The living coral coasts of the world are confined to waters in which the temperature seldom falls below 70° F. (and never for prolonged periods), for the massive structures of the reefs can be built only where the coral animals are bathed by waters warm enough to favor the secretion of their calcareous skeletons. Reefs and all the associated structures of a coral coast are therefore restricted to the area bounded by the Tropics of Cancer and Capricorn. Moreover, they occur only on the eastern shores of continents, where currents of tropical water are carried toward the poles in a pattern determined by the earth’s rotation and the direction of the winds. Western shores are inhospitable to corals because they are the site of upwellings of deep, cold water, with cold coastwise currents running toward the equator.
In North America, therefore, California and the Pacific coast of Mexico lack corals, while the West Indian region supports them in profusion. So do the coast of Brazil in South America, the tropical east African coast, and the northeastern shores of Australia, where the Great Barrier Reef creates a living wall for more than a thousand miles.
Within the United States the only coral coast is that of the Florida Keys. For nearly 200 miles these islands reach southwestward into tropical waters. They begin a little south of Miami where Sands, Elliott, and Old Rhodes Keys mark the entrance to Biscayne Bay; then other islands continue to the southwest, skirting the tip of the Florida mainland, from which they are separated by Florida Bay, and finally swinging out from the land to form a slender dividing line between the Gulf of Mexico and the Straits of Florida, through which the Gulf Stream pours its indigo flood.
To seaward of the Keys there is a shallow area three to seven miles wide where the sea bottom forms a gently sloping platform under depths generally less than five fathoms. An irregular channel (Hawk Channel) with depths to ten fathoms traverses these shallows and is navigable by small boats. A wall of living coral reefs forms the seaward boundary of the reef platform, standing on the edge of the deeper sea (see page 198).
The Keys are divided into two groups that have a dual nature and origin. The eastern islands, swinging in their smooth arc 110 miles from Sands to Loggerhead Key, are the exposed remnants of a Pleistocene coral reef. Its builders lived and flourished in a warm sea just before the last of the glacial periods, but today the corals, or all that remains of them, are dry land. These eastern Keys are long, narrow islands covered with low trees and shrubs, bordered with coral limestone where they are exposed to the open sea, passing into the shallow waters of Florida Bay through a maze of mangrove swamps on the sheltered side. The western group, known as the Pine Islands, are a different kind of land, formed of limestone rock that had its origin on the bottom of a shallow interglacial sea, and is now raised only slightly above the surface of the water. But in all the Keys, whether built by the coral animals or formed of solidifying sea drift, the shaping hand is the hand of the sea.
In its being and its meaning, this coast represents not merely an uneasy equilibrium of land and water masses; it is eloquent of a continuing change now actually in progress, a change being brought about by the life processes of living things. Perhaps the sense of this comes most clearly to one standing on a bridge between the Keys, looking out over miles of water, dotted with mangrove-covered islands to the horizon. This may seem a dreamy land, steeped in its past. But under the bridge a green mangrove seedling floats, long and slender, one end already beginning to show the development of roots, beginning to reach down through the water, ready to grasp and to root firmly in any muddy shoal that may lie across its path. Over the years the mangroves bridge the water gaps between the islands; they extend the mainland; they create new islands. And the currents that stream under the bridge, carrying the mangrove seedling, are one with the currents that carry plankton to the coral animals building the offshore reef, creating a wall of rocklike solidity, a wall that one day may be added to the mainland. So this coast is built.
To understand the living present, and the promise of the future, it is necessary to remember the past. During the Pleistocene, the earth experienced at least four glacial stages, when severe climates prevailed and immense sheets of ice crept southward. During each of these stages, large volumes of the earth’s water were frozen into ice, and sea level dropped all over the world. The glacial intervals were separated by milder interglacial stages when, with water from melting glaciers returning to the sea, the level of the world ocean rose again. Since the most recent Ice Age, known as the Wisconsin, the general trend of the earth’s climate has been toward a gradual, though not uniform warming up. The interglacial stage preceding the Wisconsin glaciation is known as the Sangamon, and with it the history of the Florida Keys is intimately linked.
The corals that now form the substance of the eastern Keys built their reef during that Sangamon interglacial period, probably only a few tens of thousands of years ago. Then the sea stood perhaps 100 feet higher than it does today, and covered all of the southern part of the Florida plateau. In the warm sea off the sloping southeastern edge of that plateau the corals began to grow, in water somewhat more than 100 feet deep. Later the sea level dropped about 30 feet (this was in the early stages of a new glaciation, when water drawn from the sea was falling as snow in the far north); then another 30 feet. In this shallower water the corals flourished even more luxuriantly and the reef grew upward, its structure mounting close to the sea surface. But the dropping sea level that at first favored the growth of the reef was to be its destruction, for as the ice increased in the north in the Wisconsin glacial stage, the ocean level fell so low that the reef was exposed and all its living coral animals were killed. Once again in its history the reef was submerged for a brief period, but this could not bring back the life that had created it. Later it emerged again and has remained above water, except for the lower portions, which now form the passes between the Keys. Where the old reef lies exposed, it is deeply corroded and dissected by the dissolving action of rain and the beating of salt spray; in many places the old coral heads are revealed, so distinctly that the species are identifiable.
While the reef was a living thing, being built up in that Sangamon sea, the sediments that have more recently become the limestone of the western group of Keys were accumulating on the landward side of the reef. Then the nearest land lay 150 miles to the north, for all the southern end of the present Florida peninsula was submerged. The remains of many sea creatures, the solution of limestone rocks, and chemical reactions in the sea water contributed to the soft ooze that covered the shallow bottoms. With the changing sea levels that followed, this ooze became compacted and solidified into a white, fine-textured limestone, containing many small spherules of calcium carbonate resembling the roe of fish; because of this characteristic it is sometimes known as “oolitic limestone,” or “Miami oolite.” This is the rock immediately underlying the southern part of the Florida mainland. It forms the bed of Florida Bay under the layer of recent sediments, and then rises above the surface in the Pine Islands, or western Keys, from Big Pine Key to Key West. On the mainland, the cities of Palm Beach, Fort Lauderdale, and Miami stand on a ridge of this limestone formed when currents swept past an old shore line of the peninsula, molding the soft oozes into a curving bar. The Miami oolite is exposed on the floor of the Everglades as rock of strangely uneven surface, here rising in sharp peaks, there dropping away in solution holes. Builders of the Tamiami Trail and of the highway from Miami to Key Largo dredged up this limestone along the rights of way and with it built the foundations on which these highways are laid.