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Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few
island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atoll An atoll (), sometimes known as a coral atoll, i ...

island
s to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where
wetland A wetland is a distinct ecosystem that is flooded by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in the soils. ...

wetland
s may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal
inlet An inlet is an indentation of a shoreline, usually long and narrow, such as a small bay or arm, that often leads to an enclosed body of water, body of salt water, such as a Sound (geography), sound, bay, lagoon, or marsh. Overview In sea coasts, ...

inlet
s that separate the islands, the longest and widest being
Padre Island Padre Island is the largest of the Texas barrier islands and the world's longest barrier island. The island is located along Texas's southern coast of the Gulf of Mexico and is noted for its white sandy beaches. Meaning ''father'' in Spanish langua ...
of Texas. Sometimes an important inlet may close permanently, transforming an island into a
peninsula A peninsula ( la, paeninsula from ' "almost" and ' "island") is a landform surrounded by water on most of its border while being connected to a mainland from which it extends. The surrounding water is usually understood to be continuous, though ...

peninsula
, thus creating a barrier peninsula. The length and width of barriers and overall morphology of barrier coasts are related to parameters including
tidal range Tidal range is the height difference between high tide and low tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon an ...
,
wave energy Wave power is the capture of energy of wind waves to do useful mechanical work, work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power (physics), power is a wave energy converte ...
, sediment supply,
sea-level trends
sea-level trends
, and
basement controls
basement controls
.Davis Jr., p. 144. The amount of vegetation on the barrier has a large impact on the height and evolution of the island. Chains of barrier islands can be found along approximately 13-15% of the world's coastlines.Smith, Q.H.T., Heap, A.D., and Nichol, S.L., 2010, "Origin and formation of an estuarine barrier island, Tapora Island, New Zealand:" ''Journal of Coastal Research,'' v. 26, p. 292–300. They display different settings, suggesting that they can form and be maintained in a variety of environments. Numerous theories have been given to explain their formation. A man-made offshore structure constructed parallel to the shore is called a breakwater. In terms of
coastal morphodynamics Coastal morphodynamics (i.e. the dynamics of beach A summer tourism at the Yyteri Beach in Pori, Finland">Pori.html" ;"title="Yyteri Beach in Pori">Yyteri Beach in Pori, Finland. A beach is a landform alongside a body of water which con ...
, it acts similarly to a naturally occurring barrier island by dissipating and reducing the energy of the waves and currents striking the coast. Hence, it is an important aspect of
coastal engineering Coastal engineering is a branch of civil engineering Civil engineering is a professional engineering Regulation and licensure in engineering is established by various jurisdictions of the world to encourage public welfare, safety, well-being a ...
.


Constituent parts

;Lower shoreface The shoreface is the part of the barrier where the ocean meets the shore of the island. The barrier island body itself separates the shoreface from the backshore and
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' and ''atoll lagoons''. They ...

lagoon
/
tidal flat , intertidal and subtidal The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illumina ...

tidal flat
area. Characteristics common to the
lower shoreface diagram. Lower Shoreface refers to the portion of the seafloor, and the sedimentary depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical and biological processes ...
are fine sands with mud and possibly silt. Further out into the ocean the sediment becomes finer. The effect of waves at this point is weak because of the depth.
Bioturbation Bioturbation is defined as the reworking of soil Surface-water- gley developed in glacial till, Northern Ireland.">Northern_Ireland.html" ;"title="glacial till, Northern Ireland">glacial till, Northern Ireland. Soil is a mixture of organic m ...
is common and many
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

fossil
s can be found in lower shoreface deposits in the geological record. ;Middle shoreface The middle shore face is located in the upper shoreface. The middle shoreface is strongly influenced by wave action because of its depth. Closer to shore the sand is medium-grained, with shell pieces common. Since wave action is heavier, bioturbation is not likely. ;Upper shoreface The upper shoreface is constantly affected by wave action. This results in development of herringbone
sedimentary structure Sedimentary structures include all kinds of features formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a b ...
s because of the constant differing flow of waves. The sand is coarser. ;Foreshore The
foreshore
foreshore
is the area on land between high and low tide. Like the upper shoreface, it is constantly affected by wave action.
Cross bedding In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...

