RELATION OF WIND TO TOPOGRAPHY 563 



mountain-like sand dune covered with vegetation. Very often such 

 finished dunes are again broken up by the wind (Fig. i). On these 

 rejuvenated dunes the usual series of development takes piace from 

 the very beginning, and they have often a peculiar character, resulting 

 from the remaining remnants of the old vegetation covering and the 

 new plant immigrants which have arrived after the complete or partial 

 destruction of the dune. 



It is a rule that the inner dunes in a complex are higher than those 

 nearer the beach because the horizontal directions of wind are de- 

 flected.^ 



Dunes do not travel extensively. Those in Brittany are said to 

 move 27 feet a year for 200 years. On the coast of Norfolk facing 

 the North Sea the dunes travel 150-80 feet a year, according to 

 Lapparent. I have not been able to find conclusive evidence regard- 

 ing a more rapid advance of dunes than 42 feet in one year, which is 

 the rate of advance of some dunes near Vera Cruz in Mexico. 



Dr. Baschin^ reports a mean drift of moving sand ridges on the 

 west coast of Fano, the northernmost of the islands of North Fries- 

 land, of about 10 feet a day. The author's explanation is that on a 

 large dune more material must be driven to the lee-side before any 

 displacement of the crest becomes evident, while the small sand 

 ridges on which his experiments were made moved more rapidly. 

 He also maintains that the slope on the leeward side is due simply 

 to the fall of the sand as the crest of the dune moves forward. I hold 

 with Bertoldy that the steep slope is due not only to the fall of the sand 

 but also to the effect of the vortex of air round a horizontal axis formed 

 on the lee-side of the dune. 



BIBLIOGRAPHY 



Andrews, E. W. "Pebbles and Drifting Sand," Trans. N. Z. Inst., Vol. XXVI, 



P- 397- 

 AuERBACH, Felix. "Die Gleichv/ichtsfiguren pulverformiger Massen," Ann. 



Physik, Leipzig ( 4 Folge), Vol. I, 1901, pp. 170-219. 

 Bolton, X. C. "Denudation," Trans. N. Y. Ac. Sci., Vol. IX, 1889-90, pp. 



110-26. 

 Bremontier, N. T. T. Recherches sur le mouvement des ondes, Paris, 1809. 



ipranz Czerny, "Die Wirkungen der Winde auf die Gestaltung der Erde," 

 Peternt. Geog. Mitt., 1876, Erganzh. 48, p. 27. 



2 Zeitschrijt der Ges. fur Erdkunde zu Berlin, No. 6, 1903. 



