Florida Peninsula Geology

The Florida peninsula is built on carbonate rock, and the Florida Geological Survey says the entire state is underlain by it (Florida Geological Survey).

The thickest part is the stack of Tertiary carbonate rock that holds the Floridan aquifer system. USGS says the sequence generally thickens seaward from the system's northern boundary, and its total thickness ranges from about 100 feet at the updip limit to over 3,300 feet, under all of Florida and parts of southeastern Georgia, Alabama and South Carolina; a 2015 USGS framework report listed on the same page also includes parts of Mississippi (USGS).

One of those rocks is the Ocala Limestone, an important part of the Floridan aquifer system, named for limestone exposed near Ocala in Marion County and made of nearly pure limestone with occasional dolostone (USGS, Ocala Limestone). The Florida Geological Survey dates it to more than 35 million years ago, when Florida was completely submerged beneath the sea (FGS).

The Floridan aquifer system is the groundwater stored in that limestone, and USGS calls it one of the most productive aquifers in the world, covering about 100,000 square miles (USGS).

USGS says the Floridan aquifer system is the primary source of drinking water for almost 10 million people, and nearly 50 percent of all withdrawals go to industry and agricultural irrigation (USGS karst aquifers). USGS adds that Floridan water-supply wells were first drilled in the late 1880s (USGS). Over roughly half of its extent it is an unconfined to thinly confined karst aquifer (USGS).

South Florida is different. USGS notes that there the Upper Floridan aquifer is brackish, so it is used as reverse osmosis source water, blended with fresh water from the shallower Biscayne aquifer and used for aquifer storage and recovery (USGS). The Miami Limestone of the southeast Florida coast forms much of that Biscayne aquifer (USGS, Miami Limestone).

In its overview of Florida's Upper Floridan and Biscayne aquifers, USGS says the hydrogeology of karst areas results in aquifers that are highly productive but extremely vulnerable to contamination (USGS).

Florida sinkholes form when slightly acidic groundwater slowly dissolves cavities in the limestone until the roof can no longer hold the sediment above, and the surface sinks or collapses into the void (Florida Geological Survey).

The Florida Geological Survey describes three types (FGS Sinkhole FAQ). Solution sinkholes form where little or no sediment covers the limestone, so the rock dissolves at or near the surface. Cover-subsidence sinkholes form where sand slowly settles into openings below, often leaving a gentle, circular, bowl-shaped low spot at the surface. Cover-collapse sinkholes occur where sediments over a void in the rock suddenly collapse, creating a hole at the ground surface.

FGS says that since the entire state is underlain by carbonate rocks, sinkholes could theoretically form anywhere, though risk is considerably higher in some regions, and areas where limestone is close to the surface or where deeper limestone has a conducive water table, stratigraphy and aquifer setting have increased sinkhole activity (FGS). USGS says the Ocala Limestone sits at or near the surface in the Ocala Karst District of the west-central to northwestern peninsula, where it shows extensive karst with tens of feet of relief (USGS).

Florida has no agency with responsibility or authority for sinkhole inspections, and the Florida Geological Survey points owners to its statewide subsidence incident reports, a certified home inspector or foundation specialist, and the Department of Financial Services for insurance questions (FGS Sinkhole FAQ).

The subsidence incident reports are on DEP's Map Direct website and GIS Open Data Portal. FGS warns that while the data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified (FGS). FGS adds that many sinkholes go unseen or unreported, and the reported data covers only 1954 to the present.

The record behind it is long. FGS has gathered Florida karst data since its start in 1907; the Florida Sinkhole Research Institute at the University of Central Florida held the files from 1982 until the Legislature ended its funding in the early 1990s, when the database returned to FGS (FGS). FGS also notes that settling ground can come from decomposing organic debris, broken sewer and drain pipes or improperly compacted soil, so not every sunken spot in Florida ground is a sinkhole.

The surface rock changes across the Florida peninsula: Ocala Limestone in the west-central karst district, phosphate-rich Bone Valley beds in Hillsborough, Polk and Hardee counties, coquina of the Anastasia Formation on the Atlantic coast and Miami Limestone in the southeast (USGS; USGS; USGS; USGS).

The Bone Valley Member is sand-sized and larger phosphate grains in quartz sand, silt and clay in Hillsborough, Polk and Hardee counties, and USGS says it provided much of U.S. phosphate production during the twentieth century (USGS, Bone Valley Member). Mining there began in 1888 and continues.

Along the Atlantic coast, the Anastasia Formation of sand and orange-brown coquina made of whole and broken mollusk shells underlies the Atlantic Coastal Ridge from St. Johns County south to Palm Beach County, and reaches as much as 20 miles inland in St. Lucie and Martin counties (USGS, Anastasia Formation).

Devil's Millhopper Geological State Park in Alachua County, a large and deep cover-collapse sinkhole designated a state geological site in 1998, is one of the few places in Florida with more than 100 feet of rock layers exposed (Florida Geological Survey).

Paynes Prairie Preserve State Park, designated in 2024, covers 23,000 acres and holds some of the best karst features in Florida: the prairie is a basin that formed as the limestone beneath it dissolved, and Alachua Sink drains it and recharges the aquifer below (FGS state geological sites). The Jennings Bluff Tract, designated in 2023 and owned by the Suwannee River Water Management District, is what FGS calls a premier example of how surface and groundwater systems are interconnected: water entering the Dead River swallet enters the Upper Floridan aquifer, travels about 10 miles south and discharges into the Suwannee River within days. Collecting fossils or artifacts on that district land is prohibited.

On the Atlantic side, USGS lists excellent Anastasia Formation exposures at Washington Oaks State Park in Flagler County, the House of Refuge on Hutchinson Island in Martin County and Blowing Rocks in Palm Beach County (USGS). Last verified: October 15, 2026, on the Florida Geological Survey and USGS websites.