ACTION OF WIND-BLOWN SANDS 1 93 



lying shallows. So far as depends upon the action of the 

 waves the coastal sand islands cannot rise more than a few 

 feet above the level of low tide ; but as soon as a beach is 

 formed, the winds operate to form sand heaps or dunes above 

 the level of the sea, which may considerably increase its 

 elevation above the ocean level. 



The wind-blown sands of these wave-built islands play 

 an important part in the history of our sandy shores. Even 

 before the barrier ridge has attained to a sufificient height 

 above the tide to give a foothold for land vegetation, the 

 formation of these dunes begins. When the tide is out a 

 broad section of the beach is left bare, the superficial sands 

 of which in an hour or two become dry enough to be blown 

 in-shore whenever the winds come strongly against the coast, 

 and moving up the slope of the beach they form a ridge 

 parallel to the coast-line. As long as the sand is on the slope 

 of the dune toward the sea, it is exposed to the wind and is 

 likely to be kept in motion. When it passes the crest of the 

 elevation it drops into a lee and comes to rest ; at least until 

 the wind changes direction so as to again obtain possession 

 of it. As soon as the land vegetation fitted to orrow in such 

 places obtains a foothold on the island, the growth of the 

 dunes is favored by the fact that the sand catches amid the 

 stems and leaves and becomes tied together by the roots, so 

 that the occasional storms from the land, in most cases less 

 energetic than those from the sea, cannot blow it back into 

 the ocean. Under these conditions the wind-blown materials 

 may accumulate in sharp ridges, which attain a height of 

 from ten to a hundred feet or more. 



Although the species of plants, such as our beach grass 

 of northern dunes or the dwarf palmettos and other plants of 



