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A dune is a
landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, mountains ...
composed of wind- or water-driven
sand Sand is a granular material composed of finely divided rock and mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural cla ...
. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat regions covered with wind-swept sand or dunes with little or no vegetation are called '' ergs'' or ''sand seas''. Dunes occur in different shapes and sizes, but most kinds of dunes are longer on the
stoss Stoss is an English word meaning "the upflow side" ("stoss side" is the opposite of lee side, and a more general term for "windward side", also applying to flows of liquids and solids). Stoss (Stoß) is also a German word meaning "shock, impact", fo ...
(upflow) side, where the sand is pushed up the dune, and have a shorter ''slip face'' in the lee side. The valley or trough between dunes is called a ''dune slack''. Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash, and in other areas where poorly cemented
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) be ...
bedrock disintegrates to produce an ample supply of loose sand. Subaqueous dunes can form from the action of water flow (
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglac ...
processes) on sand or
gravel Gravel is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel () and p ...
beds of
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 becomes dry at the end of its course without reaching another body of water. Sm ...
s,
estuaries An estuary is a partially enclosed 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 environments and maritime environmen ...
, and the sea-bed. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the
beach A summer tourism at the Yyteri Beach in [[Pori, [[Finland">Pori.html" style="text-decoration: none;"class="mw-redirect" title="Yyteri Beach in [[Pori">Yyteri Beach in [[Pori, [[Finland. A beach is a [[landform alongside a body of water whic ...
. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the [[sea. Artificial dunes are sometimes constructed to protect coastal areas. The dynamic action of wind and water can sometimes cause dunes to drift, which can have serious consequences. For example, the town of
Eucla, Western Australia Eucla is the easternmost locality in Western Australia, located in the Goldfields-Esperance region of Western Australia along the Eyre Highway, approximately west of the South Australian border. At the 2016 Australian census, Eucla had a populat ...
, had to be relocated in the 1890s because of dune drift. The modern word "dune" came into English from French around 1790, which in turn came from
Middle Dutch Middle Dutch is a collective name for a number of closely related West Germanic dialects whose ancestor was Old Dutch and was spoken and written between 1150 and 1500. Until the advent of Modern Dutch after 1500, there was no overarching standard l ...
''dūne''.


Formation

A universally precise distinction does not exist between ripples, dunes, and a
draa :''Dra is also the abbreviation for the constellation Draco.'' The Draa ( ber|Asif en Dra, ⴰⵙⵉⴼ ⴻⵏ ⴷⵔⴰ, ary|واد درعة|wad dərʿa; also spelled Dra or Drâa, in older sources mostly Darha or Dara) is Morocco's longest riv ...
, which are all deposits of the same type of materials. Dunes are generally defined as greater than 7 cm tall and may have ripples, while ripples are deposits that are less than 3 cm tall. A draa is a very large eolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes. Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces (
saltation Saltation may refer to: * Saltation (biology), an evolutionary hypothesis emphasizing sudden and drastic change * Saltation (geology), a process of particle transport by fluids * Cutaneous rabbit illusion (sensory saltation), a perceptual illusion ...
) up the stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face (or slipface). The
Bagnold formula The Bagnold formula, named after Ralph Alger Bagnold, relates the amount of sand moved by the wind to wind speed by saltation. It states that the mass transport of sand is proportional to the third power of the friction velocity. Under steady condit ...
gives the speed at which particles can be transported.


Aeolian dunes


Aeolian dune shapes

Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types).


Barchan or crescentic

Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal winds). They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below). Some types of crescentic dunes move more quickly over
desert upright=1.5|alt=see caption|Sand dunes in the [[Rub' al Khali ("Empty quarter") in the [[United Arab Emirates A desert is a barren area of [[landscape where little [[precipitation occurs and, consequently, living conditions are hostile for ...
surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in [[China's [[Ningxia Province, and similar speeds have been recorded in the [[Western Desert (Egypt)|Western Desert of
Egypt Egypt ( ; ar|مِصر ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia by a land bridge formed by the Sinai Peninsula. Egypt is a Mediterranean count ...
. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's
Taklamakan Desert The Taklamakan Desert (; zh|s=|p=Tǎkèlāmǎgān Shāmò, Xiao'erjing: , dng|Такәламаган Шамә; ug|تەكلىماكان قۇملۇقى; also spelled Taklimakan and Teklimakan) is a desert in Southwest Xinjiang in Northwest China. ...