Cross bedding
and
lamination 250px, Laminate flooring Lamination is the technique/process of manufacturing a Raw material, material in multiple layers, so that the composite material achieves improved strength of materials, strength, stability, sound insulation, visual ...
are present and coarser sands are present because of the high energy present by the crashing of the waves. The sand is also very well sorted. ;Backshore The
backshore The backshore area of a beach extends from the limit of high water foam line Foam lines are accumulations of foam, with trapped insects, exuvia, etc. found on the surface of freshwater water courses or bodies.berm A berm is a level space, shelf, or raised barrier (usually made of compacted soil In geotechnical engineering, soil compaction is the process in which stress applied to a soil causes densification as air is displaced from the pores between the ...

berm
is also found here which marks the boundary between the foreshore and backshore. Wind is the important factor here, not water. During strong storms high waves and wind can deliver and erode sediment from the backshore. ;Dunes
Coastal dunes A dune is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the s ...
, created by wind, are typical of a barrier island. They are located at the top of the backshore. The dunes will display characteristics of typical aeolian wind-blown dunes. The difference is that dunes on a barrier island typically contain coastal vegetation roots and marine bioturbation. ;Lagoon and tidal flats The
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' and ''atoll lagoons''. They ...

lagoon
and
tidal flat , intertidal and subtidal The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illumina ...

tidal flat
area is located behind the dune and backshore area. Here the water is still, which allows fine silts, sands, and mud to settle out. Lagoons can become host to an
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...
environment. This will allow high amounts of organic-rich mud to form. Vegetation is also common.


Location

Barrier Islands can be observed on every continent on Earth, except Antarctica.


Australia

Moreton Bay Moreton Bay is a bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approx ...

Moreton Bay
, on the east coast of Australia and directly east of
Brisbane Brisbane ( ) is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller low ...

Brisbane
, is sheltered from the Pacific Ocean by a chain of very large barrier islands. Running north to south they are
Bribie Island Bribie Island is the smallest and most northerly of three major sand islands forming the coastline sheltering the northern part of Moreton Bay, Queensland Moreton Bay is a Suburbs and localities (Australia), locality split between the City ...
,
Moreton Island Moreton Island (Mulgumpin) is an island upright=1.15, Great_Britain.html"_;"title="Ireland_(left)_and_Great_Britain">Ireland_(left)_and_Great_Britain_(right),_are_large_islands_of_north-west_Europe image:Small_Island_in_Lower_Saranac_Lake.jp ...
,
North Stradbroke Island North Stradbroke Island ( Jandai: ''Minjerribah''), colloquially ''Straddie'' or ''North Straddie'', is an island that lies within Moreton Bay in the Australian state of Queensland, southeast of the centre of Brisbane. Originally there was only ...
and
South Stradbroke Island The South Stradbroke Island ( Indigenous: ''Minjerribah''), colloquially ''South Straddie'', is an island that lies within Moreton Bay Moreton Bay is a bay located on the eastern coast of Australia from central Brisbane Brisbane ( ) is ...
(the last two used to be a single island until a storm created a channel between them in 1896). North Stradbroke Island is the second largest sand island in the world and Moreton Island is the third largest.
Fraser Island Fraser Island (K'gari) is the traditional land of the Butchulla people, and a heritage-listed island upright=1.15, Great_Britain.html"_;"title="Ireland_(left)_and_Great_Britain">Ireland_(left)_and_Great_Britain_(right),_are_large_islands ...

Fraser Island
, another barrier island lying 200 km north of Moreton Bay on the same coastline, is the largest sand island in the world.


United States

Barrier islands are seen most prominently on the United States' East Coast and Gulf Coast, where every state, stretching from Maine to Florida and Florida to Texas on each coast has at least part of a barrier island, stretching to more than twenty-five for Florida. This chain is international. It starts in Quebec's
Magdalen Islands The Magdalen Islands (french: Îles de la Madeleine ) are a small in the with a land area of . While part of the Province of , the islands are in fact closer to the and than to the on the Quebec mainland. The islands are part of the homeland ...

