A Geologic View of Old Florida and its Coral Reefs

While reading Grabau’s 1913 Principles of Stratigraphy, I came across this fascinating description of the geology of Florida and its coral reefs.  This is for the most part a summary of expeditions in the early 1850’s by two of this country’s pioneering geologists: Louis Agassiz and Joseph LeConte.  In addition to the outline of the geology of the reefs (a life form where geology and biology run together) it’s an interesting early description of a sparsely populated area before its dramatic transformation during the last century.

The reefs of the Florida coast form an interesting example of the fringing type on a shallow continental platform. The southern coast of Florida rises from 12 to 15 feet above the sea-level, in the form of a curving ridge, which encloses an extensive fresh-water swamp, the Everglades. The surface of this lies only two or three feet above sea-level, and is dotted over with small islands, the so-called hummocks. Some distance outside of the southern border of the land lies a row of small islands or “keys” of dead coral rock and sand, ranging from 5 to 30 miles distant from shore and continued westward in a curved line far beyond the western coast of the peninsula. The islands are low and of limited extent, that of Key West near the western end of the line being less than 4 miles long, while the longest of the islands is only 15 miles in length. The keys slope toward their northern shore, and present a steep face to the south, where they are separated from the living reefs by an open channel. Between the keys and the southern coast of the mainland the water is very shallow and navigable only to the smallest boats. This lagoon is, moreover, dotted with small, low mangrove islands. The mangrove trees growing here extend their aerial roots in all directions, forming at angle, which becomes efficient in checking sediment-laden currents and causes them to deposit their load. Hence the inner lagoon will gradually silt up, and this has already progressed so far that a considerable portion of the area forms mud flats at low tide. (Figs.81, 82.) Here, then, a clastic mud sediment rich in organic matter rests directly up on the ancient coral reef now represented by the keys, a relationship expressed in the rocks of the older geological periods by the superposition of a black carbonaceous shale above an earlier coral limestone.

Outside of the line of key sand from 3 to 15 miles distant from it is a line of living coral reefs, consisting of mounds made up of branching madrepores, Porites,etc., besides many smaller genera such as Manicina, Agaricia, etc.Corallina and Lithothamnion also add a large percentage of calcareous material to the reef.

These reefs are for the most part submerged, rising only here and there to the surface. Between them and the keys lies the outer lagoon, along a narrow channel five to six fathoms deep and navigable for small vessels. Here the sedimentation consists of coral sand and of the shells of marine organisms, thus producing a normal marine limestone, which is either in the form of a coral breccia or a more or less oolitic calcarenyte. Outside the living reef, the bottom rapidly descends to the abyssal depths of the Florida Straits (2,916feet). While nullipores, or the stony algae, luxuriate in the outer zone of the reef, where they form a distinct Nullipore zone in the face of the strong surf, the more delicate branching and brittle coral lines are confined to the channels and lagoons within the reef, where they often form thick carpets in the quiet water. Thus the shallow parts of the bottom of the ship channel between the living Florida reefs and the old reefs or keys are covered with the so-called “country grass,” one of these calcareous algae. This is especially noticeable between Fowey Rocks, Triumph Reef and Long Reef on the one side, and Soldier Key and Ragged Keys on the inside (Agassiz-2:126,127). The floor of Hawk Channel, which has a depth of from 6 to 7 fathoms, is covered with disintegrated corals and coral-lines (Pourtales; Dana-20:211), while some of the keys in the Dry Tortugas and Marquesas are wholly composed of fragments of coral lines bound together into a solid mass. Among these coral-lines a large species of Opuntia is especially noticeable.

It has been shown by Agassiz that the keys, the southern rim of the mainland, and a strip including the north shore of the Everglades and extending as far north on the eastern shore as St. Augustine, are of coral-reef origin. When this last strip was the living reef, the platform in front was being built out into the sea by the accumulations of the shells of organisms which lived in abundance in the genial waters of the Gulf Stream border. When the platform was sufficiently extended a new line of reefs came into existence, forming a barrier reef at a distance from shore where now lies the outer rim of the Florida mainland. The lagoon behind this new reef was of the character of the present outer lagoon, and received normal marine sediments, until with progressive upbuilding the outer reef was converted into a series of dead keys and a new line of reefs came into existence up on the meanwhile extended submarine platform. This new line was subsequently converted in to the present line of keys, the preceding row became the southern coastal rim, and the old lagoon behind it was converted by successive steps in to the present Everglades. The present inner lagoon channel between the mainland and the keys is gradually approaching the same fate, and it and the line of keys will in turn be added to the mainland, while the present living reef will gradually emerge as a line of island sand the lagoon behind it suffer filling up. It is not likely that a new line of reefs will form outside of the present one, as the force of the Gulf Stream will prevent further extension on the Pourtales Plateau of the submarine platform which serves as the foundation of the reef. (LeConte-59; 60.)

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