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|Vol. 1, No. 1 - March, 1953 - pp. 21-24.|
SOME THIRTY to fifty thousand years ago South Florida was a shallow submarine bank. The sediments deposited in that sea have become the limestones which underlie the peat soil of much of the Everglades, and which act as natural dams to maintain the water level inland.
Much of Everglades National Park is floored by this most recent limestone which geologists call the Miami Oolite. The term oolite from the Greek word for egg oos, refers to the minute egg-shaped particles which form this entire layer of rock as shown in Fig. 1. When sliced very thin, a section of these particles examined under the microscope shows that each individual egg consists of a series of concentric shells, like the layers of an onion, formed around a central sand grain or shell fragment as shown in Fig. 2. Each layer was laid down by precipitation of calcium carbonate from seawater in the same fashion as well water in limestone country forms the limy scale in tea kettles or water pipes. These tiny particles are not now being formed in Florida, but present day examples of this process occur on the Bahama Banks and in the Gulf of Suez. The hot, shallow, and unusually salty seawater in both of these localities is necessary for precipitation.
The minute individual egg-shaped particles, called ooliths, are similar in size and shape to sand grains, and can be moved about by water currents. In the case of the Miami Oolite the particles were formed west and northwest of the Florida Keys. Wind, waves, and currents piled them up to form a ridge extending from Fort Lauderdale to Coot Bay. In the Park this ridge is represented by the Everglades Keys which stand slightly above the surrounding seasonally wet sawgrass prairies, and on these are located pinelands and some of the oldest south Florida hammocks.
Important changes in sea level have occurred since the deposition of the oolitic sediments. As the glaciers thickened and advanced southward from Canada, the large amount of water lock-up in glacial ice caused the level of the oceans to drop. During the warm periods between the several advances of the glaciers when temperatures were similar to those of the present day, sea-level rose from the water added to the oceans by the melting of glacial ice. Our tiny ooliths were formed in a hot, shallow sea during the last warm or inter-glacial period. During the period of glacial advance which followed, sea-level stood at least twenty-feet below its present position and the processes which convert loose sediment to rock began to act on the Miami Oolite. Rainwater falling on the surface first dissolved the limestone particles and then as it percolated down through the rock the same calcium carbonate was precipitated between the grains to form a coherent rock. This sort of precipitation is similar to some crystal formations in limestone caves. The continued solution of limestone near the surface by rain water and soil acids has produced the pinnacled, honeycombed rock which may be seen today on the Everglades Keys.
Perhaps at the same time that this sand-like oolitic sediment was being solidified, some of the drainage from the Everglades cut through the low ridge as it flowed seaward. These channels or spillways broke the more or less continuous ridge into segments which in the Park form the Everglades Keys.
Limestones similar to the Miami Oolite are very common in the geologic past. They can be found among the oldest sedimentary rocks known, the Pre-Cambrian of Michigan and Montana, some 1500 to 2000 million years old, and in every succeeding period of the earth's history. In the general character of the individual ooliths, their accumulations, and their chemical compositions these ancient limestones are fossil equivalents of the Miami Oolite. Such similarity calls attention to the continuity of processes from the earliest recorded history of the earth to the present day, a principle which is the basis for the science of geology.
The presence of the Miami Oolite under a large portion of the park has a significant effect on the natural history. The high porosity of the oolitic rock provides underground storage of water for plant and animal needs during the dry season, and the chemical composition and texture of the rock exercise some control over the type of vegetation. But it is the damming action of the oolite ridge which is most important to the Park. Were it not for the arc-shaped oolitic ridge between Ft. Lauderdale and Coot Bay water levels in the southern Everglades might be considerably lower and the major drainage pattern of the Park very differently arranged.