Coquina (Dense)
“Compacted shell concrete”
Dense coquina is a well-cemented variety of coquina where shell fragments have been strongly lithified into a durable building stone. While regular coquina is soft and porous, this more compacted form has been used in remarkable fortifications that could absorb cannonball impacts without shattering.
aragonite and calcite shell fragments, calcite cement, quartz sand (minor)
St. Augustine (Florida, USA), Bahamas, eastern Australia, coastal Brazil, Mediterranean coasts
3-4
Castillo de San Marcos in St. Augustine is built from coquina — during British cannon attacks in 1702, the cannonballs simply embedded in the soft shell rock instead of shattering the walls.
Test Your Knowledge
Why was coquina effective as a fort-building material against cannon fire?
Photo Attribution
Related Specimens
Want to identify rocks yourself?