36 



GEOLOGY. 



dry and sandy shores of lakes and seas, sandy valleys, and arid sandy 

 plains. 



Along coasts, dunes are likely to be extensively developed only 

 where the prevailing winds are on shore. Thus about Lake Michigan, 

 where the prevailing winds are from the west, dunes are abundant and 

 large on the east shore, and but few and small on the west. In shallow 

 water, shore currents and storm waves often build up a reef of sand a 

 Httle above the normal level of the water. When the waves subside, 

 the sand dries and the wind heaps it up into dunes. This sequence of 

 events is in progress at many points on the Atlantic Coast. Sandy 

 Hook, New Jersey, and the '^ beaches" farther south started as barrier 









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Fig. 19. — Migration of dune sand exposing bones in a cemetery. 

 Hatteras Island, N. C. (Collier Cobb.) 



ridges. When the waves had built them above normal water-level, the 

 wind re-worked the sand, piling it up into mounds and hillocks (Fig. 1, 

 PI. IT). Such dune belts a little off shore are sometimes turned to 

 good account. They are usually separated from the mainland by a 

 shallow lagoon. Where land is valuable, the lagoon is sometimes 

 filled in, making new land, thus anticipating the result which nature 

 would achieve more slowly. This has been done at some points on the 

 western coast of Europe. 



Dunes are likely to occur along stream valleys (Fig. 2, PL II), if 

 their bottoms or slopes are of sand, and not covered by vegetation. 

 Dunes along valleys are usually on the side toward which the prevailing 

 winds blow. Thus they are more common on the east side of the 

 Mississippi than on the west. Dunes may be formed in the valley 



