Permeable limestone formations developed between 25 million and 70 million years ago and created the Floridan Aquifer and now serves as the main source of fresh water for the northern portion of Florida. This began with fluctuating sea levels compressing numerous layers of calcium carbonate, sand, and shells. The aquifer lies beneath thousands of feet of impermeable sedimentary rock from the southern tip of Florida to Lake Okeechobee.

There are rive geologic formations that make up the southern portion of Florida:

Tamiami Formation

Compressed of highly permeable light colored fossiliferous sands and pockets of quartz 150 feet thick. Named for the Tamiami Trail which follows the upper bedrock of the Big Cypress Swamp, and underlies the southern portion of the Everglades.

Caloosahatchee Formation

Found between the Tamiami Formation and Lake Okeechobee, and named for the river over it. It is much less permeable and the formation is highly calcitic and composed of sandy shell marl, clay, and sand. The water underneath the formation is typically very mineralized. Both formations developed during the Pliocene Epoch.

Anastasia Formation

Shelly limestone, coquina, and sand compose the formation. This represents a former mangrove or salt marsh. The formation is much more permeable and filled with pocks and solution holes.

Miami Limestone (Miami Oolite) and  Fort Thompson Formations

These to formations have the most influence on the Everglades. The Miami Limestone forms on the floor of the lower Everglades. It is made up of ooids (tiny formations of egg-shaped concentric shells and calcium carbonate that form around a single grain of sand). Originally, the Miami Limestone was named the Miami Oolite which comprises facies of ooids and fossilized bryozoan organisms. Due to its unique structure, it was some of the first material used in the housing in the early 20th-century in South Florida.

The rock is especially porous and stores water during the dry season in the Everglades. Consequently, the composition of this sedimentary formation affects the hydrology, plant life and wildlife above it. The Miami Limestone also acts as a dam between Fort Lauderdale and Coot Bay.

Under both formations lies the Biscayne Aquifer. This aquifer serves as the Miami metropolitan area’s fresh water source. Rainfall and stored water in the Everglades replenish the Biscayne Aquifer directly.

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