The Florida Keys form a crescentic chain of small limestone islands which extend from near Miami to Key West, a distance of about 150 miles. They are made of two main formations of Pleistocene age—the Key Largo Limestone and the Miami Limestone. The former, named and described by Sanford, is an elevated coral reef rock, and the latter, also described by Sanford, is an oölitic limestone in this area. The Key Largo Limestone is the surface rock of the Upper Keys, and the Miami Limestone covers the Lower Keys. A contact found at Big Pine Key shows that the oölitic limestone overlaps the Key Largo, and core borings show that the Key Largo underlies the oölitic cover for the entire area of the Lower Keys. The Key Largo, therefore, extends for the total length of the Keys. A core placed at Dry Tortugas, 70 miles west of Key West, encountered it at 30 feet below sea level. The Key Largo has a maximum thickness of over 200 feet.

The fact that the Keys are located about 5 miles from the seaward edge of the Florida coral reef platform creates an interesting problem. They may be (1) remnants of an ancient outer reef similar to that found today bordering the living coral reef tract, or (2) they may have become established as a line of patch reefs in the back reef zone of a pre-existing platform which extended seaward some distance, possibly as far as the present platform. It is believed that the answer to this problem can be found through an understanding of what was happening seaward and leeward of the old reef at the time its corals were flourishing. Consequently, the composition and structure of the reef platform seaward of the Keys and the geologic history of the rocks of the Florida mainland immediately behind the Keys were studied. Information thus obtained plus observations on the composition, especially the coral content, of the Keys themselves have led the authors to believe that the Keys were formed as a line of patch reefs in a back reef area which was bordered on its seaward edge by an outer reef which has since been lowered, chiefly by erosion, and covered by more recent material.

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