SAND DUNES 



283 



and then carried away again, owing to a change in the 

 condition of the river's load, so forests and houses in sandy 

 regions are sometimes buried, to be uncovered again 

 perhaps by a change in the load carried by the wind. 



Sand Dunes. Sand-laden wind generally deposits its 

 burden in mounds and ridges called sand dunes (page 258). 



A FOREST ON CAPE COD, MASSACHUSETTS, BEING BURIED IN 

 WIND-BLOWN SAND 



When once a deposition pile begins, it acts as a 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 

 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. (Figure 92.) 



