DESERT SAND-DUNES 61 



it was evident that the specimens on the sand- 

 bank were residual structures. Had I visited 

 Helwan earher in the season whilst this part of 

 the sandbank was still covered with loose sand, I 

 should no doubt have found it in regular trans- 

 verse ridges and furrows similar to those now 

 occupying the tract to windward and nearer the 

 present margin of the river. The ridges with crests 

 consisting of peaks separated by saddles are an 

 intermediate condition. I have described two 

 cases : first, that in which there was an accumula- 

 tion of sand on the lee side of each peak, and 

 second that in which the loose sand was swept away 

 under the lee of the peak, leaving an oval patch 

 of bare ground. In this latter case the tract pre- 

 sented the appearance of a somewhat undulating 

 plain pitted with holes. The dark-coloured ground 

 in the hollows was the most conspicuous feature in 

 the miniature landscape, but the peaks and steep 

 cliff on the weather side were also noticeable. To 

 the right and left of the peak there was a low 

 saddle, the walls of the pit being lower at the 

 sides than across the central longitudinal section. 

 Let us suppose the action of the wind in gouging 

 out the saddles to have continued until the bare 

 ground was reached. The sandbank would then 

 be covered by a number of crescentic mounds of 



