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71-448: A dune is a landform composed of wind- or water-driven sand . 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

142-562: A dune forms, plant 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 marram grass , sea wort grass and other sea grasses. These plants are well adapted to

213-590: 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,

284-407: A low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as 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

355-481: A method of driving over the large sand dunes found in the "sand seas" of the Libyan Desert . He wrote, "I increased speed. ... A huge glaring wall of yellow shot up high into the sky. The lorry tipped violently backwards—and we rose as in a lift, smoothly without vibration. We floated up on a yellow cloud. All the accustomed car movements had ceased; only the speedometer told us we were still moving fast. It

426-676: A mobile scouting force for desert operations against the Italian Armed Forces in Libya, which Wavell was charged with defeating in the field. During the conversation Wavell asked Bagnold what he would do if he found that the Italians were not doing anything beyond the Libyan coast in the desert interior. Bagnold replied that the new unit that he had in mind might be able to commit "acts of piracy". Wavell granted Bagnold authority to form

497-410: 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 , 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 much a dune erodes during any storm surge

568-462: 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. 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,

639-818: 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. Seif dunes are common in the Sahara. They range up to 300 m (980 ft) in height and 300 km (190 mi) in length. In the southern third of the Arabian Peninsula, a vast erg , called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km (120 mi) and reach heights of over 300 m (980 ft). Linear loess hills known as pahas are superficially similar. These hills appear to have been formed during

710-479: A unit along these lines, with it being constituted in July 1940 with the name Long Range Desert Group (L.R.D.G.). After assembling its first formation, Bagnold was the L.R.D.G.'s commanding officer until August 1941, when he handed over command to Guy Prendergast on being promoted to the post of Inspector of Desert Troops. In October 1941 he was promoted to the post of Deputy Signal Officer-in-Chief Middle East, with

781-485: Is a type of sandstone 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 colour of the rock. 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

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852-441: Is a very large aeolian 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 ) up

923-408: 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

994-445: Is by saltation , 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 . 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

1065-428: Is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include: Plants have evolved many adaptations to cope with these pressures: 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

1136-404: 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

1207-425: Is the third or vertical dimension of land surface . Topography is the study of terrain, although the word is often used as a synonym for relief itself. When relief is described underwater , the term bathymetry is used. In cartography , many different techniques are used to describe relief, including contour lines and triangulated irregular networks . Elementary landforms (segments, facets, relief units) are

1278-535: The Axis powers ' defeat in that theatre. and returned to his scientific interests, being elected to a Fellowship of the Royal Society in the same year. After the war Bagnold continued to work in the field of the geological science, and he published academic papers into his nineties. He made significant contributions to the understanding of desert terrain such as sand dunes , ripples and sheets . He developed

1349-526: The North West Frontier , India, where he was again mentioned in dispatches. In both of these locations, he spent much of his leave exploring the local deserts . After having read Ahmed Hassanein 's "Lost Oasis" he spent one such expedition in 1929 using a Ford Model A automobile and two Ford lorries exploring the vast swathe of desert from Cairo to Ain Dalla which was an area reputed to contain

1420-774: The Royal Military Academy, Woolwich . He spent three years in the trenches in France , being mentioned in despatches in 1917 and receiving the Belgian Order of Leopold in 1919. After the war Bagnold studied engineering at Gonville and Caius College, Cambridge , obtaining an MA before returning to active duty with the British Army in 1920 with the Royal Corps of Signals . He served in Cairo and

1491-509: The Universe . Examples are mountains, hills, polar caps, and valleys, which are found on all of the terrestrial planets . The scientific study of landforms is known as geomorphology . In onomastic terminology, toponyms (geographical proper names) of individual landform objects (mountains, hills, valleys, etc.) are called oronyms . Landforms may be extracted from a digital elevation model (DEM) using some automated techniques where

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1562-522: The Western Desert of Egypt . The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert . 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

1633-989: The Wollaston Medal , the highest award granted by the Geological Society of London , and in 1981 the David Linton Award of the British Geomorphological Research Group . He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1974. Other awards included the 1970 Penrose Medal by the Geological Society of America ; and the Sorby Medal from the International Association of Sedimentologists. He also received honorary D.Sc. degrees from both

