22 PHYSIOGRAPHY 



common. Fig. 13, and Pis. V- VII, pp. 10, 11 and 12 show large 

 and small dunes. 



Distribution of dunes. Dunes are found mostly near the sources 

 of abundant dry sand. They are common along much of the Atlan- 

 tic coast of the United States, where the sand is washed up on the 

 beach by the waves. After drying, it becomes the prey of the 

 wind. Winds from the west blow this sand into the sea; those 

 from other directions, but especially from the east, drift it up onto 



Fig. 13. The beginning of a dune. Sand has lodged about the tufts of 

 vegetation. Shore of Lake Michigan, south of Chicago. 



the land. Dunes abound along the eastern side of Lake Michigan, 

 and some of them are very large; but there are few on the west 

 shore. This is because the prevailing winds are from the west. 

 Where westerly winds prevail, as in most of the United States, most 

 of the dunes along valleys are on their east sides. Dunes abound 

 over thousands of square miles of land in the semi-arid parts of the 

 Great Plains, as in western Nebraska, western Kansas, and some 

 parts of Wyoming, and they reach their greatest development in 

 still drier regions, such as the Sahara. In some places, dunes are 

 the most conspicuous feature of the landscape. 



Destructiveness of wind-blown sand. The piling up of sand 

 into dunes sometimes does great damage. Farm lands, especially 

 near sea coasts, have been covered in this way, and forests of large 

 trees have been buried (Fig. 1, PI. V, p. 10). Sometimes the sand buries 

 buildings, and occasionally it causes much trouble along railways, 



