SAND DUNES AND SALT MARSHES 



slight increase in the rate of subsidence might 

 reverse this, and the sea would drown the 

 grass, and great inland bays of sea water 

 would replace the marshes. With a cessation 

 in subsidence, the estuaries would become 

 more contracted, and the fresh water vege- 

 tation would very gradually creep down upon 

 the marsh. A change to a movement of ele- 

 vation and the salt marshes would in the 

 course of time cease to exist. 



Accurate maps of the marshes made from 

 time to time, even in the brief space of the 

 white man's occupancy,— less than three cen- 

 turies,— would be of great value in showing 

 the changes that have taken place, but such 

 maps are lacking. In England, however, his- 

 tory goes farther back, and there is plenty 

 of evidence that the marsh lands of East An- 

 glia, the region of the Norfolk Broads, which 

 corresponds in many ways with this Ipswich 

 region, have become less and less invaded by 

 the sea, notwithstanding the subsidence which 

 is going on there as here. 



When the Romans devastated the country, 

 they sailed far up the great tidal estuaries, 

 which are now narrow sluggish streams mean- 

 dering through meadows of fresh grass, and 



220 