1704-522: The rescue expedition of 1884–85 to rescue General Gordon in Khartoum , Sudan . His sister was the novelist and playwright Enid Bagnold , who wrote the 1935 novel National Velvet . After Malvern College , he attended the Royal Military Academy, Woolwich . In 1915, Ralph Bagnold followed in his father's footsteps and was commissioned into the Royal Engineers, after having graduated from

1775-676: The slipface of the dunes—that face away from the winds—also move the dunes forward. Landform Landforms are categorized by characteristic physical attributes such as elevation, slope, orientation, structure stratification , rock exposure, and soil type. Gross physical features or landforms include intuitive elements such as berms , mounds , hills , ridges , cliffs , valleys , rivers , peninsulas , volcanoes , and numerous other structural and size-scaled (e.g. ponds vs. lakes , hills vs. mountains ) elements including various kinds of inland and oceanic waterbodies and sub-surface features. Mountains, hills, plateaux , and plains are

1846-458: The 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. 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

1917-656: The stoss (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 bedrock disintegrates to produce an ample supply of loose sand. Subaqueous dunes can form from

1988-525: The temporary rank of Brigadier and worked on camouflage and deception operations. He left after six months in March 1942, visiting Gaza , Brummana and Jerusalem (October 1942), Sudan and Eritrea (December 1942) and Turkey (May 1943 and August 1943) before returning to England in March 1944. On 7 June 1944 Bagnold retired from the British Army with the end of military operations in North Africa after

2059-546: The action of water flow ( fluvial processes) on sand or gravel beds of rivers , estuaries , and the sea-bed. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach . 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,

2130-420: The book Libyan Sands: Travel in a Dead World (first published 1935; reprinted by Eland in 2010). He is credited with developing a sun compass , which is not affected by magnetic iron ore deposits or by metal vehicles as a magnetic compass might be. During the 1930s his group also began the practice of reducing tyre pressure when driving over loose sand. In addition Bagnold is credited with devising

2201-407: The coastal environment of the sand dune vital to their species' survival. Over the course of time coastal dunes may be impacted by tropical cyclones 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

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2272-608: 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 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

2343-405: 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

2414-763: The data has been gathered by modern satellites and stereoscopic aerial surveillance cameras. Until recently, compiling the data found in such data sets required time consuming and expensive techniques involving many man-hours. The most detailed DEMs available are measured directly using LIDAR techniques. Igstar, cxvellie (2017), Howard, Jeffrey (ed.), "Anthropogenic Landforms and Soil Parent Materials", Anthropogenic Soils, Progress in Soil Science, Cham: Springer International Publishing, pp. 25–51, doi:10.1007/978-3-319-54331-4_3, ISBN 978-3-319-54331-4, retrieved 2022-08-12 Ralph Bagnold Brigadier Ralph Alger Bagnold , OBE , FRS , (3 April 1896 – 28 May 1990)

2485-520: The dimensionless " Bagnold number " and " Bagnold formula " for characterising sand flow. He gave a constitutive relation for a suspension of neutrally buoyant particles in a Newtonian fluid . He also proposed a model for " singing sands ". and made contributions to the science of Sedimentology . His work received a number of awards. He was the 1969 recipient of the G. K. Warren Prize from the National Academy of Sciences . In 1971 he received

2556-496: 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 and West Africa , and in parts of

2627-678: 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

2698-604: The dune and underlying soils . 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

2769-511: The dunes and provide horticultural benefits, but instead spread taking land away from native species. Ammophila arenaria , 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. As

2840-598: 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. Dune ecosystems are extremely difficult places for plants to survive. This

2911-591: The dunes. Typically these are heather , heaths and gorses . These too are adapted to the low 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 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

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2982-401: 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

3053-464: The enormous empty solitudes of the inner desert, walled off by sheer distance, lack of water, and impassable seas of sand dunes. Little did we dream that any of the special equipment and techniques we had evolved for very long-distance travel, and for navigation, would ever be put to serious use." On 10 June 1940 Italy declared war on the United Kingdom in alliance with Germany while Bagnold

3124-643: The four major types of landforms. Minor landforms include buttes , canyons, valleys, and basins. Tectonic plate movement under the Earth can create landforms by pushing up mountains and hills. Oceans and continents exemplify the highest-order landforms. Landform elements are parts of a high-order landforms that can be further identified and systematically given a cohesive definition such as hill-tops, shoulders, saddles , foreslopes and backslopes. Some generic landform elements including: pits, peaks, channels, ridges, passes, pools and plains. Terrain (or relief )