Taklamakan Desert
. See lunettes and parabolic dunes, below, for dunes similar to crescent-shaped ones.


Transverse dunes

Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest.


Seif or longitudinal dunes

Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The two slip faces make them sharp-crested. They are called ''seif'' dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations). Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along the resultant direction of sand movement (hence the name "longitudinal"). Some linear dunes merge to form Y-shaped compound dunes. Formation is debated.
Ralph Bagnold Ralph Alger Bagnold, OBE, FRS, (3 April 1896 – 28 May 1990) was an English 20th-century desert explorer, geologist and soldier. In 1932, he staged the first recorded East-to-West crossing of the Libyan Desert. His work in the field of Aeolian ...
, in ''
The Physics of Blown Sand and Desert Dunes ''The Physics of Blown Sand and Desert Dunes'' (1941) is a science book, written by Ralph A. Bagnold. The book laid the foundations of the scientific investigation of the transport of sand by wind. It also discusses the formation and movement of ...
'', suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by
vortices , revealed by colored smoke In fluid dynamics, a vortex (plural vortices/vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smo ...
in a unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. File:Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1).jpg|Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1). Most of these dunes are seif dunes. Their origin from barchans is suggested by the stubby remnant "hooks" seen on many of the dunes. Wind would be from left to right. File:ISS-31 Linear dunes in the Great Sand Sea in southwest Egypt.jpg|Large linear seif dunes in the
Great Sand Sea The Great Sand Sea is an approximately 72,000 km² sand desert (erg) in the Sahara between western Egypt and eastern Libya in North Africa. Some 74% of the area is covered by sand dunes. Geography The Great Sand Sea stretches for a length of abou ...
in southwest
Egypt Egypt ( ; ar|مِصر ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia by a land bridge formed by the Sinai Peninsula. Egypt is a Mediterranean count ...
, seen from the
International Space Station The International Space Station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russi ...

International Space Station
. The distance between each dune is 1.5–2.5 km. File:Longitudinal_dune.jpg|The average-direction-longitudinal model of seif dune formation File:Tranverse dune.jpg|alt=Transverse dune with wind blowing across crest|By contrast, transverse dunes form with the wind blowing perpendicular to the ridges, and have only one slipface, on the lee side. The stoss side is less steep. File:Cross-bedding.gif|alt=Animation of wind pushing transverse dunes along. The sand blows from the stoss side down onto the less side, where it is buried by the next layer. The dune thus moves, and a cross-section through it shown diagonal
cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original de ...

cross-bedding
|Transverse dunes lie perpendicular to the wind, which moves them forwards, producing the
cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original de ...

cross-bedding
shown here.
Seif dunes are common in the Sahara. They range up to in height and in length. In the southern third of the Arabian Peninsula, a vast erg, called the
Rub' al Khali The Rub' al KhaliOther standardized transliterations include: / . The ' is the assimilated Arabic definite article, ', which can also be transliterated as '. (; ar|ٱلرُّبْع ٱلْخَالِي (), the "Empty Quarter") is the sand desert ...
or Empty Quarter, contains seif dunes that stretch for almost 200 km and reach heights of over 300 m. Linear
loess Loess (, ; from German ''Löss'' ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of the Earth's land area is covered by loess or similar deposits. Loess is an aeolian (windborn ...
hills known as pahas are superficially similar. These hills appear to have been formed during the last
ice age#REDIRECT Ice age {{Redirect category shell|1= {{R from move {{R from ambiguous term {{R from other capitalisation {{R unprintworthy ...
under
permafrost Permafrost is ground that continuously remains below 0°C (32°F) for two or more years, located on land or under the ocean. Permafrost does not have to be the first layer that is on the ground. It can be from an inch to several miles deep under ...
conditions dominated by sparse
tundra In physical geography, tundra () is a type of biome where the tree growth is hindered by low temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless mount ...
vegetation.


