THE PHYSIOGRAPHY OF SALT MARSHES 



by 



J. A. Steers 

 University of Cambridge 



The physiographer can consider salt marshes from at least two points of 

 view:- (1) the marshland of a given coast in its entirety, and (Z) the detailed 

 features of any salt marsh. This distinction applies not only to the ordinary 

 salt marshes of temperate climates, but also to tropical mangrove swamps. 



There can be few better places to study those problems than the eastern 

 coast of the United States. The marshes in the Bay of Fundy, New England, 

 the Atlantic Coastal Plain (using D. W. Johnson's divisions), and the tropical 

 swamps of Southern Florida present an unrivalled series. 



These marshes add a fringe to the land, and so form a feature of some 

 magnitude. The deep red mud and often bare marshes of Fundy contrast with 

 the predominantly grass marshes farther south, which in their turn are divis- 

 ible into those formed nnainly of mud and silt and those made of salt peat with 

 variable amounts of silt. This difference is related to the geology of the areas 

 in which they occur. New England is a heavily glaciated district of resistant 

 rock. Streams gather less load from such a surface, and much of it is depos- 

 ited in lakes and swamps. In the coastal plain marshes to the south, on the 

 other hand, there is a greater amount of sediment, much of which is derived 

 from rivers which flow over an area of relatively weak rock. Because of the 

 lesser number of lakes and swamps through which they pass, the rivers do not 

 lose their load as they do in New England, but retain it until they reach the 

 ocean. The abundant mud in the Fundy marshes, although the region was heavily 

 glaciated, is derived from the soft red sandstones and shales, and is reworked 

 and pushed higher up the bays and estuaries by marine agencies. 



The relation of salt marshes to vertical movement of the shoreline is 

 complicated. In studying the vertical movement in marshlands, allowance must 

 be made for the full tidal range, and possibly a little more, so as to include the 

 effects of storms. If, in any marsh, the thickness of deposit does not exceed 

 that of the extreme tidal range, submergence cannot be assumed. If the thick- 

 ness is appreciably greater, then submergence is probable. But compression 

 of marsh deposits by the overrolling of a shingle bar commonly occurs and may 

 cause a fictitious appearance of submergence, which can also be simulated in 

 other ways. A rapid submergence may give a "drowned" appearance to a marsh. 

 The effects of both submergence and emergence on the vegetation of the marsh - 

 solid-land boundary must be fully considered. Submergence and the ecological 

 history of the marshland are also closely related. If a bore or cut in a marsh 

 shows, for example, brackish water or high-marsh plant remains underlying 

 those peculiar to low- or mid-marsh levels, submergence must be assumed, 

 except for a quite local occurrence which might possibly be explained by com- 



