SAND DUNES AND SALT MARSHES 



in the last sixty years. A shift in the currents 

 may in a short time wash away this work of 

 years, and may even enlarge some of the 

 creeks, but I am inclined to think that the 

 general tendency is toward a contraction 

 of the tidal estuaries by the enterprising 

 thatch. 



The third zone, that of the marsh hay, which 

 constitutes the greater part of the broad 

 marsh, is in the same way able to keep its 

 level in relation to the water by the gradual 

 deposition during high tides of fine mud and 

 sand, and by the climbing up of the grass on 

 the shoulders of its dead ancestors. In the 

 swirl of the tides the sand and mud are con- 

 stantly carried up from the sea, and while 

 the sand holds its place in tolerably swift 

 water, the mud comes to rest only in regions 

 of comparative calm. Both are dropped in 

 periods of quiet water between tides. The 

 fresh water detritus brought down by the 

 rivers here is so small in amount that it plays 

 but an insignificant part in the building up 

 of the marshes. 



In calm weather a surprising amount of sand 

 is borne along by the rising tide, floating on 

 the surface in the same way that a needle can 



218 