Star

Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the
Grand Erg Oriental The Grand Erg Oriental (English: 'Great Eastern Sand Sea') is a large ''erg'' or "field of sand dunes" in the Sahara Desert. Situated for the most part in Saharan lowlands of northeast Algeria, the Grand Erg Oriental covers an area some 600 km ...
of the Sahara. In other deserts, they occur around the margins of the
sand sea An erg (also sand sea or dune sea, or sand sheet if it lacks dunes) is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word ''ʿarq'' (), meaning "dune field". ...
s, particularly near topographic barriers. In the southeast
Badain Jaran Desert The Badain Jaran Desert () is a desert in China which spans the provinces of Gansu, Ningxia and Inner Mongolia. It covers an area of . By size it is the third largest desert in China. This desert is home to some of the tallest stationary dunes on ...
of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth.


Dome

Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas.


=Lunettes

= Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to the direction (s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, and up to 50 metres high. They also occur in
southern The name Southern may refer to: * South, a point in direction. * Southern (surname) Businesses * China Southern Airlines, airline based in Guangzhou, China * Southern Airways, defunct US airline * Southern Air, air cargo transportation company bas ...
and
West Africa West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, ...
, and in parts of the western United States, especially Texas.


Parabolic

U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of the sand in the dune migrates forward. In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms. These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of the dune and underlying
soil Soil (often stylized as SOiL) is an American rock band that was formed in Chicago, Illinois in 1997. After some independent releases, the band was the first rock group signed to J Records and achieved mainstream success with their major label debut ...
s. The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes. Most parabolic dunes do not reach heights higher than a few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes. Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds, the effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction. The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are the corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because the nose is usually made up of loose sand without much if any vegetation. A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a ''zibar''. The term zibar comes from the
Arabic Arabic (, ' or , ' or ) is a Semitic language that first emerged in the 1st to 4th centuries CE.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Wats ...
word to describe "rolling transverse ridges ... with a hard surface". The dunes are small, have low relief, and can be found in many places across the planet from
Wyoming Wyoming () is a state in the Mountain West region of the United States. The 10th largest state by area, it is also the least populous and least densely populated state in the contiguous United States. It is bordered by Montana to the north an ...

Wyoming
(United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they don't become more than 10 metres high. The dunes form at about ninety degrees to the prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest.


Reversing dunes

Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions. The minor slipfaces are usually temporary, as they appear after a reverse wind and are generally destroyed when the wind next blows in the dominant direction.


Draas

Draas are very large-scale dune bedforms; they may be tens or a few hundreds of metres in height, kilometres wide, and hundreds of kilometres in length. After a draa has reached a certain size, it generally develops superimposed dune forms. They are thought to be more ancient and slower-moving than smaller dunes, and to form by vertical growth of existing dunes. Draas are widespread in sand seas and are .


Dune complexity

All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (lager dunes on which smaller dunes of same type form), and complex (combinations of different types). Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.


Dune movement

The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above, the sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles.


