The Everglades

The Everglades is one of the lowest, youngest and most geologically stable parts of the North American continent. South Florida's geology, coupled with a warm, wet subtropical climate, provided conditions for the development of a huge, freshwater marshland ecosystem. The geology of south Florida's Everglades is a result of marine and freshwater processes that have alternated with the rise and fall of sea level. At high sea level, limestone was deposited and beaches and dunes were created. The Atlantic Coastal Ridge, for example, resulted from marine deposition that occurred during an interglacial age when sea level was as much as 25 ft above the present level. At low sea level, the limestone was dissolved and eroded by drainage of acidic freshwater to create the riddled solution features that are characteristic of the region.

With the recession of glaciers in northern North America at the end of the Pleistocene Epoch, sea level rise began and has continued to the present day. The rising sea level retarded runoff and downward leakage in south Florida and, with abundant rainfall, helped establish the broad expansion of wetlands in the region. The rise in sea level slowed about 3,200 years ago, and this slow rise favored the expansion of coastal and freshwater wetlands. The Atlantic Coastal Ridge helped retain freshwater in the Everglades Basin and this allowed thick layers of peat to accumulate within the northern parts of the Basin. Parts of the present-day Everglades area had become short-term flooded calcitic mud marshes by about 6,500 years ago. Peat deposition began in the Everglades area about 5,000 years ago, which indicates that conditions favorable to long-term flooding had begun. Eventually, Everglades's peat lands covered nearly 2 million acres.

South Florida is underlain by a huge volume of shallow marine carbonate sediments. The deeper sediments, which exceed 20,000 ft, are almost pure limestone, dolomite and anhydrite, and were deposited from Cretaceous through early Tertiary time as a carbonate platform. During much of this time, south Florida was isolated from the mainland by the deep water of the Suwannee Strait. Later, as the strait filled, south Florida was connected to the mainland and clastic sediments were transported to the south to form the younger Tertiary deposits that consist of shallow marine sandy limestone, marls, and sands.

` The marine carbonate sediments in south Florida contain three major aquifer systems: the Floridan, the intermediate, and the surficial. The confined Floridan aquifer system is at or near the land surface in central Florida but dips deeply beneath the surface to the south. The semi confined, intermediate aquifer system overlies the Floridan and serves as a confined unit for the Floridan. The surficial aquifer system includes the highly permeable Biscayne aquifer. The Biscayne aquifer is more than 200 ft thick under parts of the Atlantic Coastal Ridge and wedges out about 40 mi to the west in the Everglades. The shallow aquifer of southwest Florida is about 130 ft thick along the Gulf Coast and wedges out in the eastern Big Cypress Swamp. The surficial aquifers are recharged by abundant rainfall.

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