Magdalen Islands
and ends in Mexico.
Padre Island Padre Island is the largest of the Texas barrier islands and the world's longest barrier island. The island is located along Texas's southern coast of the Gulf of Mexico and is noted for its white sandy beaches. Meaning ''father'' in Spanish langua ...
, Texas, is the world’s longest barrier island; other well-known islands on the Gulf Coast include
Galveston Island Galveston Island ( ) is a barrier island on the Texas Gulf Coast in the United States, about southeast of Houston, Texas, Houston. The entire island, with the exception of Jamaica Beach, Texas, Jamaica Beach, is within the city limits of the City ...
in Texas and
Sanibel Island Sanibel is an island and city in Lee County, Florida, United States. The population was 6,469 at the 2010 census, with an estimated 2018 population of 7,402. It is part of the Cape Coral-Fort Myers, Florida Metropolitan Statistical Area. The isl ...
in Florida. Those on the East Coast include
Miami Beach Miami Beach is a coastal in , , . It was incorporated on March 26, 1915. The is located on natural and s between the and , the latter of which separates the Beach from the mainland city of . The of , comprising the southernmost of Miami ...

Miami Beach
, Palm Beach, and
Cape Canaveral , image = cape canaveral.jpg , image_size = 300 , caption = View of Cape Canaveral from space in 1991 , map = Florida#USA , map_width = 300 , map_caption = Location in Florida , location = Florida, United S ...
in Florida;
Cape Hatteras Cape Hatteras is a bend in Hatteras Island, one of the barrier islands of North Carolina North Carolina () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. North Carolina is the List of ...

Cape Hatteras
in North Carolina;
Assateague Island Assateague Island is a long barrier island located off the eastern coast of the Delmarva Peninsula facing the Atlantic Ocean. The northern two-thirds of the island is in Maryland while the southern third is in Virginia. The Maryland section ...
in Virginia and Maryland;
Absecon Island Absecon Island is a barrier island located on the Jersey Shore of the Atlantic Ocean in Atlantic County, New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeaster ...
in New Jersey, where
Atlantic City Atlantic City, often known by its initials A.C., is a coastal Resort town, resort city in Atlantic County, New Jersey, Atlantic County, New Jersey, United States, known for its casinos, Boardwalk (entertainment district), boardwalk, and beache ...
is located; and
Jones Beach IslandImage:Wpdms ev26188 jones beach island.jpg, 300px, Jones Beach Island Jones Beach Island is one of the Outer barrier, outer barrier islands off the southern coast of Long Island in the U.S. state of New York (state), New York. It is named for Majo ...
and
Fire Island Fire Island is the large center island of the outer barrier islands parallel to the south shore of Long Island Long Island is a densely populated island in the southeast part of the U.S. state In the United States The Unit ...

Fire Island
, both off
Long Island Long Island is a densely populated island in the southeast part of the U.S. state In the , a state is a , of which there are currently 50. Bound together in a , each state holds al jurisdiction over a separate and defined geographic ...
in New York. No barrier islands are found on the Pacific Coast of the United States due to the rocky shore and short continental shelf, but barrier peninsulas can be found. Barrier islands can also be seen on Alaska's Arctic coast.


Canada

Barrier Islands can also be found in Maritime Canada, and other places along the coast. A good example is found at
Miramichi Bay Miramichi Bay is an estuary An estuary is a partially enclosed Coast, coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river ...
,
New Brunswick ("Hope restored") , image_map = New Brunswick in Canada 2.svg , Label_map = yes , coordinates = , capital = Fredericton Fredericton (; ) is the capital city of the Canadian provinc ...

New Brunswick
, where Portage Island as well as Fox Island and Hay Island protect the inner bay from storms in the
Gulf of Saint Lawrence The Gulf of St. Lawrence (French language, French: ''Golfe du Saint-Laurent'') is the outlet of the North American Great Lakes via the St. Lawrence River into the Atlantic Ocean. The gulf is a semi-enclosed sea, covering an area of about and conta ...

Gulf of Saint Lawrence
.


Mexico

Mexico's Gulf Coast has numerous barrier islands and barrier Peninsulas.


New Zealand

Barrier islands are more prevalent in the north of both of New Zealand's main islands. Notable barrier islands in New Zealand include
Matakana Island
Matakana Island
, which guards the entrance to
Tauranga Harbour Tauranga Harbour is the natural tidal harbour that surrounds Tauranga CBD and the Mount Maunganui area of Tauranga Tauranga () is a coastal metropolitan city in the Bay of Plenty region and the fifth most populous city of New Zealand, wit ...
, and Rabbit Island, at the southern end of
Tasman Bay / Te Tai-o-Aorere Image:NZ-Tasman B.png, 160px, Location of Tasman Bay Tasman Bay / Te Tai-o-Aorere, originally known as Blind Bay, is a large V-shaped bay at the north end of New Zealand's South Island. Located in the centre of the island's northern coast, it str ...
. See also Nelson Harbour's
Boulder Bank The Boulder Bank ( Māori: ''Te Taero a Keropa'' or ''Te Tāhuna o Tama-i-ea'') is a very unusual naturally formed landform in Nelson, New Zealand Nelson ( mi, Whakatū) is a city on the eastern shores of Tasman Bay / Te Tai-o-Aorere. Nelso ...
, below.


