levels within the enclosed areas. The natural levee barriers restricted movement of ground water 

 as well as surface runoff. 



Anaerobic conditions between the ridges were favorable for preservation of organic matter and 

 formation of peat deposits. Peat and organic soils were able to accumulate vertically because of 

 natural subsidence. Radiocarbon dates of buried peat deposits indicate that over the past 1,000 

 years this area has subsided approximately 0.18 cm (0.060 ft) per year (Gagliano et al. 1979). 

 Auger borings taken by the authors demonstrate that peat accumulations in the area range up to 

 3 m in thickness. 



Geologists have long been interested in Mississippi Delta peat deposits because they provide 

 modern analogs for ancient coal beds and because they are important in dating and unraveling the 

 late Quaternary geological history of the deltaic plain. Following the 1958 lead of the master 

 deltaic geologist, Harold N. Fisk, a number of important papers concerning the distribution and 

 character of peats and other organic-rich wetland deposits have been published. Among the most 

 relevant of these are papers by Frazier and Osanik (1969) and Coleman (1966). Building on these 

 earlier studies, Kosters et al. (1987) concluded that "true peat and organic-rich beds in the 

 Mississippi Delta are of freshwater forested swamp or herbaceous marsh (floating) origin. Saline 

 marshes produce only organic-poor sediments." 



A considerable part of the total study area was occupied by trembling prairie or floating marshes 

 in the 19th and early 20th centuries. In his study of the marshes of Louisiana, O'Neil (1949) 

 indicated that there were approximately 101,000 ha of floating marshes, and classified a large part 

 of the present study area as floating marsh. Floating marshes are made up of thick (up to 1 m) 

 mats of cane-like root stalks and other plant materials that float. The mats support a diverse flora 

 of emergent freshwater species dominated by Panicum hemitomon (paille fine or canouche). 



Two different explanations have been offered for the origin of the mats. Russell (1942) 

 suggested that they formed by floating aquatic vegetation extending out into lakes from the 

 shoreline. O'Neil (1949) noted that small mats may form in this way, but he thought that the 

 extensive mats formed from marshes that were originally anchored into the underlying clay pan 

 (mineral sediment substrate). He reasoned that as subsidence lowered the clay pan, the buoyant 

 nature of the mats caused them to eventually break away and adjust to fluctuating water levels. 

 Geological data favor O'Neil's hypothesis (see, for example, Fisk 1958; Frazier and Osanik 1969). 



The natural levee ridges and the mineral substrate between them formed large, shallow vessels 

 that contained the peat and organic muck deposits. The soft, poorly consolidated, supersaturated 

 organic deposits and soils were literally held in place by the natural levee ridge, analogous to a pie- 

 shell configuration. The organic sediments made up the soft filling of the pie and the mineral 

 deposits formed the shell which held them in place. Within this shell there was considerable 

 variation in the consistency of organic soils, ranging from firm, woody, and grassy peats to organic 

 ooze lying below floating marsh mats. 



The authors conclude that the skeletal framework and hydrologic effects of natural levee ridges 

 and lake rims are essential components for the formation of peat and organic deposits and for 

 maintaining the types of vegetation that sustain them. These ridges are essential to the formation 

 of floating marshes. 



Some researchers consider tidal introduction of sediment to be necessary for marsh accretion 

 to occur at a high enough rate to keep up with subsidence (Mendelssohn et al. 1983; Gosselink 



36 



