FLORIDA KEYS: HOW IT ALL BEGAN

The Keys were formed near the edge of the Florida Plateau. The Florida Keys are the exposed portions of an ancient coral reef, with very little sand. The northernmost island arising from the ancient reef formation is Elliott Key, in Biscayne National Park. North of Elliott Key are several small transitional keys, composed of sand built up around small areas of exposed ancient reef. The Florida Keys have taken their present form as the result of the drastic changes in sea level associated with ice ages. Beginning some 130,000 years ago the Sangamon interglacial raised sea levels to approximately 25 feet above the current level. All of southern Florida was covered by a shallow sea. Several parallel lines of reef formed along the edge of the submerged Florida plateau, stretching south and then west from the present Miami area to what is now the Dry Tortugas. This reef formed the Key Largo limestone that is exposed on the surface from Soldier Key to the southeast portion of Big Pine Key and the Newfound Harbor Keys. The types of coral that formed Key Largo limestone can be identified on the exposed surface of these keys

Starting about 100,000 years ago the Wisconsin glaciation began lowering sea levels, exposing the coral reef and surrounding marine sediments. By 15,000 years ago the sea level had dropped to 300 to 350 feet below the contemporary level. The exposed reefs and sediments were heavily eroded. Acidic water, which can result from decaying vegetation, dissolves limestone. Some of the dissolved limestone redeposited as a denser cap rock, which can be seen as outcrops overlying the Key Largo and Miami limestones throughout the Keys. The limestone that eroded from the reef formed oolites in the shallow sea behind the reef, and together with the skeletal remains of bryozoans, formed the Miami limestone that is the current surface bedrock of the lower Florida peninsula and the lower keys from Big Pine Key to Key West. To the west of Key West the ancient reef is covered by recent calcareous sand.

ENVIRONMENT

The Keys are in the subtropics between 24 and 25 degrees north latitude. The climate and environment are closer to that of the Caribbean than the rest of Florida, though unlike the Caribbean's volcanic islands, the Keys were built by plants and animals. The Upper Keys islands are remnants of large coral reefs, which became fossilized and exposed as sea level declined. The Lower Keys are composed of sandy-type accumulations of limestone grains produced by plants and marine organisms. The natural habitats of the Keys are upland forests, inland wetlands and shoreline zones. Soil ranges from sand to marl to rich, decomposed leaf litter. In some places, "caprock" (the eroded surface of coral formations) covers the ground. Rain falling through leaf debris becomes acidic and dissolves holes in the limestone, where soil accumulates and tree roots find purchase. The climate is considered to be tropical and the Keys are the only frost-free place in Florida. There are two main seasons: hot, wet, and humid from about June through October, and somewhat drier and cooler weather from November through May. Many plants grow slowly or go dormant in the dry season. Some native trees are deciduous, and drop their leaves in the winter or with spring winds.

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