Europe

Barrier islands can be observed in the Baltic Sea and are a distinct feature of the Wadden Islands, which stretch from the Netherlands to Denmark.
Lido di Venezia The Lido, or Venice Lido ( it, Lido di Venezia), is an barrier island in the Venetian Lagoon, northern Italy; it is home to about 20,400 residents. The Venice Film Festival takes place at the Lido late August/early September. Geography The ...
and
Pellestrina Pellestrina is an island upright=1.15, Great_Britain.html"_;"title="Ireland_(left)_and_Great_Britain">Ireland_(left)_and_Great_Britain_(right),_are_large_islands_of_north-west_Europe image:Small_Island_in_Lower_Saranac_Lake.jpg.html" ;"title ...

Pellestrina
are notable barrier islands of the
Lagoon of Venice The Venetian Lagoon ( it, Laguna di Venezia; vec, Łaguna de Venesia) is an enclosed bay of the Adriatic Sea The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkans. The Adriatic is the northernmost arm of the ...
which have for centuries protected the city of
Venice Venice ( ; it, Venezia ; vec, Venesia or ) is a city in northeastern Italy Italy ( it, Italia ), officially the Italian Republic ( it, Repubblica Italiana, links=no ), is a country consisting of delimited by the and surrounding ...

Venice
in Italy.
Chesil Beach Chesil Beach (also known as Chesil Bank) in Dorset, Dorset, England is one of three major Shingle beach, shingle beach structures in Britain.A. P. Carr and M. W. L. Blackley, "Investigations Bearing on the Age and Development of Chesil Beach, Dor ...

Chesil Beach
on the south coast of England developed as a barrier beach.


Processes

;Migration and overwash Water levels may be higher than the island during storm events. This situation can lead to overwash, which brings sand from the front of the island to the top and/or landward side of the island. This process leads to the evolution and migration of the barrier island. ;Critical width concept Barrier islands are often formed to have a certain width. The term “critical width concept” has been discussed with reference to barrier islands, overwash, and washover deposits since the 1970s. The concept basically states that overwash processes were effective in migration of the barrier only where the barrier width is less than a critical value. The island did not narrow below these values because overwash was effective at transporting sediment over the barrier island, thereby keeping pace with the rate of ocean shoreline recession. Sections of the island with greater widths experienced washover deposits that did not reach the bayshore, and the island narrowed by ocean shoreline recession until it reached the critical width. The only process that widened the barrier beyond the critical width was breaching, formation of a partially subaerial flood shoal, and subsequent inlet closure. For the present discussion, critical barrier width is defined as the smallest cross-shore dimension that minimizes net loss of sediment from the barrier island over the defined project lifetime. The magnitude of critical width is related to sources and sinks of sand in the system, such as the volume stored in the dunes and the net long-shore and cross-shore sand transport, as well as the island elevation. The concept of critical width is important for large-scale barrier island restoration, in which islands are reconstructed to optimum height, width, and length for providing protection for estuaries, bays, marshes and mainland beaches.


Formation theories

Scientists have proposed numerous explanations for the formation of barrier islands for more than 150 years. There are three major theories: offshore bar, spit accretion, and submergence. No single theory can explain the development of all barriers, which are distributed extensively along the world's coastlines. Scientists accept the idea that barrier islands, including other barrier types, can form by a number of different mechanisms.Davis Jr., p. 147 There appears to be some general requirements for formation. Barrier island systems develop most easily on wave-dominated coasts with a small to moderate tidal range. Coasts are classified into three groups based on
tidal range Tidal range is the height difference between high tide and low tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon an ...
: microtidal, 0–2 meter tidal range; mesotidal, 2–4 meter tidal range; and macrotidal, >4 meter tidal range. Barrier islands tend to form primarily along microtidal coasts, where they tend to be well developed and nearly continuous. They are less frequently formed in mesotidal coasts, where they are typically short with tidal inlets common. Barrier islands are very rare along macrotidal coasts. Along with a small tidal range and a wave-dominated coast, there must be a relatively low gradient shelf. Otherwise, sand accumulation into a sandbar would not occur and instead would be dispersed throughout the shore. An ample sediment supply is also a requirement for barrier island formation. The last major requirement for barrier island formation is a stable sea level. It is especially important for sea level to remain relatively unchanged during barrier island formation and growth. If are too drastic, time will be insufficient for wave action to accumulate sand into a dune, which will eventually become a barrier island through
aggradation Aggradation (or alluviation) is the term used in geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, a ...

aggradation
. The formation of barrier islands requires a constant sea level so that waves can concentrate the sand into one location.


