114 THE STUDY OF PLANT COMMUNITIES * Chapter V 



determined by the angle of rest of the sand. Because wind fre- 

 quently changes direction, dunes are rarely stable for long and 

 present a constantly shifting pattern. Along sea coasts they tend 

 to move inland as sand is carried from the windward side and 

 dropped down the lee side. 



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FlG. 59. Planted grasses and brush fences set up on shifting sand as part 

 of a dune-stabilization program developed by the Civilian Conservation 

 Corps. The attempt was partially successful but was not followed up with 

 later work, which would have added to its success.— Photo by C. R Korstian. 



A dune is never completely stable unless covered with a con- 

 tinuous mat of vegetation. Should this mat be broken for any rea- 

 son, a "blowout" results, which may enlarge and start again the 

 shifting of the entire dune. Many cottage owners have learned this 

 to their sorrow when— as has happened on Lake Michigan dunes— 

 they have returned after a single year to find their summer homes 

 almost completely buried under a shifting dune that had been stable 

 for years. The encroachment of dunes on forest areas is not un- 



