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Barrier Islands

Barrier spits and islands are long, narrow landforms off the coast of the United States that are separated from the mainland by bays or lagoons. They formed near the end of the last Ice Age, as the rising seas winnowed away the cliffs, headlands and river deltas along the coast and inundated previously dry areas. 

Barriers are found along the Atlantic coast, the Gulf of Mexico coast, and in certain regions of the Alaskan coast. The presence of significant amounts of sediment, in concert with wave and current action and stable sea level, results in the formation of barriers in many coastal regions.

Barrier systems along the northeast coast of the United States formed by the erosion of sea cliffs and the accretion, or deposition, of that sediment in open water. For example, Fenwick Island and Assateague Island developed through this process. In their particular case, the growing Atlantic Ocean eroded sediment from the Delaware headlands, which was deposited by currents as an elongated body of sand.

These sediment-moving currents are known as longshore currents because they move along the coast, parallel to the shore. The process of sediment traveling in longshore currents is known as longshore sediment transport. Longshore sediment transport is responsible for the formation of many barriers along the Atlantic Coast.

Sea-level changes after the last Ice Age caused barriers to develop in other ways as well. Along the southern Atlantic coast, rising sea waters flooded coasts and river valleys,separating the tallest dunes from the mainland and reworking river deltas. The isolated dunes remained as barrier islands. The ocean also moved and shaped relict delta sediments into barriers, such as those found along the capes of the Carolinas.

Coastal Conditions

Barriers require large amounts of sediment and stable sea level to take shape. The Atlantic coastal plain is very large and contains many rivers that carried sediment to the coast in the period of lowered sea level during the last Ice Age. This sediment, along with that eroded from headlands and sea cliffs, provided extensive material for barrier formation. Because the continental shelf along the Atlantic coast is wide and not very steep, these sediments remained in the coastal zone and could be shaped by ocean waves and currents.

When sea level was rising rapidly about 15,000 years ago, the waters moved too quickly to form barriers. Sea-level rise stabilized about 3,000 years ago. This allowed sediments to be shaped into barriers without being swept out to deeper water. 

The shape of the Atlantic coast, as well as the rate of sea-level rise after the Ice Age, therefore determined the formation of barrier spits in this region. 

Off the coast of west central Florida, barriers enclose mangrove marsh areas, which progressively dominate the northern coast of western Florida. Farther northwest, where the coastal trend changes from north-south to east-west along the Florida Panhandle and Alabama coast, barriers re-develop due to the sediment available from Ochlockonee Bay. The quartz sand in the area drains from the Appalachian Mountains, and waves and currents re-work the sand into barriers that are separated by inlets.

West of the Mississippi River delta, different barriers have formed along the muddy coast. Offshore of western Louisiana are cheniers, which are barriers that form when waves push sandy or shelly sediment landward onto a marsh surface. The result is a low-lying feature with ridges of sand and silt overlaying marshes and lakes, blurring the distinction between coast and ocean. The Mississippi River provides the sediment to form these unique coastal features.

In the western Gulf of Mexico, long, sandy barriers form once again, including Padre Island and Mustang Island along the coast of eastern Texas. The barriers in this area feature large dune fields where wind shapes the sandy beaches into tall ridges.

Barriers along the Gulf coast are composed of different sediments and formed by a variety of mechanisms, but all are influenced by the large amount of sediment, relatively low wave energy and broad continental shelf found in the Gulf region.

Gulf Coral Coast

Live coral reefs surround the Florida Keys in southwestern Florida. Hardbottom habitats in this area feature rock colonized by calcifying algae, octocorals, stony corals and sponges. Patch reefs dominated by stony corals are found near Key Largo, Key West and Elliott Key. Bank reefs, also know as forereefs, extend seaward from other reef types.

Located immediately west of the Florida, the Tortugas Banks are extensive coral reefs growing on a foundation of Pleistocene-era limestone. These reefs have a high degree of coral cover but low coral diversity.

More reef areas are found on the West Florida Shelf in the Gulf of Mexico, extending west and north of the Florida Keys and the peninsula. Pulley Ridge is a rocky underwater feature located west of the Keys that is colonized by a vibrant reef ecosystem. Farther north are the Florida Middle Grounds, which comprise carbonate ledges westward of the bend in Florida's coastline. The Florida Middle Grounds are the northernmost coral reefs in the continental U.S. They support algae, sponges, urchins and many species of coral and fish.

Mangrove Coast

Mangrove coasts are those where mangrove plants have rooted in the shallow water of bays, and sediments around their roots have built up to sea level, thus extending the coast. Mangroves include several species of low trees and shrubs that thrive in the warm, shallow, saltwater environments of the lower latitudes.

Mangroves have the ability to form unique intertidal forests that are characterized by dense entangled networks of arched roots, called prop roots, that facilitate trapping of fine sediments, thereby promoting accretion and the development of marshlands. The prop roots and upright exposed roots called pneumatophores also allow the plants to withstand occasional wave action and allow oxygen to reach the roots in anaerobic soils.

The most extensive mangrove coast in the United States is the southwest shore of Florida in the Everglades National Park and the region of Florida Bay. Mangrove islands and marshes continue at intervals along the western coast of Florida. Barriers replace the mangrove coast along the Florida Panhandle and Alabama coast, but farther west mangrove coastal areas and swamps redevelop in the Mississippi River delta region.

Gulf Delta Coast

The Mississippi River has built a series of deltas over time, resulting in overlapping lobes of deltaic sediment. The current delta, the Balize, or "Bird Foot," is approximately 1,000 years old. This delta is southeast of New Orleans, but the river is slowly shifting its course to the Atchafalaya distributory. This long-term process of river mouth shifting has taken place repeatedly in the Holocene period.

Most of the greater Mississippi River delta consists of marshland and mud flats with numerous shallow lakes and intertwining channels. The marsh features aquatic plants and an extensive amount of waterfowl. The large amount of sediment from the river has influenced the growth of barriers islands and cheniers. Mangrove coasts and marshes are also common in this area.

Barrier Formation

Barrier formation
Source: National Park Service

Maryland's Atlantic Coast

For most of their existence, Fenwick Island and Assateague Island on the Atlantic Coast were connected as one barrier split, which is a barrier attached to the mainland.

In 1933, however, they were separated by a hurricane. Fenwick Island remains connected to the Delmarva Peninsula and is technically still a barrier spit.

Assateague Island is a true barrier island.

Gulf Coast Barriers

The low wave energy conditions in the Gulf of Mexico allowed sediment to be shaped into the extensive barriers found along the coast. The many rivers draining into the Gulf, particularly the Mississippi River, provide great quantities of sediment, including sand, silt and mud. The white barrier beaches of the Florida Panhandle, the marshy cheniers in Louisiana and the wind-swept islands along the Texas coast are the result of these conditions.

Coastal Plain

The coastal plain is the horizontal or gently sloping strata of sediments generally representing a strip of sea bottom that emerged in recent geological time and may extend many kilometers inland.