Coastal dunes

Coastal dunes form when wet sand is deposited along the coast and dries out and is blown along the beach. Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate. Obstacles—for example, vegetation, pebbles and so on—tend to slow down the wind and lead to the deposition of sand grains. These small "incipient dunes or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or
rhizome Lotus rhizome peeled and sliced In botany and dendrology, a rhizome (, from grc|rhízōma|script=Latn (ῥίζωμα) - "mass of roots", from (ῥιζόω) "cause to strike root") is a modified subterranean plant stem that sends out roots and ...
. Models of coastal dunes suggest that their final equilibrium height is related to the distance between the water line and where vegetation can grow. Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group. Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks. Different locations around the globe have dune formations unique to their given coastal profile. Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types. Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food. Birds are also known to utilize coastal dunes as nesting grounds. All these species find the coastal environment of the sand dune vital to their species' survival. Over the course of time coastal dunes may be impacted by
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and/or squalls. Dependin ...
s or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency. During a storm event, dunes play a significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a
storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the norm ...
, will retreat or erode. To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation. How a much a dune erodes during any storm surge is related to its location on the coastal shoreline and the profile of the beach during a particular season. In those areas with harsher winter weather, during the summer a beach tends to take on more of a convex appearance due to gentler waves, while the same beach in the winter may take on more of a concave appearance. As a result, coastal dunes can get eroded much more quickly in the winter than in the summer. The converse is true in areas with harsher summer weather. There are many threats to these coastal communities. Some coastal dunes, for example ones in San Francisco, have been completely altered by urbanization; reshaping the dune for human use. This puts native species at risk. Another danger, in California and places in the UK specifically, is the introduction of invasive species. Plant species, such as ''
Carpobrotus edulis ''Carpobrotus edulis'' is a ground-creeping plant with succulent leaves in the genus ''Carpobrotus'', native to South Africa. It is also known as Hottentot-fig (though this nickname is considered a racist term), ice plant, highway ice plant, or p ...
'', were introduced from South Africa in an attempt to stabilize the dunes and provide horticultural benefits, but instead spread taking land away from native species. ''
Ammophila arenaria ''Ammophila arenaria'' is a species of flowering plant in the grass family Poaceae. It is known by the common names marram grass and European beachgrass. It is one of two species of the genus ''Ammophila''. It is native to the coastlines of Europe ...
'', known as European beachgrass, has a similar story, though it has no horticulture benefits. It has great ground coverage and, as intended, stabilized the dunes but as an unintended side effect prevented native species from thriving in those dunes. One such example is the dune field at Point Reyes, California. There are now efforts to get rid of both of these invasive species.


Ecological succession on coastal dunes

As a dune forms, plant [[Ecological succession|succession occurs. The conditions on an ''embryo dune'' are harsh, with [[salt spray from the sea carried on strong winds. The dune is well drained and often dry, and composed of calcium carbonate from seashells. Rotting [[seaweed, brought in by storm waves adds nutrients to allow [[pioneer species to colonize the dune. For example, in the [[United Kingdom these pioneer species are often [[Ammophila (Poaceae)|marram grass, [[sea wort grass and other sea grasses. These plants are well adapted to the harsh conditions of the foredune, typically having deep roots which reach the [[water table, [[root nodules that produce [[nitrogen compounds, and protected [[stoma, reducing [[transpiration. Also, the deep roots bind the sand together, and the dune grows into a [[foredune as more sand is blown over the grasses. The grasses add [[nitrogen to the soil, meaning other, less hardy plants can then colonize the dunes. Typically these are [[Calluna|heather, [[Heath (habitat)|heaths and [[gorses. These too are adapted to the low [[Soil#Soil moisture|soil water content and have small, prickly leaves which reduce transpiration. Heather adds [[humus to the soil and is usually replaced by [[coniferous trees, which can tolerate low [[soil pH, caused by the accumulation and decomposition of organic matter with nitrate leaching. Coniferous forests and [[heathland are common [[climax community|climax communities for sand dune systems. Young dunes are called [[yellow dunes and dunes which have high humus content are called [[grey dunes. Leaching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks' soil to be waterlogged where only marsh plants can survive. In Europe these plants include: creeping willow, cotton grass, [[yellow iris, reeds, and rushes. As for vertebrates in European dunes, [[natterjack toads sometimes breed here.


Coastal dune floral adaptations

Dune ecosystems are extremely difficult places for plants to survive. This is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include: * Little available soil moisture * Little available [[soil organic matter/nutrients/water * Harsh winds * Salt spray * Erosion/shifting and sometimes burial or exposure (from shifting) * Tidal influences There are many adaptations plants have evolved to cope with these pressures: * Deep taproot to reach water table ([[Abronia umbellata|Pink Sand Verbena) * Shallow but extensive root systems *[[Rhizomes * Prostrate growth form to avoid wind/salt spray (''Abronia spp''., Beach Primrose) * [[Krummholz growth form ([[Cupressus macrocarpa|Monterrey Cypress-not a dune plant but deals with similar pressures) * Thickened cuticle/Succulence to reduce moisture loss and reduce salt uptake (''Ambrosia/Abronia spp., Calystegia soldanella'') * Pale leaves to reduce insolation (''Artemisia/Ambrosia spp.'') * Thorny/Spiky seeds to ensure establishment in vicinity of parent, reduces chances of being blown away or swept out to sea (''Ambrosia chamissonis'')


Gypsum dunes

In deserts where large amounts of limestone mountains surround a [[closed basin, such as at [[White Sands National Park in south-central [[New Mexico, occasional storm runoff transports dissolved limestone and [[gypsum into a low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as [[Selenite (mineral)|selenite. The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea.


