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Coquina Rock
| Article
# : |
12398 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
1 / 1987 |
596 Words |
| Author
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Pamela G. Miller-McDonald
Pamela G. Miller-McDonald is a free-lance photographer
currently living in Elizabethtown, Pennsylvania. |
Coquina rock is a buff-colored limestone, abundant on the beaches near St. Augustine, Florida. One of the main components of coquina rock is a small mollusk, Donax variabilis, commonly called the coquina clam. This bivalve, though very small, is an edible shellfish making its home on the tide lines from New York to Florida and Northern Mexico. While coquina rock contains significant amounts of Donax variabilis, sand is the other basic component, and some deposits contain a small amount of coral.
During the Pleistocene era, Donax grew by the billions in the warm sea. For centuries, shells accumulated as a beach deposit, and through the action of plate tectonics, these deposits were elevated and subsequently exposed to weathering. Rain, containing dissolved carbonic acid and soil acids, percolated through the loose shells and sand slowly dissolving the lime carbonate from the deposit. When the water evaporated, the shells and sand were bound together by this natural lime cement into a compact matrix.
Coquina rock has always been a part of northern Florida's culture. The earliest settlers of North America, the Spanish, used it as a building material. They named it coquina, which is Spanish for shell or shell-life. The Spanish found that when it was exposed to air, it hardened and could be cut into blocks with a saw or an ax.
Coquina deposits are usually found along Atlantic beaches on a slight ridge behind the line where the vegetation and sand meet. Historically, one of the main sources of coquina was several quarries on Anastasia Island, a 15-mile-long barrier island three-quarters of a mile off the coast of St. Augustine.
St.Augustine's oldest fort, Castillo de San Marcos, was constructed in 1672, with coquina rock from one of these quarries. The Castillo, the oldest masonry for still standing in the continental United States, was virtually impregnable. Coquina rock, being a limestone, has air pockets, and unlike wood, stone, or brick structures does not completely shatter under cannonball bombardment.
The rock had another advantage. It replaced vulnerable wood that could be burned, or would rot in the humid climate. Many other early structures made of coquina rock such as cathedrals, the city gates of St. Augustine, homes and public buildings continue in use today.
Although coquina rock is a durable and beautiful building material,
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