Named by geologist Samuel Sanford in 1909, the late Pleistocene Key Largo Limestone is a coral reef comprised of fossils deposited during an interglacial period in the late Pleistocene Epoch when sea level was higher. The formation is exposed along the upper Florida Keys and is part of the Biscayne Aquifer. This geologic unit was deposited in a shallow marine environment. Older (and deeper) parts of the Key Largo Limestone formed in deeper water. In contrast, the younger limestone formed in shallower and warmer waters that contained patchy coral reefs. The Key Largo Limestone is estimated to range between 50 to 174 feet thick as observed in well borehole records.
The Key Largo Limestone is a white to light gray, moderately hard limestone composed of coral heads encased in a matrix of sand-sized grains. These grains are composed of the mineral calcite (calcium carbonate, or CaCO3). There are five primary coral species found within the Key Largo
Limestone: star coral (Montastrea annularis), porous coral (Porites astreoides), and brain corals (Diploria stigosa, Colpophyllia natans, Diploria labyrinthiformis). These corals make up approximately one-third of the limestone formation. Fossil mollusks, echinoids, bryozoans, and the fossils of other marine organisms are also found in the Key Largo Limestone.
Quarry walls containing beautiful exposures of Key Largo Limestone can be viewed at Windley Key Fossil Reef Geological State Park, where visitors can stand inside a fossil reef and see the same rocks that comprise part of the Biscayne aquifer of the upper Florida Keys. When the reef was alive (just like offshore today), sea level was higher than present and would have stood tens of feet above the heads of park visitors. Fossil corals and other ancient marine life are preserved in these rocks and record a unique part of Florida’s geologic history.
When you visit Windley Key Fossil Reef Geological State Park, you will notice layers of red soils, known as terra rossa, within some of the quarry walls. The story behind these soils is fascinating. Over thousands of years, naturally acidic rainwater dissolved some of the limestone, leaving behind a residue consisting mainly of African dust that was blown across the Atlantic Ocean and deposited along with the carbonate sediments. Agricultural fields on the Florida mainland west of Miami are named the “Redlands” because of the color added by this dust component.
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