Nabkha dunes

A [[nabkha, or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals.


Sub-aqueous dunes

Sub-aqueous ([[underwater) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural [[Channel (geography)|channels such as rivers and estuaries, and also form in engineered [[canals and pipelines. Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical [[bedform construction. In the case of sub-aqueous barchan dunes, sediment is lost by their extremities, known as horns. These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in [[wavelength and height. The shape of a dune gives information about its formation environment. For instance, rivers produce asymmetrical ripples, with the steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments. Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river [[flooding.


Lithified dunes

A [[Lithification|lithified (consolidated) sand dune is a type of
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) be ...
that is formed when a marine or aeolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the color of the rock. [[Cross-bedding|Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in [[Zion National Park in the western United States. A slang term, used in the [[Southwestern United States|southwest US, for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the [[Old West because their steel-rimmed wagon wheels could not gain traction on the rock.


Desertification

Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move is by
saltation Saltation may refer to: * Saltation (biology), an evolutionary hypothesis emphasizing sudden and drastic change * Saltation (geology), a process of particle transport by fluids * Cutaneous rabbit illusion (sensory saltation), a perceptual illusion ...
, where sand particles skip along the ground like a [[bouncing ball. When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as [[creep (sand dunes)|creep. With slightly stronger winds, particles collide in mid-air, causing [[sheet flows. In a major [[dust storm, dunes may move tens of metres through such sheet flows. Also as in the case of snow, sand [[avalanches, falling down the [[slipface of the dunes—that face away from the winds—also move the dunes forward. Sand threatens buildings and crops in Africa, the Middle East, and China. Drenching sand dunes with oil stops their migration, but this approach uses a valuable resource and is quite destructive to the dunes' animal habitats. [[Sand fences might also slow their movement to a crawl, but geologists are still analyzing results for the optimum fence designs. Preventing sand dunes from overwhelming towns, villages, and agricultural areas has become a priority for the [[United Nations Environment Programme. Planting dunes with vegetation also helps to stabilise them.


Conservation

Dune habitats provide niches for highly specialized plants and animals, including numerous [[rare species and some [[endangered species. Due to widespread human population expansion, dunes face destruction through land development and recreational usages, as well as alteration to prevent the encroachment of sand onto inhabited areas. Some countries, notably the United States, Australia, Canada, New Zealand, the United Kingdom, Netherlands, and Sri Lanka have developed significant programs of dune protection through the use of [[sand dune stabilization. In the U.K., a [[Biodiversity Action Plan has been developed to assess dunes loss and to prevent future dunes destruction.


