It is formed from "incompletely consolidated sedimentary rock...formed of billions of small clam-like seashell, called Coquina, or cockleshell. Overall composition of most Coquina rock is a mixture of these and other shells"
According to www.coquinarock.com, it is geologically known as Donak Variabilis.
According to www.coquinarock.com/geology.php, Since "Coquina rock, as it exists today, was formed along the East Coast of Florida, formations can be found near the coast, from Palm Beach, Florida, then Northward to South of Jacksonville, Florida. Coquina rock can be found as far as 20 miles inland, and most deposits follow the Eastern Coast, and along the now I-95 corridor."
Spanish colonists were the first to come to the US, landing in Saint Augustine, the oldest city/settlement in the country. The Spanish used coquina to build their fort, the "Castillo de San Marcos". Since they were the first to find and utilize coquina, it should come as no surprise that "coquina" is a spanish word that means “cockle shell” or “tiny shell”.
As if the Spanish had any other options, coquina was the only stone accessible on Florida's coast, but what they feared would be a disadvantage, turned out to be an enormous benefit. Due to it's conglomerate mixture and "light and porous nature" it's millions of "microscopic air pockets" makes it compressible; "cannon balls fired at the walls of the Castillo burrowed their way into the rock and stuck there, much like a bb would if fired into Styrofoam. So the thick coquina walls absorbed or deflected projectiles rather than yielding to them, providing a surprisingly long-lived fortress" (www.nps.gov).
I've been to the castillo, these are some of the pictures I took...