Thousands of years ago, the tiny coquina clam donax variabilis flourished in the shallow waters of coastal Florida. The small pink, lavender, yellow, or white shells people see along the beach there are the same type of shells that protected the city hundreds of years ago. The Spanish people who built the fort knew it had powerful qualities that would make an excellent fort. When the clam died, the shells accumulate in layers, getting thicker year after year, century after century, eventually forming submerged layers of walls several feet thick. Over time, the shells became covered with soil and trees. Rain water, dead vegetation and soil created carbon dioxide, then becoming carbonic acid. When the acid ran downward, it dissolved the calcium in the shells. This produced calcium carbonate. This calcium solidified in the lower layers, similar to the way caves are formed. It essentially “glued” the shell fragments together into a porous type of limestone making coquina (Spanish for tiny shell).

Built by the Spanish to defend Florida and the Atlantic trade route, Castillo de San Marcos is the oldest masonry fortification in the United States. And it is built out of seashells! The seashells held up under numerous attacks including three days under cannon fire. The coquina stone is only found in a couple places in the world. Conditions had to be perfect for coquina formation along the east coast of Florida. The Spanish knew about this rock and used it to their advantage.  

I imagine a soldier picking up a chunk of it and saying:

“This is a magic rock right here.”

“What do you mean, it is a magic rock?”

“I heard they are building the fort out of it.”

“I heard that too because it is stronger than wood but why do you think it is magic?”

“A fort built out these seashells will last for hundreds of years.”

“I don’t know about that. I don’t think it will hold up when it is hit by a canon.”

“You are mistaken my friend.”

The people of St. Augustine were primarily soldiers, not stonemasons, so for many years the rock sat on the shorelines unused.

“The problem is that people don’t appreciate this rock or see what it can do.”

“I think we should just build a wooden fort…”

“That right there is the problem with a lot of people, they have no foresight.”

“Wood is everywhere and it is easier to work with.”

“At first maybe but we need more than a wooden fort to protect St. Augustine and to keep the British from taking over Florida.”

The British settled to the north, in the Carolinas so Spanish Florida was only a short sail away. The British wanted to use St. Augustine as a base of operations to attack the Spanish treasure fleets and colonies of the Spanish Caribbean. The Spanish had to defend themselves so they began construction on the Castillo de San Marcos in 1672. The coquina stone was quarried at the nearby present-day Anastasia State Park on Anastasia Island. Military engineers and stonemasons were brought from Spain to help with the construction.

“This is amazing,” I imagine one engineer saying.

“You can only find this in a couple places in the world so this might be the strongest fort ever built.”

Convicts and additional soldiers were brought from Cuba to help with the construction. 

“I don’t see why they had to bring in these guys.”

“Let’s have them handle the oyster shells.”

“You two over there, you need to get the oyster shells and burn them into lime.”

“Sir?”

“You heard me. After you burn the shells, mix it with sand and water to make mortar.”

“Yes, sir.”

The walls rose, one by one. No one had ever built a fort or a building out of coquina so most people had no idea if the walls would stay in place.

“They will stay. These walls will still be standing hundreds of years after we die.”

“I hope you are right.”

“I am right,” a stonemason said to a couple soldiers building the fort.

“I don’t know how seashells are going to last under canon fire?”

“You don’t know anything about these shells,” the stonemason said.

They built the walls an average of 12 feet high but the thickness is what is unique about them; the walls on the ocean side are19 feet thick. In 1695, the first phase of construction was completed. Several years later, Governor James Moore of Charleston led his English forces against St. Augustine. He captured the town and set his cannon up amongst the houses to bombard the fortress. 

“PREPARE YOURSELF SOLDIERS!”

“Yes, sir!”

“This fort may go down,” the sergeant said.

“It is made out of seashells,” a soldier said in a sarcastic tone.

“I know, who’s idea what that?

“I don’t know but right now I am really wishing their made this fort out of wood.”

Then a strange thing happened that most of the soldiers did not expect. Instead of the fort shattering like most of the men anticipated, the coquina stone compressed, absorbing the shock of the cannon. 

“Woo-hoo!”

“Take that!”

“You thought you could take us down!”

“This fort has been blessed.”

“The shells are magic!”

“You guys are idiots. Someone knew what they were doing when they built this,” the sergeant said.

“Sarg, you sure about that?”

“No,” he said and the all started laughing in enjoyment of the entire situation.

The cannon balls bounced off the fort like they were bouncy balls. The few that did actually penetrate the walls, only sank in a few inches. 

“The shell rock actually worked!”

“Look the cannon are bouncing off the walls! I can’t believe it,” a soldier said.

Decades later, in 1740, General Oglethorpe tried to take down St. Augustine. His troops tried to attack the Castillo for 27 days and once again the walls held firm. The idea to use the seashells as a building material turned out to be brilliant. Years later when the Spanish fortified the southern approaches to St. Augustine by building Fort Matanzas they also used coquina stone like the Castillo. Only the Castillo de San Marcos survived the destruction of the city. Castillo de San Marcos is now a National Monument preserves and interprets more than 450 years of cultural intersections.

One can say that if it wasn’t for coquina, the British would have captured St. Augustine earlier than 1763.  They gained Florida by treaty at that time but if the British gained Florida earlier, who knows what would have happened?  It could have changed the course of history and the American Revolution. I can’t imagine America part of Great Britain as a commonwealth. All hail the coquina, the rock that saved St. Augustine.

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