components to the observed sea level 

 change varies from place to place, but 

 Redfield {1S67) and Emery and Uchupi 

 (1972) were able to use l^C-dated peat 

 from salt marshes along the Atlantic 

 and Gulf of Mexico coasts to arrive at 

 an estimate of about 0.8 mm/yr for the 

 eustatic rise in sea level during the 

 past 4,0C0 years (Figure 3). Thus, 

 most of the change in sea level 

 observed in the Northeastern United 

 States during the last 2,000 to 3,000 

 years (Table 1) appears to be due to 

 an absolute increase in the level of 

 the sea rather than to land 

 subsidence. 



Aggradation and Accretion--the Shaler 

 Model 



THOUSANDS 

 5 4 3 



OF YEARS AGO 



2 1 



About 

 offered his 

 salt-marsh 

 observed, N. 

 a model for 



30 years after Kudge 

 explanation for the thick 

 peat accumulation he 

 ,S. Shaler (1886) developed 

 marsh formation based on a 

 different set of observations. Shaler 

 emphasized the gradual accumulation of 

 sediments in shallow coastal waters, 

 particularly where seagrasses might 

 accelerate the depositional process. 

 As the water became shallower, the 

 seagrass beds would be replaced by mud 

 flats which would, in turn, be 

 colonized by Spartina alterniflora , 

 the only grass to survive in the low 

 intertidal zone. The presence of the 

 grass would further enhance sediment 

 deposition, and the roots and rhizomes 

 would contribute to peat formation. 

 This process would continue until the 

 sediment accumulated almost to the 

 limit of the high tide. By this 

 process, the marsh would build up and 

 out from the shore as sediments were 

 redistributed along the coast. 



In his classic description and 

 analysis of the New England coastline, 

 Johnson (1925) discussed the problem 

 of salt marsh formation in some 

 detail, and established "criteria 

 for testing the Mudge and Shaler 

 Theories." According to Kudge, 

 sections through the marsh should show 



6 



Figure 3. Age and depth of salt-marsh 

 peat in different areas: A (Cape Hat- 

 teras to Mexico plus Bermuda), B (Cape 

 Cod to Cape Hatteras), C (eastern 

 Massachusetts), D (Bay of Fundy). In 

 area A, the last 3,500 years are 

 assumed to reflect the eustatic rise 

 in sea level, so that the deviation of 

 each of the other curves from A is due 

 to local land subsidence. For example, 

 in area C local subsidence continued 

 until about 2,500 years ago at a rate 

 of 0.3 mm/yr. When combined with the 

 eustatic rise of 0.8 mm/yr, the result 

 is the 1.1 mm/yr of relative rise 

 shown (Redfield 1967, as modified by 

 Emery and Uchupi 1972). 



