Florida Keys Geography

The Florida Keys are a chain of over 40 main keys in Monroe County, linked by the Overseas Highway and running from Key Largo past Marathon and the Seven Mile Bridge to Key West, with the Dry Tortugas beyond (FDEP).

Key Largo is the first and longest key, and it holds John Pennekamp Coral Reef State Park, which FDEP calls the first underwater state park in the country (FDEP Monroe County guide). Marathon, a 10-mile-long island, is the heart of the Middle Keys, and the Lower Keys begin once a driver crosses the Seven Mile Bridge west of Marathon. Monroe County is the southernmost county in Florida, its mainland portion is predominantly Everglades National Park, and FDEP lists its coastal cities as Key Largo, Islamorada, Marathon, Big Pine and Key West.

The road ends at Key West, but the island chain does not. Dry Tortugas National Park lies almost 70 miles (113 km) west of Key West, covers 100 square miles that are mostly open water, and holds seven small islands reachable only by boat or seaplane (National Park Service).

The Upper, Middle and Lower Keys labels used on this page follow the Monroe County coastal guide published by the Florida Department of Environmental Protection, which places Key Largo at the start of the chain, Marathon in the Middle Keys and everything past the Seven Mile Bridge in the Lower Keys.

The Florida Keys are limestone: the Upper Keys are the remains of ancient coral reefs and the Lower Keys are old sand bars, both formed when seas stood higher about 125,000 years ago (NOAA Florida Keys National Marine Sanctuary).

NOAA names the two dominant rock formations in the Keys as Key Largo Limestone and Miami Oolite. The U.S. Geological Survey describes the Key Largo Limestone as a fossil coral reef much like the present reefs offshore from the Keys, exposed at the surface from Soldier Key on the northeast to the Newfound Harbor Keys near Big Pine Key (USGS, Key Largo Limestone). It is a white to light gray coralline limestone of coral heads encased in a calcarenitic matrix, and it is highly porous and permeable, part of the Biscayne Aquifer.

West of Big Pine Key the surface rock changes. USGS maps the Miami Limestone in the southern Florida Keys from Big Pine Key to the Marquesas Keys, and notes that from Big Pine Key toward the mainland it is replaced by the Key Largo Limestone (USGS, Miami Limestone). USGS notes that the unit was formerly called the Miami Oolite and has two facies, an oolitic facies of sandy, oolitic limestone and a bryozoan facies, and that the highly porous Miami Limestone forms much of the Biscayne Aquifer.

The Keys became dry land when sea level fell during the last ice age about 100,000 years ago, exposing reefs and sand bars that fossilized into rock (NOAA).

From about 15,000 years ago, melting glaciers and polar ice caps flooded land and, with tidal influence, changed the geography of the Keys and the waters around them (NOAA Habitats).

NOAA explains that while seas were low the Florida land mass was much larger than it is today, and the area now called Florida Bay was forested (NOAA). Today a boat can travel within minutes from the mangrove and seagrass flats of Florida Bay to coral reefs and deep water on the ocean side.

The islands remain low. Monroe County describes the Keys as a chain of islands barely above the sea, where many streets flood at times throughout the year, especially during the extreme fall and spring high tides called king tides (Monroe County).

Windley Key Fossil Reef Geological State Park, designated a state geological site in 1998, provides an exposure of the Key Largo Limestone where visitors can stand inside a fossil reef (Florida Geological Survey).

The Florida Geological Survey notes that the Windley Key rock is the same rock that makes up the aquifer of the upper Florida Keys. USGS describes an exceptional exposure of the Key Largo Limestone at the Windley Key quarry site, where large coral heads show in a series of old quarries (USGS). Windley Key is one of nine state geological sites that FGS lists across Florida (FGS state geological sites).

Farther west, Dry Tortugas National Park is mostly open water around seven small islands at the far western end of the chain (NPS), and FDEP lists Bahia Honda State Park in the Lower Keys for its beaches and clear water (FDEP).

Florida Keys National Marine Sanctuary protects 4,539 square miles of water around the Keys, from south of Miami west to the Dry Tortugas but excluding Dry Tortugas National Park, and its shoreward boundary is the mean high-water mark (NOAA).

The sanctuary was designated on November 16, 1990, when President George H.W. Bush signed the law establishing it after a series of boat groundings on the reefs of the island chain, folding in the older Key Largo and Looe Key sanctuaries to protect 3,800 square miles of Keys waters (NOAA, The Sanctuary). Two older refuges now sit inside it. The Great White Heron National Wildlife Refuge, established in 1938, is a spread of low, mostly mangrove keys over 264 square miles of open water. The Key West National Wildlife Refuge covers over 375 square miles of open water and 2,019 acres of land.

The two official sources disagree on size. FDEP's Monroe County coastal guide still describes the sanctuary as protecting 2,900 nautical square miles (FDEP), while NOAA, which administers the sanctuary jointly with the state of Florida, gives 4,539 square miles on its own page (NOAA).

Rising seas are already reaching Keys roads: sea level at the Key West gauge rose about 3.9 inches from 2000 to 2017, and Monroe County plans for 21 to 54 inches by 2070 (Monroe County).

The nearer Monroe County planning number is 10 to 17 inches of rise from 2000 to 2040, of which about 4 inches had already occurred through 2017 (Monroe County).

The Monroe County Board of County Commissioners adopted the Unified Regional Sea Level Rise Projections of the Southeast Florida Regional Climate Change Compact as the basis for its adaptation planning, and the longest projection reaches 40 to 136 inches by 2120 (Monroe County). The county says rising seas have begun affecting some roads and other county infrastructure in the Keys, as well as some homes and businesses.

The county's response starts with elevation. Its Office of Sustainability runs a mobile LiDAR project to improve elevation data, since the county must first have accurate data on the exact elevation of all 300 miles of county roads, and a countywide roads analysis that began in 2018 identifies roads at risk of inundation, including nuisance flooding (Monroe County resiliency page). The Key West gauge figure on the county page runs only through 2017.

NOAA says the Florida Keys have more than 77,000 residents, up to 5.5 million visitors a year and a local economy of nearly $5.0 billion (NOAA). Last verified: October 15, 2026, on the Monroe County, NOAA, USGS, NPS and FDEP websites.