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The southern half of Florida is the exposed part of a huge plate of shallow-water marine limestone that extends far under the shallow portions of the Gulf of Mexico but terminates abruptly at the Straits of Florida. This limestone plate is 15,000 feet thick in Dade County and zones within it are sources of water, potential sources of petroleum and natural gas, and potential reservoirs for the disposal of waste.

Limestone crops out in most of south Dade County (see map). Hard limestone forms the Coastal Ridge and outcrops in the Rocky Glades. Soft limestone or marl underlies most of the Everglades and the Coastal Marsh. Peat covers the limestone in the northwest part of the area and underlies Mangrove Swamp along the coast. Patches are quart sand are scattered on the Costal Ridge and in the Transverse Glades. Shelly sand and mud are exposed on the Keys (Islands) in the northeast, and underlie much of the Bay.

Cross Section B-B' West of Homestead to Elliot Key

A thin layer of soft limestone covers the hard limestone throughout most of the area (see map below for location of section). Vertical lines are drill hole sites. [larger image]

The Biscayne aquifer ranges from about 10 feet thick in the western part of the area to about 120 feet thick at the coast. [larger image]

The surface of most of south Dade County is hard limestone, soft limestone, and peat. Small patches of quartz sand, shelly sand, and mud are present in some areas. Note: Barrier islands not mapped. [larger image]

Chemical analyses of six hard and soft limestones are presented in the table below. The hard limestone is very pure. The soft limestone contains considerable peat. The elements that make up clays, quartz sand, metallic minerals, and nutrients are conspicuously sparse. Certain elements of importance to animal and plant health, such as arsenic and mercury are probably present in trace amounts, below delectability by the methods used.

The hard limestone and sandy limestone below it, which is not exposed at the surface here, together constitute the principal source of water, the Biscayne aquifer.

Another aquifer, the Floridan aquifer, is separated from the Biscayne by about 800 feet of rock that does not yield water. Much thicker than the Biscayne, the Floridan aquifer contains only brackish and salt water.

The composition of rocks and soils influence mineral resource use, water quality, the nutritional value of farm produce, and the safety of the environment.

The chemical analyses below are representative of the limestone in Dade County. They are relatively pure except for peat components in the soft limestone (marls), reflected in high organic matter, and sparse quartz sand, shown by silica (SiO₂). Elements making up clays, metallic minerals, and nutrients (K₂ O₂, P₂ O₅) are generally low. Such limestones provide little residuum upon weathering. The farmed marls require much fertilizer, containing iron as well as conventional additives. Potentially harmful elements such as arsenic and mercury are insignificant in these rocks.

The limestone near the surface are coarse-gained shelly rocks containing many cavities. Many are capable of holding and transmitting much water. These rocks make up the Biscayne aquifer, which is as much as 120 feet thick under Miami but thins to a few feet in western Dade County. The rocks below are finer grained, less permeable, and seal off aquifers.

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