The rocks beneath the Big Cypress Swamp are among the oldest in South Florida. Six million years ago a shallow sea covered this area. Sediments of silt and sand and particles of calcium deposited on the bottom of this sea gradually cemented into limestone. Today this rock is called the Tamiami Formation.

The Tamiami Formation is also found in the northwest corner of Everglades National Park. Here, fresh water flowing out of Big Cypress mixes with salt water from the Gulf of Mexico in a highly productive mangrove estuary. The resulting nutrient-rich soup supports a marine nursery for pink shrimp, snook, and snapper.

Other rocks beneath the Everglades were formed during the time of the Great Ice Age. Although no glaciers developed in Florida, their effects were felt here. As glaciers in other areas of the world expanded, much of the earth's water supply was trapped in the ice. Sea levels in South Florida lowered as much as 300 feet below present levels.

The Great Ice Age was actually four shorter ice ages with periods of warming in between. During these warmer "interglacial" stages, the ice melted and returned to the sea. The last interglacial stage occurred about 100,000 years ago. At its peak, the sea level in South Florida rose 100 feet above present levels.

 The rocks beneath the southeast section of the Park were formed in this sea. Calcium carbonate settling out of the water coated tiny bits of shell or sand in layer upon layer. The resulting spherical grains of limestone are called ooids. The Atlantic Coastal Ridge which runs from Mahogany Hammock northeast to Miami was formed as long shore currents pushed the ooids up into a long ridge. The ooids later cemented into rock known as Miami Oolite. Miami Oolite also covers most of the area east of Everglades National Park and most of Florida Bay.

In quieter waters covering the central portions of the Park, tiny moss animals called Bryozoans flourished. As they died their calcium skeletons settled to the bottom. These sediments later cemented into rock known as the Miami Bryzoan Limestone.

As in most areas of South Florida, subtle changes in elevation result in dramatic changes in vegetation communities. Pine forests are found on the high ground of the Atlantic Coastal Ridge. Where fire has been excluded, pines give way to hardwood hammocks. In wetter areas near the end of the ridge, dwarf pond cypress grow. South of the ridge sawgrass prairies take over again. A narrow band of mangroves fringe the southeast coast, and the shallow waters of Florida Bay today provide an abundant food supply for great numbers of wading birds.