Examples


Africa

* [[Alexandria Coastal Dunefields, in the [[Eastern Cape, [[South Africa * Witsand Nature Reserve in the [[Kalahari Desert, [[South Africa * The white dunes of [[De Hoop Nature Reserve, [[South Africa * The dunes of the [[Suguta Valley, a desert part of the [[Great Rift Valley in northwestern [[Kenya * The dunes of the [[Danakil Depression, northeastern [[Ethiopia toward the border with [[Eritrea * The dunes of [[Sossusvlei in the greater [[Namib-Naukluft National Park, [[Namibia * The coastal dunes of [[Iona National Park in the southwesternmost part of [[Angola * [[Khawa, Botswana|Khawa dunes in the [[Kgalagadi Transfrontier Park, the southwesternmost part of [[Botswana * [[La Dune Rose in the city of [[Gao in northern [[Mali near the [[Niger River * [[Erg Aoukar in southeastern [[Mauritania extending into [[Mali * [[Erg Chech in southwestern [[Algeria and northern [[Mali * [[Erg Chebbi and [[Erg Chigaga in southern [[Morocco *
Grand Erg Oriental The Grand Erg Oriental (English: 'Great Eastern Sand Sea') is a large ''erg'' or "field of sand dunes" in the Sahara Desert. Situated for the most part in Saharan lowlands of northeast Algeria, the Grand Erg Oriental covers an area some 600 km ...
in northeastern [[Algeria and southern [[Tunisia * [[Grand Erg Occidental in western [[Algeria * The [[Idehan Ubari and the [[Idehan Murzuq in southwestern [[Libya * The [[Corralejo dunes in the [[Canary Islands, [[Spain * The [[Rebiana Sand Sea in southeastern [[Libya * The
Great Sand Sea The Great Sand Sea is an approximately 72,000 km² sand desert (erg) in the Sahara between western Egypt and eastern Libya in North Africa. Some 74% of the area is covered by sand dunes. Geography The Great Sand Sea stretches for a length of abou ...
in southeastern [[Libya and southwestern
Egypt Egypt ( ; ar|مِصر ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia by a land bridge formed by the Sinai Peninsula. Egypt is a Mediterranean count ...
* The Selima Sand Sheet in northwestern [[Sudan * The dunes of the [[Bayuda Desert in northern [[Sudan * The dunes of the [[Lompoul desert|Lompoul Desert in northwestern [[Senegal * The coastal dunes of [[Bazaruto Island, [[Mozambique * [[Djurab Desert|Erg du Djourab in northern [[Chad * The dunes of the [[Mourdi Depression in northeastern [[Chad * The dunes of [[Termit Massif Reserve|Tin Toumma Desert, in southeastern [[Niger * [[Great Bilma Erg|Grand Erg de Bilma in the [[Ténéré, in northern [[Niger * The dunes of [[Oursi Department|Oursi in the [[Sahel Region, northern [[Burkina Faso * [[Tanzania's Shifting Sands near [[Olduvai Gorge


Asia

* The dunes in the [[Thar Desert in [[India and [[Pakistan * [[Tottori Sand Dunes, [[Tottori Prefecture, [[Japan * [[Rig-e Jenn in the Central Desert of [[Iran. * [[Dasht-e Lut|Rig-e Lut in the Southeast of [[Iran. * The [[Ilocos Norte Sand Dunes in the [[Philippines, particularly [[Paoay Sand Dunes. * [[Moreeb Dune in [[Liwa Oasis, [[United Arab Emirates, used as an arena for drag motor sports and [[Sandboarding. * Gumuk Pasir Parangkusumo near [[Parangtritis beach in [[Yogyakarta, [[Indonesia. * Mui Ne, Vietnam. * [[Wahiba Sands, [[Oman * [[Teri (geology)|Teri, red dune complex in southern India * The dunes of the
Taklamakan Desert The Taklamakan Desert (; zh|s=|p=Tǎkèlāmǎgān Shāmò, Xiao'erjing: , dng|Такәламаган Шамә; ug|تەكلىماكان قۇملۇقى; also spelled Taklimakan and Teklimakan) is a desert in Southwest Xinjiang in Northwest China. ...

Taklamakan Desert
in southwest [[Xinjiang in [[Northwest China *[[Tukulans of the [[Central Yakutian Lowland, Russia


