Commemorated in 1924, Fort Matanzas National Monument is a United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 National Monument run by the National Park Service

National Park Service

The National Park Service is the U.S. federal agency that manages all national parks, many national monuments, and other conservation and historical properties with various title designations...

. The Monument consists of a 1740 Spanish fort, Fort Matanzas, and about 100 acres (0.4 km²) of salt marsh

Salt marsh

A salt marsh is an environment in the upper coastal intertidal zone between land and salt water or brackish water, it is dominated by dense stands of halophytic plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh...

 and barrier island

Barrier island

Barrier islands, a coastal landform and a type of barrier system, are relatively narrow strips of sand that parallel the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen...

s along the Matanzas River

Matanzas River

Matanzas River is a body of water located in St. Johns and Flagler counties in Florida. It is a narrow saltwater bar-bounded estuary sheltered from the Atlantic Ocean by Anastasia Island....

 on the northern Atlantic coast of Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

. It is operated by the Park Service in conjunction with the Castillo de San Marcos National Monument

Castillo de San Marcos

The Castillo de San Marcos site is the oldest masonry fort in the United States. It is located in the city of St. Augustine, Florida. Construction was begun in 1672 by the Spanish when Florida was a Spanish territory. During the twenty year period of British possession from 1763 until 1784, the...

 and several sites in the city of St. Augustine

St. Augustine, Florida

St. Augustine is a city in the northeast section of Florida and the county seat of St. Johns County, Florida, United States. Founded in 1565 by Spanish explorer and admiral Pedro Menéndez de Avilés, it is the oldest continuously occupied European-established city and port in the continental United...

.

Fort Matanzas guards Matanzas Inlet

Matanzas Inlet

Matanzas Inlet is a channel in Florida between barrier islands connecting the Atlantic Ocean and the south end of the Matanzas River. It is south of St. Augustine, in the southern part of St. Johns County, at coordinates...

, the southern mouth of the Matanzas River

Matanzas River

Matanzas River is a body of water located in St. Johns and Flagler counties in Florida. It is a narrow saltwater bar-bounded estuary sheltered from the Atlantic Ocean by Anastasia Island....

, which can be used as a rear entrance to the city of St.