a four- 

 days-old moon. This crescentic form of aeolian 

 sandhill, with a height of 20 or 30 feet, is well 

 known in many deserts. In those of Central Asia 

 it is called a barchan, in Peru a medano. The 

 height of those on the sandbank sometimes attained 

 3 feet. From the situation where I found them 



