Coquina and Tabby: Building Materials from the Sea

Children often build basic sandcastles at the shore with wet sand. Near St. Augustine, Florida, the oldest city in the country, a real fortress made from material related to sand still stands today. The Castillo de San Marcos is the oldest stone fort in the continental United States. It began around 1695 and stands on the western shore of Matanzas Bay.

The fort’s walls consist mainly of coquina, a form of natural concrete from the sea. Coquina is an uncommon limestone made from fragments of shells belonging to ancient mollusks and other sea animals. Minerals from those shells eventually bind the pieces into solid rock. The same name applies to a small clam common on Florida beaches, and the colors of its shells appear in the fort.

As a construction material, coquina is light and easy to obtain because it is native to the Florida coast. It is simple to shape and highly resistant to damage. It can stop bullets and is almost proof against cannon balls. Many tiny air pockets allow it to compress easily, so cannon balls aimed at the fort often became stuck in the walls instead of breaking through. This property helps explain why the fort remains standing after 300 years.

Coquina can be seen in its natural condition at a state park about 30 miles south of St. Augustine. A similar material called tabby is coquina made by people. It is produced by combining lime from burned oyster shells with sand, water, ash, and more shells. Starting in the 1600s, Spanish and English settlers used tabby to build houses, other buildings, and roads in the coastal areas of the Carolinas, Georgia, and Florida. Several tabby structures still survive, including Fort Pulaski in Georgia, and they demonstrate the inventiveness of early settlers and the resources of the ocean.

Why was coquina a useful material for building the fort and other coastal structures?