70 WAVES OF SAND AND SNOW 



not conforming at all to the direction of the wind. 

 The surface of the tract of sand showed, however, 

 two sets of undulations, one of which seemed to 

 be transverse waves due to westerly winds, and 

 one to be waves similarly produced by the northerly 

 winds now prevalent. 



Apparently the accumulation of a mass of blown 

 sand in this place was due to diminution of mobility 

 by moisture. Between Ismailia and Abu Hammad 

 I saw other sand-dunes produced apparently by 

 the same cause. South of the Wady Toumilat is 

 a plain dotted with scrub and free from sand- 

 dunes, but a line of them fringes the south border 

 of the moist Wady which bounds the plain. Sand- 

 grains can drift freely and swiftly over the dry 

 plain, but are checked at the margin of the moist 

 valley, and grow into dunes. Moist ground is 

 apparently the origin of the dunes near Ismailia, 

 on the west of Lake Timsah, and in the other 

 flat districts which I visited in the neighbourhood 

 of the Nile delta moist ground seemed to have 

 determined the position of the dunes. In deserts 

 where there are great variations of wind-exposure 

 the positions of dune-tracts are no doubt often 

 determined by shelter, but I have not visited these 

 localities. 



The influence of moist ground appeared to 



