ago, a dramatic change in deposition began that transformed the regime of the 
calcareous platform into one of siliceous sands and clays. This geologic revolution was 
the result of the southward spread of siliceous deposits derived from the Appalachian 
Mountains and distributed by rivers and movement along shorelines. The template, fully 
formed when the Gulf Stream-Florida Current system developed some 10 to 15 Ma, had 
shaped the southeast margin of the siliceous deposits into the gentle curve that is now 
surmounted by the Florida Keys and offshore reefs. The return of the calcareous 
deposit regime that would form the Florida Keys, the mainland, and surrounding shallow 
sea floors came with the onset of glaciation in the Northern Hemisphere. During a 
highstand of sea level about 125 ka, coral reefs and their associated calcareous 
deposits that are now the Upper and Middle Florida Keys accumulated preferentially 
along the southeast edge of the previously deposited siliceous sediments. At about the 
same time, an arc of calcareous sand banks and tidal channels grew from Ft. Lauderdale 
to Homestead and Flamingo and formed the present southernmost Atlantic Coastal Ridge. 
Farther south, similar sand banks accumulated and now form the low-relief limestone 
islands from Big Pine Key to Key West and the Marquesas. The topography of reefs and 
sand shoals was fossilized by cementation that occurred when they were above sea 
level for some 110,000 yrs. Then, a rising sea progressively began to flood the 
limestone landscape at about 8 ka. A new rim of reefs and shoals formed seaward of the 
Keys, and on the western, protected side of the Upper Keys, shallow Florida Bay and 
Barnes and Card Sounds were born. The major physiographic and hydrographic 
environments - the Florida reef tract, Biscayne Bay, and Florida Bay—are the large- 
scale template inheritances, but there are smaller scale ones as well. For example, two 
of the major subdivisions of the modern reef tract, the areas seaward of the Upper and 
Middle Keys, are determined by the degree of continuity of the islands formed 125 ka. 
Where an island is linear such as Key Largo, the offshore reef tract is shielded from the 
inhospitable waters of Florida Bay, and reef communities flourish. Where islands are 
discontinuous as in the Middle Keys, tidal exchange between the Gulf of Mexico and 
Atlantic Ocean produces a mobile sand substrate unfavorable for the establishment of 
corals and associated reef builders. In addition, the inter-island channels allow 
discharges of inhospitable waters from shallow, restricted Florida Bay to reach the 
reef tract, which further deter development of reefs offshore. These factors, the 
mobile sand substrate from cross-shelf tidal currents and the inimical waters from 
Biscayne Bay, may explain the absence of significant reef development seaward of the 
Safety Valve south of Key Biscayne. 
1994 0 
Gorte, R. W. (1994) The Florida Bay economy and changing environmental conditions. CRS 
Report for Congress. 94-435 ENR. Congressional Reserch Service, The Library of 
Congress. 19 pp. 
[DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] Florida Bay is a large, triangular 
coastal lagoon located at the southern tip of Florida, between Everglades National Park 
and the Florida Keys. Substantial changes in the vegetation of this shallow saltwater 
bay have occurred within the past decade. The mangroves that ring the mudflat islands 
are dying. The seagrass that carpets much of the Bay began dying in 1987, and the 
dieoff now affects nearly a quarter of the Bay. This seagrass dieoff is linked to blooms 
of blue-green algae that are, in turn, linked to a sponge dieoff. Finally, diatom blooms 
have become increasingly common in the western portions of the Bay since 1979. Many 
are concerned that these changes in the vegetation of Florida Bay are affecting the 
resident fauna. Florida Bay contains resident and transient populations of bottlenose 
dolphins and provides habitat for the endangered manatee, several endangered sea 
turtle species, and along the north shore, the endangered American crocodile. The Bay 
provides feeding and nesting habitat for many bird species, but the number of ospreys 
and of wading bird colonies has declined. The Bay also provides nursery grounds for 
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