LEAFLET 14 3
ment is ideal for the growth of coral: a shallow-water shelf, subtropical latitude, and the warm Gulf Stream nearby.
The geological history of the Florida Keys began about three million years ago, when a shallow sea covered what is now south Florida. During the next 2.8 million years, often called the Pleistocene Ice Ages, world sea levels underwent many fluctuations of several hundred feet, both above and below present sea level, in response to the waxing and waning of the great glaciers. Colonies of coral became established in the shallow sea along the rim of the broad, flat Floridan Plateau. The subtropical climate allowed the corals to proliferate, forming reefs. As sea levels fluctuated the corals maintained footholds along the edge of the plateau: their reefs grew upward when sea levels rose, and their colonies retreated to lower depths along the plateau's rim when sea levels fell. During times of rising sea levels, dead reefs provided good substrates for new coral growth. In this manner, during successive phases of growth, the Key Largo Limestone accumulated up to 200-feet thick in places. The Key Largo Limestone is a white to tan limestone that is primarily the skeletal remains of corals, with invertebrate shells, marine plant and algal debris, and lime-sand. The Key Largo Limestone varies irregularly in thickness from about 75 feet to over 200 feet. In the Lower Keys the Key Largo Limestone is covered by the Miami Limestone. The last major drop in sea level exposed the ancient reefs, which are the present Keys.
During reef growth, carbonate sand banks periodically accumulated behind the reef in environments similar to the Bahamas today. One such lime-sand bank covered the southwestern end of the coral reefs and, when sea level last dropped, the exposed lime-sand or ooid bank formed the Lower Keys. This white to light tan granular rock, the Miami Limestone, is composed of tiny ooliths, lime-sand and shells. Ooliths may be up to 2.0 mm in diameter and are made of concentric layers of calcium carbonate deposited around a nucleus of sand, shell, or other foreign matter. Throughout the Lower Keys the Miami Limestone lies on top of the coralline Key Largo Limestone, and varies from a few feet up to 35 feet in thickness. The northwest-southeast aligned channels between islands of the Lower Keys were cut in the broad, soft, oolite bank by tidal currents. Then, as today, the currents flowed rapidly into and out of the shallow bay behind the reefs, keeping the channels scoured clean.
Exposures of the Key Largo Limestone and Miami Limestone can be seen in many places along the Keys: in canal cuts, at shorelines, and in construction spoil piles.