Castillo de San Marcos and the Stone That Swallowed Cannonballs
Near St. Augustine, Florida
The Castillo de San Marcos is the oldest masonry fort in the continental United States, and it owes its survival to a humble building material: coquina. For more than a century the Spanish had defended Saint Augustine with a succession of nine wooden forts, and every one of them rotted or burned. After the English privateer Robert Searles sacked the town in sixteen sixty-eight, Spain finally committed to stone.
Construction began in sixteen seventy-two and finished twenty-three years later, in sixteen ninety-five. Coquina, Spanish for little shells, is a soft rock of seashells cemented together by nature. It was quarried on Anastasia Island, just across the bay, and ferried over.
Its softness turned out to be its genius. When enemy cannonballs struck the walls, they did not shatter the stone. Instead they sank in and stopped, as if the fort were swallowing them.
Soldiers later dug the iron balls back out of the walls. The Castillo proved it twice. In seventeen oh two, Carolina's governor James Moore besieged the town for two months.
His guns could not breach the walls, and a Spanish relief fleet from Havana forced him to burn his own ships and flee overland. In seventeen forty, the British general James Oglethorpe bombarded the fort for twenty-seven days while some thirteen hundred residents sheltered inside. He too gave up.
The fort was never taken by force, under any flag.
