WORK OF THE WIND 



Si 



ing grasses, shrubs, and trees on the gentle slopes in order that they 

 may hold the sand with their roots. This has been done successfully 

 in San Francisco, in Provincetown (Massachusetts), and elsewhere. 



Beneficial Effect of Dunes. — Dunes are not, however, always a 

 detriment to man. A writer states that the people of Holland and 

 Denmark " deal as carefully with their dunes as if dealing with eggs, 

 and talk of their fringe of sand hills as if it were a border set with 

 pearls. They regard these as their best defense against the sea." 

 (Kahl.) As this implies, Holland depends to a large degree for its 



SCALE OF MILES 



Fig. 30. — Map of the Grand Calumet River. The river formerly entered Lake 

 Michigan at the east, but was cut off by sand dunes and now enters the lake at Chicago. 



protection from the sea upon sand dunes, which are from one to three 

 miles wide and from 40 to 50 feet high. The sand of certain dunes in 

 England (Padstow), which consists largely of shell fragments, is used 

 to some extent for a fertilizer. 



Material of Dunes. — The material of sand dunes varies but is 

 usually quartz sand. However, in the Bermudas, Bahamas, and por- 

 tions of England, dunes are composed of shell sand (CaCOs). In the 

 Bermudas chese sands are cemented by the rain water which dissolves 

 the calcium carbonate and later redeposits it, thus forming stratified 

 eolian rock. When the shallow, alkaline lakes of portions of New 

 Mexico (Otero Basin) dry up, they leave on their beds thin sheets 

 of various salts, chiefly gypsum. These soon curl up into leaves which, 

 when blown together, are broken into gypsum and salt sands. The 

 winds carry the light gypsum out to the plains, where it gathers in a 