Offshore bar theory

In 1845 the Frenchman
Elie de Beaumont Elie and Earlsferry is a coastal town and former royal burgh in Fife Fife (, ; gd, Fìobha, ; sco, Fife) is a council area, Historic counties of Scotland, historic county, registration county and lieutenancy areas of Scotland, lieutenanc ...
published an account of barrier formation. He believed that waves moving into shallow water churned up sand, which was deposited in the form of a submarine
bar Bar or BAR may refer to: Food *Bar (establishment) A bar is a long raised narrow table or bench designed for dispensing beer or other alcoholic beverage, alcoholic drinks. They were originally chest high, and a bar, often brass, ran the len ...
when the waves broke and lost much of their energy. As the bars developed vertically, they gradually rose above sea level, forming barrier islands.


Spit accretion theory

American geologist
Grove Karl Gilbert Grove Karl Gilbert (May 6, 1843 – May 1, 1918), known by the abbreviated name ''G. K. Gilbert'' in academic literature, was an American geologist. Biography Gilbert was born in Rochester, New York and graduated from the University of Roches ...

Grove Karl Gilbert
first argued in 1885 that the barrier sediments came from longshore sources. He proposed that sediment moving in the breaker zone through agitation by waves in
longshore drift Longshore drift from longshore current is a geological process that consists of the transportation of sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of ...

longshore drift
would construct spits extending from
headlands A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a Cape (geography), ca ...
parallel to the coast. The subsequent breaching of spits by storm waves would form barrier islands.


Submergence theory

William John McGee William John McGee, LL.D. (April 17, 1853 – September 4, 1912) was an American inventor, geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as we ...

William John McGee
reasoned in 1890 that the East and Gulf coasts of the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

United States
were undergoing submergence, as evidenced by the many drowned river valleys that occur along these coasts, including Raritan,
Delaware Delaware ( ) is a state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States, bordering Maryland to its south and west; Pennsylvania to its north; and New Jersey and the Atlantic Ocean to its east. The state takes i ...
and
ChesapeakeChesapeake often refers to: *Chesapeake people, a Native American tribe also known as the Chesepian * The Chesapeake, a.k.a. Chesapeake Bay *Delmarva Peninsula, also known as the Chesapeake Peninsula Chesapeake may also refer to: Populated place ...
bays. He believed that during submergence, coastal ridges were separated from the mainland, and lagoons formed behind the ridges. He used the
Mississippi–Alabama barrier islands The Mississippi–Alabama barrier islands are a chain of barrier islands in the Gulf of Mexico along the coasts of Mississippi and Alabama enclosing the Mississippi Sound. The major islands are Cat Island (Mississippi), Cat Island, Ship Island (Mi ...
(consists of
Cat The cat (''Felis catus'') is a domestic Domestic may refer to: In the home * Anything relating to the human home A home, or domicile, is a space used as a permanent or semi-permanent residence for an individual, group or family ...
,
Ship A ship is a large watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functional propertie ...
,
Horn Horn usually refers to: *Horn (acoustic), a conical or bell shaped aperture used to guide sound ** Horn (instrument), collective name for tube-shaped wind musical instruments *Horn (anatomy), a pointed, bony projection on the head of various anima ...
, Petit Bois and Dauphin Islands) as an example where coastal submergence formed barrier islands. His interpretation was later shown to be incorrect when the ages of the coastal stratigraphy and sediment were more accurately determined. Along the coast of
Louisiana Louisiana (Standard French Standard French (in French: ''le français standard'', ''le français normé'', ''le français neutre'' eutral Frenchor ''le français international'' nternational French is an unofficial term for a standard ...

Louisiana
, former lobes of the
Mississippi River The Mississippi River is the second-longest river and chief river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and b ...

