VEGETATION OF THE STRAND. 597 



ning in the movement of the dune. 'As the top of the windward 

 side is first blown off, this lowers the gradient on the windward 

 side, until finally it becomes low, having about 5 per cent gradi- 

 ent, while the gradient on the leeward side is that which fine 

 sand, where not disturbed by wind or wave action, takes under 

 the influence of gravity and is about a 30 per cent gradient. 

 These wandering dunes are often very complex; since several 

 dunes may come together, troughs may be ploughed through the 

 dune by the wind, forming gullies, in which case plants are un- 

 covered. Then smaller dunes behind larger ones may be pro- 

 tected from the prevailing winds which come from the water. 

 These may move in any direction by lesser winds. 



1090. Effect of wandering dunes on the landward flora. 

 These wandering dunes often produce very great changes in the 



Fig. 522- 



Encroachment of a dune on old and long-established oak-dune at Dune Park. 

 Dead oak-trees at the margin. (After Cowles.) 



landward flora, since they encroach upon the plant societies, 

 and high ones often bury the vegetation, or the mere branches 

 of tree-tops may be projecting through. In many cases this 

 kills the trees, and as the dune moves onward the dead trunks, 



