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Geology and Hydrogeology of the Florida Keys

Robert B. Halley, H.L. Vacher and Eugene A. Shinn

This chapter was originally published in Geology and Hydrology of Carbonate Islands. Developments in Sedimentology 54, edited by H. L. Vacher and T. Quinn ©1997 Elsevier Science B.V. All rights reserved.

*Please note that any references to chapters within this publication are from Geology and Hydrology of Carbonate Islands. Developments in Sedimentology 54.


Pleistocene Geology
Holocene Geology
Water Resources
Case Study
Concluding Remarks

The Florida Keys, which border the southeastern tip of the Florida peninsula (Fig. 5-1), are low-lying islands composed of Pleistocene limestone. They form an arcuate chain extending from Soldier Key (15 km southeast of Miami) south and west to Key West, a distance of 240 km. The Keys are divided into the Upper Keys, from Bahia Honda northward, and the Lower Keys, from Big Pine Key to Key West. Technically, the Holocene mud islands of Florida Bay (Fig. 5-1), the sandy islands west of Key West (the Marquesas and Dry Tortugas), and ephemeral islands and rocks of the reef tract are all also Florida "keys". The mud islands of Florida Bay are discussed in the next chapter. This chapter concerns the islands formed of Pleistocene limestone. These islands, which are crossed when driving from Miami to Key West, are typically regarded as "the Florida Keys."

The Florida Keys were largely ignored during the sixteenth, seventeenth, and eighteenth centuries, although the waters just offshore provided a major shipping thoroughfare to and from the New World. For three centuries, the islands were notorious for their treacherous reefs, pirates and Caloosa Indians, and the scarcity of water and fertile soil. After Florida was ceded by Spain to the United States in 1821, Key West became an important military outpost guarding the entrance to the Gulf of Mexico. The island began to grow as a trading center between the Gulf and Atlantic coasts and between Cuba and the United States. Trading, fishing, and recovering goods from shipwrecks provided livelihood for Keys residents, and boosts to the economy were derived from the Civil, Spanish-American, and World Wars. The Overseas Railway and Overseas Highway, completed in 1912 and 1938, respectively, provided the backbone of transportation in the Keys.

map showing the location and lithology of the Florida Keys
Fig. 5-1. Map showing the Florida Keys, their lithology, and location relative to mainland and reef tract. [larger image]
Bridged transportation, together with a water pipeline from the mainland built to supply the military in Key West during World War II, set the stage for post-war development. With the advent of widespread air-conditioning and mosquito spraying, the Keys have developed into one of the most popular tourist destinations in North America. The beauty of the area's coral reefs and clear blue water, the excitement offered by sports fishing and diving, and the diversity of the region's wildlife, all combine to make the Florida Keys one of the premier natural wonders of the United States.

The outstanding and fragile character of ecosystems on and around the Florida Keys has prompted State and Federal efforts to protect and preserve the remaining public portions of the region. The northernmost Florida Keys lie within Biscayne National Park. Florida Bay, northwest of the Keys, lies almost entirely within Everglades National Park. The Dry Tortugas, 110 km west of Key West, lie in Fort Jefferson National Park. The remainder of the Keys are within the Florida Keys National Marine Sanctuary. Numerous smaller refuges and preserves are found throughout the islands.

During the last decade, there has been increasing concern for the environmental well-being of the region, including the Everglades and other South Florida terrestrial ecosystems. Large-scale declines in bird and fish populations, infestations of exotic biota, and mortality of seagrasses and corals have raised questions about the effects of agricultural and urban development on South Florida's native ecosystems. Changes brought about by water-management practices have received the most attention, particularly the draining of wetlands that began at the turn of the century. Currently, a massive Federal and State program is being developed to restore the water flow patterns and cycles of remaining natural areas to something comparable to their predevelopment condition (Holloway, 1994; Culotta, 1995).

A geological showcase

The pre-bridges geological literature of the Florida Keys and surrounding environments includes works by some of America's most celebrated and influential natural scientists. Louis Agassiz, whose Etudes sur les glaciers (1840) in Switzerland opened the concept of the Ice Ages, did a study of the reefs some five years after immigrating to the United States to become, in 1847, Professor of Zoology at Harvard College, where he established what grew to be the Museum of Comparative Zoology. His son, Alexander Agassiz, wrote specifically of the Keys ("The elevated reef of Florida," Agassiz, 1894) from a visit in the winter of 1893; this visit was among the first of his expeditions specifically to study coral reefs [see Chap. 1], coming after expeditions to the Bahamas and the coast of Cuba in the winter of 1893 and Bermuda in the spring of 1894, and before the Great Barrier Reef in 1896.

From 1908 to 1915, T. Wayland Vaughan studied the corals, reefs, sediments, and organism-sediment relations while based at the Carnegie Institution's Marine Biological Laboratory in the Dry Tortugas. This work was part of a line of investigation that included one of the seminal papers in paleoecology (Vaughan, 1940; cf., Ladd, 1957), which he gave as his Presidential address to the Geological Society of America, after retiring as Director of Scripp's Institution of Oceanography. One of the concluding sentences of that paper - "There should be continuous shuttling from studies of the modern to studies of the ancient and back again from the ancient to the modern" (Vaughan, 1940, p. 466) - describes the comparative approach to carbonate geology that has been possible in the Keys and nearby Bahamas.

As the best known marine carbonate depositional setting in the continental United States, the Keys and surrounding environments became a showcase for geologic field trips for academic and industrial groups. Particularly noteworthy are the field trips led by Robert N. Ginsburg from his lab on Fisher Island. Ginsburg named this lab, which was part of the University of Miami (Rosenstiel School of Atmospheric and Marine Sciences), the T. Wayland Vaughan Laboratory of Comparative Sedimentology (see Ginsburg, 1995, for an account). A huge number of carbonate geologists in the U.S. petroleum industry in the 1970s and 1980s participated in these trips, and many, having later entered the academic ranks, are bringing subsequent generations to this geological showcase.

The classic field trip guide is Ginsburg (1964), and it is still instructive. The book by Multer (1977) compiles much of the literature of the 1960s and 1970s and is designed for students and visitors. The field guide to the reefs by Shinn et al. (1989) incorporates work done from the U.S. Geological Survey Fisher Island Station over the 15 years of its existence. Shinn (1988) and Randazzo and Halley (1997) give summaries specifically on the geology of the Keys.

Related information:

SOFIA Project: Determination of Groundwater-Flow Direction and Rate Beneath Florida Bay, the Florida Keys and Reef Tract

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