Europe

* The [[Dyuni|Dunes of Dyuni, near [[Pomorie, [[Bulgaria, vast area of sand dunes in the Burgas Province * The [[Dune of Pilat, not far from [[Bordeaux, [[France, is the largest known sand dune in Europe * The [[Arbus, Sardinia|Dunes of Piscinas, in the south west of [[Sardinia island. * [[Sands of Forvie within the [[Ythan Estuary complex, [[Aberdeenshire, [[Scotland. * [[Oxwich Bay|Oxwich Dunes, near [[Swansea, is on the [[Gower Peninsula in [[Wales. * [[Winterton Dunes – [[Norfolk, [[England * [[Słowiński National Park, [[Poland * [[Akrotiri Peninsula (Cyprus)|Akrotiri Sand Dune, [[Limassol District|Lemesos, [[Cyprus * [[Råbjerg mile, [[Northern Jutland, [[Denmark * [[Thy National Park, [[North Denmark Region, [[Denmark * [[The Dunes of Corrubedo Natural Park|Dunes of Corrubedo, [[Spain * [[Cresmina Dune, [[Portugal * [[Northern Littoral Natural Park, [[Portugal * [[Salir do Porto|Dune of Salir, [[Portugal * [[São Jacinto Dunes Natural Reserve, [[Portugal * Rëra e Hedhur in [[Shëngjin, [[Albania * [[Hoge Veluwe National Park, [[Veluwe, [[Netherlands * [[Dunes of Texel National Park, [[Texel, [[Netherlands * [[Zuid-Kennemerland National Park, [[North Holland, [[Netherlands * [[Ammothines Lemnou, [[Lemnos, [[Greece * Dunes of the [[Curonian Spit, [[Lithuania and [[Russia * [[Oleshky Sands, [[Ukraine * [[Ullahau, [[Sweden, Big Parabel Dune and dune system


North America

* Victoria Island Sand Dunes,160 km North West of Cambridge Bay, Nunavut, Canada. Approximately 600 square kilometers, the largest in Canada, third largest in North America and the largest in the Arctic. There are two lakes with direct access to the Dunes from float planes. *Herring Cove, Race Point and The Province Lands bicycle path in [[Provincetown, Massachusetts as part of the [[National Park Service|US National Park Service of the [[Cape Cod National Seashore. * Great Kobuk Sand Dunes, [[Kobuk Valley National Park, [[Alaska * The Athabasca Sand Dunes, located in the [[Athabasca Sand Dunes Provincial Park, [[Saskatchewan. * Th
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in [[California. * The [[Kelso Dunes in the [[Mojave Desert of [[California. *[[Eureka Valley Sand Dunes and [[Places of interest in the Death Valley area#Mesquite Flat Sand Dunes|Mesquite Flat Sand Dunes in [[Death Valley National Park, [[California. *[[Great Sand Dunes National Park, [[Colorado. *[[White Sands National Park, [[New Mexico. *[[Little Sahara Recreation Area, [[Utah. *[[Sleeping Bear Dunes National Lakeshore, [[Michigan, on the east shore of [[Lake Michigan. *[[Indiana Dunes National Park, [[Indiana, on the south shore of [[Lake Michigan. *[[Warren Dunes State Park, [[Michigan, on the east shore of [[Lake Michigan. *[[Grand Sable Dunes, in the [[Pictured Rocks National Lakeshore, [[Michigan. *[[Samalayuca Dune Fields|Samalayuca Dunes, in the state of [[Chihuahua (state)|Chihuahua, [[Mexico *[[Algodones Dunes near [[Brawley, California. *[[Guadalupe-Nipomo Dunes, on the central coast of [[California. *[[Monahans Sandhills State Park near [[Odessa, Texas. *[[Beaver Dunes State Park near [[Beaver, Oklahoma. * The Killpecker sand dunes of the [[Red Desert (Wyoming)|Red Desert in southwestern
Wyoming Wyoming () is a state in the Mountain West region of the United States. The 10th largest state by area, it is also the least populous and least densely populated state in the contiguous United States. It is bordered by Montana to the north an ...

Wyoming
. *[[Jockey's Ridge State Park – on the [[Outer Banks, [[North Carolina. * The Great Dune found in [[Cape Henlopen State Park in [[Lewes, Delaware. *[[Oregon Dunes National Recreation Area near [[Florence, Oregon, on the Pacific Coast. *[[Bruneau Dunes State Park – [[Owyhee Desert, [[Idaho *[[Hoffmaster State Park – [[Muskegon, [[Michigan *[[Silver Lake State Park (Michigan)|Silver Lake State Park — a sand dunes that allows off-road vehicle use located in [[Mears, Michigan.


