Coquina

Admittedly, coquina is not a Potomac River area rock but having found myself in St. Augustine, Florida recently, and encountering this interesting stone I thought it might write something about it.

The National Park Service has a free brochure which does an excellent job of describing how this stone is formed so I will quote them:

“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 clams 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 the air and rain. Eventually the shells 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”. (National Park Service, Castillo de Dan Marcos National Monument)

The most interesting structure the early Spanish settlers in St. Augustine constructed from coquina is the Castillo de San Marcos a rather huge fortress begun in 1672 and which has successfully protected the city since 1695, more than 300 years! Having no experience with this building material, the Spanish “over built” the walls which are an average of 12 feet thick!

In 1702 the British attached the fort and to the delight of its defenders, cannon balls simply bounced off the walls or were harmlessly embedded in them. The millions of air pockets in the coquina, similar to styrofoam, dissipated the force of the cannon balls rendering them impotent.