| El Castillo de San Marcos St. Augustine, FL |
| One of the most amazing things about El Castillo de San Marcos is that its construction began in 1695 and is almost entirely made out of Coquina, a soft porous limestone, composed essentially of shells and coral, used as a building material. The Castillo still stands today. |
| This is a close-up of the Coquina. You can see the various layers of sea shells. |
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| Birds-eye view maps of El Castillo de San Marcos. |