3195-517: The geological record . All these dune shapes 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). 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

3266-593: 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

3337-408: 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

3408-486: The last ice age under permafrost conditions dominated by sparse tundra vegetation. 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 of the Sahara. In other deserts, they occur around

3479-521: The margins of the sand seas , particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth. Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas. Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to

3550-684: The mythical city of Zerzura . After a brief period of half-pay , he left the Army in 1935, but rejoined upon the outbreak of the Second World War. Bagnold and his travelling companions were early pioneers in the use of motor vehicles to explore the desert. In 1932 Bagnold explored the Mourdi Depression , in present-day Chad , and found implements dated to the Palaeolithic period in the valley. Bagnold wrote of his travels in

3621-654: 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 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 (United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they do not become more than 10 metres high. The dunes form at about ninety degrees to

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3692-439: The prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest. 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

3763-482: 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 , in The Physics of Blown Sand and Desert Dunes , 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 in

3834-477: The ridge crest. 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

3905-492: The role of vegetation in the development of dune systems and salt marshes , and the work of corals and algae in the formation of coral reefs . Landforms do not include several man-made features, such as canals , ports and many harbors ; and geographic features, such as deserts , forests , and grasslands . Many of the terms are not restricted to refer to features of the planet Earth , and can be used to describe surface features of other planets and similar objects in

3976-415: 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

4047-423: 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 form when wet sand is deposited along the coast and dries out and

4118-434: The sea. 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 ( underwater ) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as

4189-644: The smallest homogeneous divisions of the land surface, at the given scale/resolution. These are areas with relatively homogeneous morphometric properties, bounded by lines of discontinuity. A plateau or a hill can be observed at various scales, ranging from a few hundred meters to hundreds of kilometers. Hence, the spatial distribution of landforms is often scale-dependent, as is the case for soils and geological strata. A number of factors, ranging from plate tectonics to erosion and deposition (also due to human activity), can generate and affect landforms. Biological factors can also influence landforms—for example, note

4260-402: 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 . A lithified (consolidated) sand dune

4331-603: 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 gives the speed at which particles can be transported. 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 dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on

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4402-678: 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 , were introduced from South Africa in an attempt to stabilize

4473-745: The terrain of the planet Mars , the Bagnold Dunes on Mars' surface were named after him by the organisation. He returned to the forces in the Second World War , in which he founded the behind-the-lines reconnaissance , espionage and raiding unit the " Long Range Desert Group ", serving as its first commanding officer in the North Africa campaign . Bagnold was born in Devonport , England. His father, Colonel Arthur Henry Bagnold (1854–1943) ( Royal Engineers ), participated in

4544-504: The town of Eucla, Western Australia , 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 dūne . A universally precise distinction does not exist between ripples, dunes, and draas , 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

4615-467: 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

4686-533: The western United States, especially Texas. 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

4757-399: 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 . Models of coastal dunes suggest that their final equilibrium height is related to

4828-479: The wind next blows in the dominant direction. 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 well-represented in

4899-621: Was an English 20th-century desert explorer, geologist and soldier. Bagnold served in the First World War as an engineer in the British Army. In 1932, he staged the first recorded East-to-West crossing of the Libyan Desert . His work in the field of Aeolian processes was the basis for the book The Physics of Blown Sand and Desert Dunes , establishing the discipline of aeolian geomorphology, combining field work observations, experiments and physical equations. His work has been used by United States' space agency NASA in its study of

4970-462: Was in Cairo due to an accident involving a troopship collision that he was on interrupting his journey elsewhere. Upon hearing the news and realizing that North Africa was about to become a theatre of war, he requested an interview with General Archibald Wavell , Commander-in-Chief Middle East. Having secured it, Bagnold suggested that Wavell use his knowledge of the terrain in North Africa to establish

5041-586: Was incredible ..." However, noted Fitzroy Maclean , "too much dash had its penalties. Many of the dunes fell away sharply at the far side and if you arrived at the top at full speed, you were likely to plunge headlong over the precipice. ... and end up with your truck upside down on top of you." A recently discovered silent film documents Bagnold's explorations and is available via the British Film Institute . Bagnold wrote, "Never in our peacetime travels had we imagined that war could ever reach

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