COQUINAS

Peninsular Florida was formed about 180 million years ago (ma) when the super continent Pangea broke apart and formed the continents North America and South America/Africa.  Since then Florida has been underwater many times as the global climate alternated between ice ages and global warming. Sea levels were as much as 150 feet higher than today. The most recent time Florida was underwater was about 2.5 ma.

As a result, layers of coral, shellfish and fish skeletons piled up creating a limestone base hundreds (in some places thousands) of feet thick. Sand and clay from land erosion were mixed in.

This history is quite obvious when observing any excavation or deep grading that occurs when new housing developments are started. Once through the thin top soil layer, shells of all sizes and shapes are exposed and available for the picking. These shells must be at least 2.5 million years old, and probably much older!

One such development is under construction about a mile from my home. Deep “borrow pits” are used to supply the fill to raise the level of home sites for good drainage. (The pits will eventually fill with rain water and become lakes in the community) One pit is exceptionally deep at about 25 feet below the surface. I thought it would be interesting to go explore the exposed sub soil and see what I could find and identify. My thinking is that the deeper the hole, the further back in time the sediment was deposited.

Borrow Pit

In some areas the color of the sediment was brown and looked like a clay based soil. There were no rocks.  In other areas, where shells predominated, the color was almost white.  Most of the shells were very small or tiny fragments of shells. It was here where I decided to concentrate my search. I was not to be disappointed.

Sea shell is the common name for the exoskeleton of animals belonging to the phylum Mollusca (or mollusks). With over 85,000 species known, it is the largest marine phylum.   The two most common classes of mollusks are the univalves (single shells) or snails, and bivalves (two shells) such as clams, oysters, and scallops. The beauty and charm of these shells belies the very aggressive nature of many of these mollusks when alive. Some will pry open the shell of its prey, or drill a hole in the shell and suck out the living victim’s flesh. Many have highly toxic poisons that paralyze their prey. Although rare, humans have been known to die from mollusk stings.

The most common bivalve in Florida is the coquina. It is a small, rarely larger than ½ inch, beautifully colored clam which can be found live at any beach, by digging in the sand as a wave retreats. They are quite edible and considered a treat by many, although it would take dozens to make even an appetizer portion. As at the beach, these tiny mollusks proliferated the exposed loose subsoil of the construction dig.

 

I was able to collect and identify 10 different species of shells and one nice sized coquina rock. I brought them home and photographed them.

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Alphabet Cone

Branham's Tulip

Florida Crown Conch

Lightning Welk

Milk Conch

Shark's Eye

Quahog Clam

Venus Clams

The term “coquina” also refers to a sedimentary rock composed primarily of shell and shell fragments. These limestone rocks vary in hardness from loose packed to building quality solidity. They are used in landscaping, erosion prevention fill and beach nourishment.

Coquinas at Caspersen Beach

It was also my objective today to harvest one of these smaller rocks from the pit, bring it home, and crack it open to discover ancient remains of who knows what…. maybe an extinct trilobite fossil or two. (Tribolites went extinct 250 ma, so that would be extremely unlikely.)

Coquina Rock

Splitting this coquina rock turned out to be harder than I planned. After attempting a hammer and chisel, I took the rock outside, put it on the ground, and hit it repeatedly with a 2 pound maul. I was able to split off pieces. What hidden ancient marine treasures did I find? Unfortunately, none. The inside of this coquina looked exactly like the outside. Small shells and shell pieces glued together by organic sediment.

Splitting a Coquina Rock

The process of building shell and limestone layers on the sea floor is ongoing today. The main chemical of this deposition is Calcium Carbonate, CaCO3. Carbon is being trapped in the shells of untold trillions of mollusks and permanently stored in this sediment. This is one of the few permanent carbon sinks in the world today and helps to balance the increasing carbon emission sources. The protection of our oceans is critical. Mollusks exist only in the relatively shallow waters near land masses. And these are the same waters that are most susceptible to pollution.  We need international treaties to stop the poisonous runoff and discharges that are rampant in the developing and newly industrialized countries. 

 It is amazing to think that these lowly mollusks play such an important part in the world's ecological balance.