The ancient reefs that now underlie the Florida Keys are believed to have developed between 150,000 and 200,000 years ago during the last warm period of the Pleistocene - the Sangamon Interglacial. Sea level was approximately twenty-five feet higher than it is today, and warm ocean water bathed all of southern Florida. Living corals thrived on a platform reef that covered a shallow, submerged plateau at the edge of the continental shelf. This giant reef tract, reaching from what is present-day Miami to the Dry Tortugas, is now the Key Largo limestone that forms the surface bedrock of the Upper Keys from Soldier Key to Big Pine Key.
An extensive lagoon spread behind the Sangamon ref, covering what is now the freshwater Everglades. Bryozoans (mosslike creatures) thrived in these waters as encrustations on underwater plants. Ooids...and skeletal remains of the bryozoans eventually consolidated to form the underlying layers of bedrock of our present-day Everglades and Florida Bay.
Some of the oolitic sediment drifted south to cover the coral at the lower, deeper end of the great reef tract. This sediment is now Miami limestone, the surface bedrock of the Lower Keys from Big Pine Key to Key West. The numerous channels separating the islands of the Lower Keys are the result of tidal water snaking through ancient oolitic sands on a course between the Florida Straits and the Gulf of Mexico...
by Jeff Ripple, author, The Florida Keys - The Natural Wonders of an Island Paradise.
A historical postcard from the Florida State Historical Photo Archive depicts the opening of the Overseas Highway to Key West, Florida Caption Reads: Off to Sea with No Seasickness over the New Overseas Highway on the Way to Key West Florida