388 FIRST YEAR SCIENCE 



barrier to the wind and thus causes its own further growth. 

 In great deserts where the wind is generally from one 

 direction these sand dunes sometimes grow to a height of 

 several hundred feet, but usually they are not more than 

 20 or 30 feet high. 



They generally have a gentle slope on the windward 

 side and a steep slope on the leeward side. The sand is 



A FOREST ON CAPE COD. 

 The trees are being engulfed in wind-blown sand. 



continually being swept up the windward side over the 

 crest, thus causing the dune to move forward in the direc- 

 tion in which the prevailing wind blows. (^Fig. 117.) 



Dunes make travel difficult, as both in climbing and 

 descending the traveler sinks into the yielding sand. Al- 

 most 'no plant life can find lodgment in these shifting 

 sand piles, so the wind continually finds loose sand on 

 which to act, and a dune country is always a region of 

 shifting sands. As the dunes move in the direction of 

 the prevailing wind they sometimes invade a fertile coun- 



