Coquina is a sedimentary rock composed mostly of seashell material which becomes cemented over time.  I found this sample serving as a wall on St George Street in St Augustine, Florida.  My post last week featured the Castillo de San Marcos fort, which was originally constructed out of coquina. 

Fort builders favored conquina stonework because of its ability to absorb cannonball impacts.  In fact, often an embedded cannonball was recovered and then fired back at attacking forces.  For the best viewing experience, click to view a high-resolution version.

Cheers,

C. S.