South America

* [[Lençóis Maranhenses National Park in the state of [[Maranhão, [[Brazil * [[Jericoacoara National Park, in the state of [[Ceará, Brazil * [[Genipabu in [[Natal, Rio Grande do Norte|Natal, Brazil * [[Medanos de Coro National Park near the town of [[Coro (city)|Coro, in [[Falcón State, [[Venezuela * Duna Federico Kirbus in [[Catamarca Province, [[Argentina * [[Villa Gesell in [[Buenos Aires Province, Argentina * Cerro Blanco in [[Nazca Province, [[Peru * [[Huacachina in [[Ica Region, [[Peru * Cerro Medanoso in [[Atacama Region, [[Chile * [[Colún Beach, [[Valdivian Coastal Reserve in Chile * [[:es:Cerro Dragón (Chile), urban dune in Iquique, Chile


Oceania

* [[Simpson Desert sand dunes in the [[Northern Territory, [[Queensland and [[South Australia, [[Australia * [[Fraser Island in Queensland, Australia * [[Cronulla sand dunes in [[New South Wales, Australia * [[Stockton Beach in New South Wales, Australia * Te Paki sand dunes near [[Cape Reinga, New Zealand


World's highest dunes


Sand dune systems

:(coastal dunes featuring succession) * [[Athabasca Sand Dunes Provincial Park, [[Alberta and [[Saskatchewan * [[Ashdod Sand Dune, [[Israel * [[Bamburgh Dunes, [[Northumberland, England * [[Bradley Beach, New Jersey|Bradley Beach, [[New Jersey * [[Circeo National Park, a Mediterranean dune area on the southwest coast of the [[Lazio region of [[Italy * [[Cronulla sand dunes, [[New South Wales|NSW, Australia * [[Crymlyn Burrows, [[Wales * [[Dawlish Warren, [[Devon, England * [[Fraser Island, [[Queensland Australia, largest sand island in the world * [[Indiana Dunes National Park, [[Indiana * [[Kenfig Burrows, [[Wales * [[Margam burrows, [[Wales * [[Murlough Sand Dunes, [[Newcastle, County Down|Newcastle, [[County Down|Co Down, [[Northern Ireland * [[Morfa Harlech sand dunes, [[Gwynedd, [[Wales * [[Newborough Warren, [[North Wales * [[Oregon Dunes National Recreation Area, near [[North Bend, Oregon * [[Penhale Sands, [[Cornwall, England * [[Sleeping Bear Dunes National Lakeshore, [[Michigan * [[Sandy Island Beach State Park, [[Richland, New York * [[Studland, [[Dorset, [[England * [[Thy National Park, [[North Denmark Region, [[Denmark * Winterton, [[Norfolk, England * [[Woolacombe, Devon, England * [[Ynyslas Sand Dunes, [[Wales


Extraterrestrial dunes

Dunes can likely be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Dunes are common on [[Mars and in the equatorial regions of [[Titan (moon)|Titan. Titan's dunes include large expanses with modal lengths of about 20–30 km. The regions are not topographically confined, resembling sand seas. These dunes are interpreted to be longitudinal dunes whose crests are oriented parallel to the dominant wind direction, which generally indicates west-to-east wind flow. The sand is likely composed of hydrocarbon particles, possibly with some water ice mixed in. Dunes are a popular theme in science fiction, featuring in depictions of dry [[Desert planets appearing as early as the 1956 film ''[[Forbidden Planet'' and [[Frank Herbert's 1965 novel ''[[Dune (novel)|Dune''. The environment of the desert planet [[Arrakis (also known as Dune) in the [[Dune (franchise)|''Dune'' franchise ''Dune'' in turn inspired the ''[[Star Wars'' franchise, which includes prominent theme of dunes on fictional planets such as [[Tatooine, [[Geonosis, and [[Jakku.


See also

* [[Antidune * [[Devil's stovepipe * [[Ice dune * [[List of landforms * [[Masseira * [[Médanos (geology)|Médanos * [[Sandhill * [[Singing sand * [[Strand plain * [[Sand wave


Notes


References

*


Further reading

* * * * * * *


External links


Coastal Sand Dunes
* {{Authority control [[Category:Dunes| [[Category:Aeolian landforms [[Category:Coastal and oceanic landforms [[Category:Coastal geography [[Category:Sedimentology