Today's St. Marks evidently has its roots in American commercial activity that took hold beneath the walls of the fort upon acquisition of Spanish Florida by the U.S. in 1821—before the settlement moved slightly up the St. Marks River to the present position. Various articles in publications like "Florida Historical Quarterly" relate how the fort site later held a government "naval" hospital to meet yellow fever emergencies in the merchant marine. And just afterward Confederate batteries were established on the site in the Civil War. Their earthworks remain and are interpreted in the historic state park. But the site also exhibits old Spanish stonework, and not far away (though inaccessible), just down St. Marks River are shallow Spanish quarries where this limestone was evidently obtained in the 1730s.