Fort Matanzas National Monument: A Spanish Watchtower Built of Coquina

From the National Park Service's Geodiversity Atlas.

Fort Matanzas National Monument lies 23 km (14 mi) south of St. Augustine on Florida's northeast Atlantic coast, in St. Johns County. Proclaimed on October 15, 1924, it covers nearly 121 hectares (300 acres) and preserves Fort Matanzas, an 18th-century Spanish fortified watchtower.

Fort Matanzas. NPS.

The fort

The monument has two parts on either side of the Matanzas River: the southern tip of Anastasia Island to the east, and the northern end of Rattlesnake Island to the west, where the fort stands.

The Spanish built Fort Matanzas from 1740 to 1742. It is the best-preserved part of the outer defences that once guarded St. Augustine from enemies to the south. As well as keeping enemy ships away from the city, it secured Matanzas Inlet, and served as a rest stop, a coast guard station, and a place where ships bound for St. Augustine could get advice on navigating the river.

The geology

The monument lies in the Atlantic Coastal Plain, a region of fairly flat land that steps gently down to the sea in a series of well-developed marine terraces. Each terrace marks an old shoreline, formed as sea level rose and fell from the Pleistocene to the present.

• Rocks: Quaternary deposits — the Pleistocene Anastasia Formation and undivided Holocene sediments.
• Settings they record: beaches, swamps and dunes.

Fort Matanzas is built of coquina quarried from the Anastasia Formation, a rock full of fossil shells. The nearby Castillo de San Marcos is built of the same stone.

A cannon on Fort Matanzas's gun deck, looking over the marshes. NPS.

Region, maps and reports

The monument is part of the Coastal Plain province and shares its geologic history with a much wider region. The park's own pages describe its beaches and its wetlands, marshes and swamps.

Sources

• National Park Service, "NPS Geodiversity Atlas—Fort Matanzas National Monument, Florida." https://www.nps.gov/articles/nps-geodiversity-atlas-fort-matanzas-national-monument-florida.htm — which cites Colquhoun (1969), National Park Service (2016d), Richards and others (1971) and Scott and others (1986).
• Rewritten in hubnx's own words.