Seashells against Cannonballs!

On a trip to Florida, I visited the Washington Oaks Gardens State Park to check out the coquina rock formation on the beach. This type of rock, whose name is derived from the Spanish name for “shell”, is only found in a few places around the world, but it is plentiful in Florida. What makes this rock unique is that it is made up of millions of tiny shells that have been cemented together. If you inspect the rock you can easily make out the individual shells.

Outcrop of Coquina Rocks on the beach (left) and closeup of the coquina rock (right)

The way this rock was made is that over thousands of years the small mollusks that made the shells died and through wave action the shells got sorted and deposited in ever deepening layers. When the sea receded, the layers of shells were exposed and rain percolating through the top layers dissolved some of the calcium in the shells and deposited it in the deeper layers gluing the shells together and forming the rock.

Waves crashing against the rocks (left) and canyon-like appearance of the rock outcrop (right)

The action of the waves erodes some areas of the rocks in a circular fashion creating pools of sea water. As the erosion of the rock progresses the bottom of the pools is breached, and this gives rise to round holes in the rocks. 

Water pool created by erosion (left) and circular orifice in the rock created from a collapsed water pool (right)

Besides its geological characteristics, the coquina rock in Florida is also interesting for historical reasons. Spain had founded the city of Saint Augustine on the East Florida coast in 1565. By the 1600s the Spaniards began fearing the British would invade the city, and they decided to construct a fort that still stands to this day called “Castillo San Marcos”. As the only rock they could find in the area was coquina, the Spaniards erected the fort employing this rock. So essentially the Spaniards built a fort made of shell fragments glued together by erosion to ward off a British invasion!

Castle walls (left) and cannon firing demonstration (right)

View of the castle walls (left) and view out of one of the sentry towers (right). You can see the shells in the rock.

The British attacked Saint Augustine on 2 occasions, in 1702 and 1740. They subjected the castle to heavy cannonball fire trying to breach its walls, but to their surprise, the castle walls did not shatter when hit by the cannonballs. Rather the cannonballs either bounced off the walls or were absorbed into the coquina rock and the castle held its ground against the invaders. It was only in 1763 that the British gained St. Augustine (and all of Florida) as a result of the treaty of Paris.
 
So what happened? How could rocks made out of glued seashells resist cannonballs fired at them?
 
The impact response of coquina has been studied. Because of its make up, coquina is a very porous rock that deforms easily when something impacts against it. While other rocks experience brittle fractures and shatter as a result of an impact, coquina has a high capacity to absorb energy from projectile impacts. And this ability has not merely allowed the Castillo San Marcos to resist the British sieges, but it has also allowed the fort to resist hurricanes and tropical storms, and remain standing for more than 300 years.
 
Not bad for a rock made out of glued seashells, eh?

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The photographs are by the author and they can be used with permission.