SAND DUNES AND SALT MARSHES j 



in the last sixty years. A sMft in the currents 

 may in a short time wash away this work oi 

 years, and may even enlarge some of tht 

 creeks, but I am inclined to think that the 

 general tendency is toward a contractior 

 of the tidal estuaries by the enterprising 

 thatch. 



The third zone, that of the marsh hay, whicl 

 constitutes the greater part of the broac 

 marsh, is in the same way able to keep its 

 level in relation to the water by the gradua 

 deposition during high tides of fine mud anc 

 sand, and by the climbing up of the grass oi 

 the shoulders of its dead ancestors. In tht 

 swirl of the tides the sand and mud are con- 

 stantly carried up from the sea, and whih 

 the sand holds its place in tolerably swif1 

 water, the mud comes to rest only in regions 

 of comparative calm. Both are dropped it 

 periods of quiet water between tides. Tht 

 fresh water detritus brought down by tht 

 rivers here is so small in amount that it playj 

 but an insignificant part in the building uj 

 of the marshes. 



In calm weather a surprising amount of sane 

 is borne along by the rising tide, floating or 

 the surface in the same way that a needle car 



218 



