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Barrier Islands

All along much of the Atlantic coast of North America stretch barrier islands, long, narrow strips of sand built up over time by ocean currents, waves, and wind. Anastasia Island and Rattlesnake Island, where Fort Matanzas is located, are both barrier islands.

Facing the ocean are sand dunes covered by hardy plants such as sea oats, railroad vine, and beach sunflower. The deep roots of sea oats and other grasses help hold the dunes in place. This is why people should not pick these plants.

Just behind the dunes is an area of coastal scrub consisting of other salt-tolerant plants such as scrub live oak, wax myrtle, sabal palm, and saw palmetto.

The coastal forest or hammock, is located in the interior of the island. (It is thought that the word "hammock" comes from an Indian word for "shady place.") One of the primary plants of the hammock is the live oak tree (Quercus virginiana), which can be seen growing in the park's picnic area and along the nature trail.

The Fort Matanzas nature trail winds through a coastal hammock. Look for the remnants of old dunes now covered by shade-loving plants like the live oak, sweet bay, eastern red cedar and shrubs like wax myrtle, yaupon holly and paw paw.

On the back side of the barrier island is the salt marsh, a low, wet area flooded twice a day by the tides. The primary plants are spartina grass, sea ox-eye

Oak Tree

daisy, and glasswort. This is the home of many types of crabs, oysters, and clams and serves as a "nursery" for the young of many types of fish.

Besides providing habitat for wildlife, the barrier islands are important in another way. They protect the mainland from the direct force of storms and hurricanes.

The sloping beaches and sandy dunes absorb the energy of the waves like a coquina fort absorbs the shock of a cannon ball hit. The sand gives and shifts, the energy is dispersed, and the mainland receives less of the storm's force.

The thick vegetation of the scrub and hammock breaks the force of the wind, thus further protecting the mainland from the storm. The marshes act like sponges and absorb rising flood water.

At one time, a thick hammock ran the whole length of Anastasia Island. Where are all the trees now? As more people move to the coast, as more marshes are filled in, as more trees are cut down, as people make roads and paths through the dunes, how well will the barrier islands be able to protect the animals which call them home. How well will the barrier islands be able to protect the mainland from the next big storm?

Barrier Island

The Changing Landscape of Matanzas

Barrier Islands move. After all, they are made of sand and are shaped by wind and waves.

Currents move along the east coast of Florida from the north to the south. These currents gradually wear away the north end of the islands and deposit the sand at the south end, where the tidal current moving out through the inlet, between two barrier islands, stops the flow of the longshore current. Storms, especially hurricanes and nor'easters, pound the shoreline, sometimes making new inlets or over-washing the island. Tidal currents often form hooks and sandbars at inlets where there are no jetties or seawalls.

This is what has happened at the Matanzas Inlet, the only "natural" inlet left on the east coast of Florida. The inlet is not dredged or marked. There are no groins or jetties, and the sand comes and goes as nature wills. Every year the hook on the inside of the inlet grows a little, and the island creeps south.

We know by comparing charts from the present time with those from the mid-1700s that over the past two and a half centuries, Anastasia Island has "migrated" south, moving the inlet about 1/2 mile (650 meters) south of where it was when Fort Matanzas was built in 1740-1742. Standing on the gun deck of Fort Matanzas, a soldier would be looking directly out into the ocean. (See drawings below)

It's a part of nature for these islands to move. Often, however, people try to control the movement of barrier islands by building jetties at inlets or piers or groins vertical to the shore. These might catch the sand drifting down from the north, but then the shoreline south of the jetty is starved for sand and begins to wash away. This is what is happening at the north end of Anastasia Island and what happened at Cape Hatteras, North Carolina making it necessary to move the lighthouse as the ocean washed closer.

Progression of Anastasia Island


 
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