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Did you know?
The Castillo de San Marcos in Florida is the oldest masonry fort in the US. It was built by the Spanish in 1672, and it is made of a stone called coquina, which consists of ancient shells that bonded together to form a sedimentary rock much like limestone.
![Picture The Castillo de San Marcos is Built of Stones Made of Ancient Shells](https://unbelievable-facts.com/wp-content/uploads/2022/05/Castillo-de-San-Marcos-450x300-c-default.jpg)
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