26 GENESIS AND DEVELOPMENT 



The drifting of sand bj' ■wind takes place according to following 

 principles: If the force of the wind is great, the grains do not move 

 on the surface but are lifted by the wind to a certain height. The larger 

 grains make jiimps, and touch the ground from time to time, while the 

 smaller grains often are carried forward in form of clouds at a con- 

 siderable height from the ground. At a velocity of 4.5 m. in the second, 

 grains of 0.25 mm. diameter slide on the ground, but at a velocity of 

 15 m., grains of 1 mm. diameter are lifted high in the air. As a 

 corollary of this fact it follows that the movement of the grains depends 

 on their volume. The greater part of the sand grains have an irregular 

 flat form, and hence their movement is not rolling but sliding. Thai 

 of the largest take place spasmodically and only during stronger gusts. 

 According to SolcoJoff, a wind of a velocity of 10 — 12 m. in a second 

 cannot carry grain.s of 100 — 150 cubic mm. When the wind is not too 

 strong, the grains slide along the surface, but when they are lifted up 

 during the strong gusts, and fall down at a certain angle, they again 

 rebound at the same angle. Magen has proved that coarser sand grains 

 are sometimes lifted up to 2 m. height, and in such a case they are 

 carried up to 12 m. from their original place. 



Eidges or rim formed ripples advance almost entirely b}' the sliding 

 of the larger grains of the top layer of the crest, and Cornish estimates 

 the progress of the ridges at one foot per hour. Helmann found in 

 Chiwa that the ripples on the lee-side of the dunes move almost with 

 the same rapidity as on the windward side, and he was not able to 

 interpret this phenomenon. It has been ascertained that the movement 

 of the dunes landward goes on at the average rate of 4.30 m. a year, 

 and that the quantity of sand thus transported is about 75 cubic m. to 

 the running m. of the length of the dune. 



The winds have a greater power at a higher elevation than near the 

 surface, and consequently more sand is removed from the summit than 

 the wind is able to Lift from the ground. This difEerence in the strength 

 of wind exercises a modifying influence on the development of the 

 dunes. The effect of the wind is to diminish the maximum slope, but as 

 the formation of dunes is mainly regulated by the supply of sand, the 

 varying angle of the windward slope depends upon the varying density 

 of the sand shower pushed forth toward the summit. In cases where 

 the supply of sand has become scarce or exhausted, the front slope 

 of the dune soon will be almost as steep" as the lee side, that is, approach 

 the natural limit of the angle of rest. 



The leeward slope of the dune varies but slightly, provided a reverse 