Mississippi River
delta have been reworked by wave action, forming beach ridge complexes. Prolonged sinking of the marshes behind the barriers has converted these former vegetated wetlands to open-water areas. In a period of 125 years, from 1853 to 1978, two small semi-protected bays behind the barrier developed as the large water body of Lake Pelto, leading to
Isles Dernieres Last Island (Official name: Isle Dernière, often misspelled as Îsle Dernière, Isle Dernier, L'Îsle Dernière, Île Dernière, etc. ) was a barrier island and a pleasure resort southwest of New Orleans on the Gulf Coast of Louisiana, United Stat ...

Isles Dernieres
's detachment from the mainland.


Boulder Bank

An unusual natural structure in
New Zealand New Zealand ( mi, Aotearoa ''Aotearoa'' (; commonly pronounced by English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon Engl ...

New Zealand
may give clues to the formation processes of barrier islands. The
Boulder Bank The Boulder Bank ( Māori: ''Te Taero a Keropa'' or ''Te Tāhuna o Tama-i-ea'') is a very unusual naturally formed landform in Nelson, New Zealand Nelson ( mi, Whakatū) is a city on the eastern shores of Tasman Bay / Te Tai-o-Aorere. Nelso ...
, at the entrance to
Nelson Haven Image:Mudflats of Nelson Haven at low tide.jpg, upright=1.1, Water channels in Nelson Haven Nelson Haven is an extensive area of mudflats northeast of Nelson, New Zealand. It is separated from Tasman Bay / Te Tai-o-Aorere by the Boulder Bank and ove ...

Nelson Haven
at the northern end of the
South Island The South Island, also officially named , is the larger of the two major islands of New Zealand upright=1.25, Whakaari / White Island in the Bay of Plenty New Zealand consists of many islands, estimated around six hundred, mainly remnan ...

South Island
, is a unique 13 km-long stretch of rocky substrate a few metres in width. It is not strictly a barrier island, as it is linked to the mainland at one end. The Boulder Bank is composed of
granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from grani ...
from Mackay Bluff, which lies close to the point where the bank joins the mainland. It is still debated what process or processes have resulted in this odd structure, though longshore drift is the most accepted hypothesis. Studies have been conducted since 1892 to determine the speed of boulder movement. Rates of the top-course gravel movement have been estimated at 7.5 metres a year.


Ecological importance

Barrier islands are critically important in mitigating ocean swells and other storm events for the water systems on the mainland side of the barrier island, as well as protecting the coastline. This effectively creates a unique environment of relatively low energy,
brackish water Brackish water, also sometimes termed brack water, is water occurring in a natural environment having more salinity Salinity () is the saltiness or amount of dissolved in a body of , called (see also ). It is usually measured in g/L or g/kg ...
. Multiple wetland systems such as lagoons, estuaries, and/or marshes can result from such conditions depending on the surroundings. They are typically rich habitats for a variety of flora and fauna. Without barrier islands, these wetlands could not exist; they would be destroyed by daily ocean waves and tides as well as ocean storm events. One of the most prominent examples is the Louisiana barrier islands.Stone, G.W., and McBride, R.A., 1998, "Louisiana barrier islands and their importance in wetland protection: forecasting shoreline change and subsequent response of wave climate:" ''Journal of Coastal Research,'' v. 14, p. 900–915.


See also

* North Frisian Barrier Island *
Outer Banks The Outer Banks (frequently abbreviated OBX) are a string of barrier islands and Spit (landform), spits off the coast of North Carolina and southeastern Virginia, on the east coast of the United States. They line most of the North Carolina coas ...
*
Virginia Barrier Islands The Virginia Barrier Islands are a continuous chain of long, narrow, low-lying, sand and scrub barrier islands separated from one another by narrow inlets and from the mainland by a series of shallow marshy tidal bays along the entire coast of the ...
* New York Barrier Islands *
Texas barrier islands The Texas barrier islands are a chain of barrier islands in the Gulf of Mexico along the coast of Texas. The islands enclose a series of Estuaries of Texas, estuaries along the Texas coast and attract tourists for activities such as recreational fis ...
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Sea Islands The Sea Islands are a chain of tidal and barrier islands Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. T ...
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Long Beach Island Long Beach Island (colloquially known as LBI, The LBI Region or simply The Island) is a barrier island Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave an ...
* Bald Head Island


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References

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External links

{{coastal geography Physical oceanography Coastal geography Hydrology Coastal and oceanic landforms Oceanographical terminology Islands by type