SAND DUNES AND SALT MARSHES 



by the rush of the brook to the sea. In the 

 estuary the steep side of the ripple-mark is 

 up-stream on the flood and down-stream on 

 the ebb tide. In the bed of both water and 

 wind stream the grains of sand are pushed 

 along in parallel ridges up a gradual slope 

 until they drop over and come to rest on the 

 steep sheltered side. 



In a gentle wind the ripple-marks advance 

 so slowly that one is unconscious of any 

 change, but in stronger blasts the changes are 

 very manifest. On a blustering March day 

 with a keen wind from the northwest I 

 watched some ripple-marks that were four 

 inches apart from crest to crest, and found 

 that they were advancing at the rate of a foot 

 in eight and a half minutes. 



The most common form of dune at Ipswich 

 is one whose longest axis runs from east to 

 west across the prevailing winds, and these 

 again may be divided into two classes. Both 

 advance to the south like waves before the 

 boreal blasts, but the commoner, unlike the 

 water wave, presents its crest to the storm 

 and retreats backward. The sharp, steep side 

 of the dune is undercut and worn away by the 

 wind, and streams out on the sweeping slope 



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