A Fort Made Out of Seashells?
Who would think that a fort made of seashells would last 300 years? Who would think that a fort made out of seashells would last three days under cannon fire? But the Castillo de San Marcos, made of local coquina stone, did just that. What exactly is this strange rock? How was it formed, and where did it come from? And how did this rock shape the history of St. Augustine?
How Coquina is Formed
Thousands of years ago, the tiny coquina clam donax variabilis lived in the shallow waters of coastal Florida, as they still do today. These are the small pink, lavender, yellow, or white shells one sees along the beach at the waterline. As the resident clam died, the shells accumulated in layers, year after year, century after century, for thousands of years, forming submerged deposits several feet thick. During the last ice age, sea levels dropped, exposing these shell layers to air and rain. Eventually, the shell became covered with soil, then trees and other vegetation. Rain water percolating through the dead vegetation and soil picked up carbon dioxide and became carbonic acid, the same ingredient that makes soda fizz.
As this weak acid soaked downward, it dissolved some of the calcium in the shells, producing calcium carbonate, which solidified in lower layers, much like how flowstone and stalactites are formed in caves. This material "glued" the shell fragments together into a porous type of limestone we now call coquina, which is Spanish for "tiny shell".