date for the Pleistocene epoch in 

 Florida (Cooke 1945). Subsequent 

 oscillations of sea level in re- 

 sponse to glacial formation and 

 melting are primary factors in 

 determining Pleistocene rock forma- 

 tions underneath the Everglades, 

 Taylor Slough, and Florida Bay. 

 Much of the area is underlain by 

 marine sedimentary sequences punctu- 

 ated by fresh water limestones and 

 subaerial exposure surfaces (Perkins 

 1977). The wedge of overlying 



Pleistocene sediment, which attains 

 a thickness of approximately 61 

 meters (200 ft) in the lower Keys, 

 pinches out northward against topo- 

 graphically higher Miocene and 

 Pliocene sediments, such as the 

 Tamiami and Caloosahatchee Forma- 

 tions mentioned earlier (Perkins 

 1977). 



Sea level prior to the initial 

 Pleistocene glacial melt lay at 

 approximately 82.3 meters (270 ft) 

 above the present shore line. Dry 

 land on the Floridan Plateau was 

 restricted to a few small islands 

 lying in what is now Polk County, 

 and another group in the vicinity of 

 the Trail Ridge area near Jackson- 

 ville. Subsequent sea level fluctu- 

 ations gradually left more and more 

 of the Floridan Plateau exposed. As 

 Cooke (1945) explains, this succes- 

 sive dampening of sea level rise is 

 probably the result of sea floor 

 spreading which concurrently in- 

 creased the global volume of the 

 oceans. Table 10 lists the names 

 of recognizable sea level fluctu- 

 ations of the Pleistocene in Florida 

 and the respective heights to which 

 they extended above present day sea 

 level. 



The most ancient of the Pleis- 

 tocene rock layers in south Florida 

 is the Fort Thompson Formation. 

 Sellards (1919) first applied this 



Silver Bluff (tentative) 



Table 10. Recognized sea level fluc- 

 tuations of the Pleistocene 

 in Florida (adapted from 

 Cooke 1945). 



name to the formation, which con- 

 sists of beds of fresh water marl 

 and limestone alternating with beds 

 of marine shell marl, in the vicin- 

 ity of Fort Thompson on the Caloosa- 

 hatchee River (Cooke 1945). The 

 Fort Thompson includes three sepa- 

 rate and distinct marine shell beds, 

 the youngest of which is the Coffee 

 Mill Hammock marl. The three marine 

 beds are separated by two thin beds 

 of soft marl containing shells of 

 fresh water snails. The tops of the 

 fresh water beds have been hardened 

 into brittle limestone, but are per- 

 forated by solution holes which are 

 filled with marine shells from suc- 

 ceeding strata. The Fort Thompson 

 Formation is of special importance 

 to the human population of the 

 southeast coast for it forms part of 

 what is commonly referred to as the 

 Biscayne Aquifer, the sole drinking 

 water source for much of the south- 

 east coast. Figure 22 from Dubar 

 (1974) shows the Fort Thompson For- 

 mation in relation to more recent 

 Pleistocene strata in south Florida. 

 Toward the east coast the formation 

 is relatively thicker than toward 

 the west coast, where it thins out 

 about half way across the peninsula. 



51 



