WORK OF THE WIND 



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ridges, perpendicular to the direction of the wind. The waves of the 



sea have the same origin as these ridges, but the molecules of the 



water are not carried along by 



the wind as are the grains of 



sand, but after completing their 



orbits (p. 199), return to their 



original positions. 



Migration of Sand Dunes. — 

 Dunes are constantly migrating 

 unless the sand of which they are 

 composed is prevented from blow- 

 ing by grass or other vegetation 

 (Fig. 28). The forward move- 

 ment is accomplished by the force 

 of the wind, which shifts the sand 

 of the dunes to the leeward. 

 This can be seen on a windy day, 

 when the crests of the dunes ap- 

 pear to smoke. The rate at 

 which dunes move varies, de- 

 pending largely upon the velocity 

 of the wind and the height of the 

 dune, small dunes migrating the 

 faster. In Denmark the rate is 

 from three to twenty feet a year ; 

 in France, on the Bay of Biscay, 

 the sands have advanced at a rate 

 estimated from 15 to 105 feet a 

 year, burying in their progress 

 forests, farms, vineyards, villages, 

 and churches (Fig. 29). Some 

 of these, after being buried for 

 years, have been again uncovered 

 by the further advance of the 

 dunes. The church of Lege, 

 taken down at the end of the 

 seventeenth century and rebuilt 

 two and a half miles inland, had again to be removed 160 years 

 afterwards, showing an advance of the sands at a rate of 81 feet 

 a year. (Wheeler.) On the south side of Lake Michigan forests 



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Fig. 27. — Diagrams showing the form 

 of sand dunes. In A the wind blows from 

 the upper left-hand corner. The supply 

 of sand and strength of wind are moder- 

 ate. In B the direction of the wind is 

 from the upper right-hand corner. The 

 supply of sand is large and the winds 

 moderate. Under these conditions dunes 

 transverse to the wind are formed. When 

 the supply of sand (C) is large and the 

 winds strong, dune ridges parallel to the 

 direction of the wind are formed. 



