CHAPTER 2. PHYSICAL ENVIRONMENT 



2.1 PROTECTIVE BARRIERS AND SEDIMENTS 



Salt marshes require muddy or sandy 

 sediments in areas which receive tidal 

 flushing, but which are protected from the 

 full force of breaking waves. Although 

 old and compact marsh peat is somewhat 

 resistant to erosion by wave action and is 

 sometimes seen on beaches where old marsh 

 sediments are exposed by sand movements, 

 the formation of a marsh requires a 

 quieter environment for the accumulation 

 of sediment and growth of marsh plants. 

 Small marshes protected by rocky outcrops 

 or headlands can be found in Maine and the 

 Canadian Maritimes. By far, the majority 

 of salt marshes are protected by sand 

 structures, e.g., barrier beaches and 

 islands and spits. Redfield (1972) 

 illustrated vividly how the Great 

 Barnstable Marsh grew through historical 

 time and how sediments and peat 

 accumulated as sea level rose (Figure 2). 

 The barrier beach that protects the marsh 

 grew out from one edge of an indentation 

 in the coast, converting it into a 

 protected bay now filled with marsh. The 

 close connection between barrier formation 

 and marsh existence is further shown by 

 the response of Georgia marshes to changes 

 in their protective barriers over the 

 recent geological past (Pomeroy and 

 Wiegert 1981). 



Growth of sandy barriers since the 

 retreat of continental glaciers, coupled 

 with changes in sea level, has created 

 large areas in which marshes have formed 

 over most of the eastern United States. 

 Sea level has been rising between 1 and 3 

 mm yr- 1 over the past few thousand years. 

 (A detailed description of long- and 

 short-term changes in sea level and their 

 causes can be found in Nixon 1982.) The 

 marsh level keeps pace with sea level rise 

 through both the accumulation of sediment 

 and, to a lesser extent, the accumulation 



of organic matter. The sediment is trans- 

 ported to the marsh by rivers and coastal 

 circulation which brings marine sediment 

 into estuaries and sheltered embayments. 

 Water movement slows in the protected 

 areas; the flow has less capacity to carry 

 particles which then settle to the bottom. 

 Thus, the basin becomes progressively 

 shallower until it can be colonized by 

 marsh plants. Plant stems further impede 

 flow and concentrate sediment accumulation 

 along the edge of the marsh. This process 

 leads to higher elevations (or levees) 

 along the marsh face and the edges of 

 tidal creeks. Such levees are quite 

 evident in Georgia marshes. 



Not only do marshes expand into the 

 estuary or bay as a result of sediment 

 accumulation, but they also extend into 

 the adjacent land as the sea level rises. 

 The central portion of the marsh generally 

 keeps pace with sea-level rise. Thus, the 

 parts of Barnstable Marsh with the deepest 

 peat are some distance from the present 

 landward edge. All of the area between 

 the deepest peats and the present upland 

 represent former land now buried beneath 

 salt marsh. In many cases, a barrier 

 built at the back end of the marsh 

 prevents marsh growth over the upland. A 

 railroad line forms such a barrier in 

 Great Sippewissett Salt Marsh. A similar 

 situation exists for almost every salt 

 marsh in an urban setting. In most of 

 these cases, as sea level rises, the marsh 

 cannot extend over the upland. Since the 

 barrier beach does move inland with the 

 rising water, the marsh, if it has already 

 reached the inland barrier, gets 

 progressively smaller. 



In a creek bank in Barnstable Marsh, 

 Redfield found a cavity which held a cache 

 of cobbles. He interpreted this as the 

 remains of a small boat ballasted with 

 small stones that had been abandoned on 



