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New Uses for Old Rock

  • March 14, 2015
  • By KIDS DISCOVER

Built in the 1600s by the Spanish, Florida’s Castillo de San Marcos is the oldest masonry fort in the U.S. It’s made of coquina (“little shells”), a rare limestone with tiny air pockets that provided a unique defense: Cannonballs would get stuck in the walls without breaking them. (Diane Uhley / Shutterstock)

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