above sea level during middle and late Pliocene time and earliest (Nebraskan stage) 
Pleistocene time. They may have been very slightly tilted toward the west at the time 
of their emergence. The Fort Thompson formation (including the Coffee Mill Hammock 
marl member at the top) consists of three thin marine shell beds separated from one 
another by two freshwater limestones or marls, each of the younger beds filling 
solution holes in the older. The total thickness at the type locality is about 8 ft. The 
marine beds are interpreted as deposits formed during Aftonian, Yarmouth, and 
Sangamon interglacial stages, when the region was flooded by the sea to depths 
apparently as great as 270, 215, and 100 ft. The solution holes and the freshwater 
limestones and marls apparently were formed during the Kansan and lllinoian glacial 
stages, when the sea temporarily withdrew to considerable distances below its present 
level. The Anastasia formation (predominantly sand and shells), the Key Largo 
limestone (an extinct coral reef), and the Miami oolite are contemporaneous Pleistocene 
formations which apparently accumulated on and along the southeastern coast mainly 
during the Sangamon interglacial stage and therefore are equivalent to only part of the 
Fort Thompson formation developed in the Everglades and the Caloosahatchee River 
area. The Penholoway and Talbot formations, which are coastal terrace deposits, 
consist of sand swept down from the north by longshore currents during the middle and 
late parts of the same interglacial stage. A thin shift of sand, the Pamlico formation, 
was spread over part of the shallow sea floor during a mid-Wisconsin invasion by the 
sea. The accumulation of these Pleistocene deposits to various thicknesses on the 
nearly level Pliocene surface formed a very shallow basin (the Lake Okeechobee 
Everglades depression) adjacent to highlands on the north and partly enclosed by a low 
coastal ridge of oolite on the east and a slightly higher plain on the west. The northern 
and lowest part of this basin is now occupied by Lake Okeechobee, which, before 
drainage and diking operations changed it, overflowed southward across the open 
Everglades more or less as a sheet flow that imposed an aligned drainage pattern on the 
organic deposits of the Everglades. Tests made in ground water investigations of the 
Miami area indicate that the Tamiami formation is among the most productive water 
bearing formations ever investigated by the US Geological Survey. Its coefficient of 
permeability is about 35,000, which indicates that through a section of the formation a 
mile wide and a foot thick 35,000 gallons of water a day, at 60°F, would pass through 
under a hydraulic gradient of one foot. Large areas of salty ground water in the 
northern part of the Everglades are considered to be remnants of seawater left during 
Pleistocene sea invasions and now altered by dilution with freshwater and by chemical 
reactions, mainly of the base-exchange variety, with the enclosing rocks. 
1945 - 1959, 1991 
Strong, A. M., and G. T. Bancroft (1994) Patterns of deforestation and fragmentation of 
mangrove and decidous seasonal forests in the upper Florida Keys. Bull. Mar. ScL 
54(3):795-804. 
The forested ecosystems of the Florida Keys contain a tropical flora with many species 
found nowhere else in the conterminous US. These forests have gone through three 
periods of anthropogenic perturbations resulting in forests that are smaller, more 
fragmented, and have an altered species composition. We digitized 1991 aerial 
photographs of the remaining mangrove and deciduous seasonal forests of the Upper 
Keys (Ragged Keys to Long Key) to determine the changes in forest coverage from the 
original condition [as determined from 1945 - 1959 aerial photographs]. Forty-one 
percent of the original 4,816 ha of deciduous seasonal forests and 15% of the original 
8,306 ha of mangrove forests have been cleared for development. Losses were 
greatest on those keys accessible from U.S. 1, intermediate on keys accessible from 
Rt. 905, and least on keys accessible only by boat. Mean forest size decreased from 
50.7 ha to 2.6 ha and from 67.5 ha to 28.1 ha for deciduous seasonal and mangrove 
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