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deposits are also described as aeolian 5 aeolian processes are most important in areas where there is little or no vegetation 1 however aeolian deposits are not restricted to arid climates they are also seen along shorelines along stream courses in semiarid climates in areas of ample sand weathered from weakly cemented sandstone outcrops and in areas of glacial outwash 6 loess which is silt deposited by wind is common in humid to subhumid climates much of north america and europe are underlain by sand and loess of pleistocene age originating from glacial outwash 6 the lee downwind side of river valleys in semiarid regions are often blanketed with sand and sand dunes examples in north america include the platte arkansas and missouri rivers 6 wind erosion edit sand blowing off a crest in the kelso dunes of the mojave desert california dust blown by the wind from an iowa field that was not planted 1890 effects of wind erosion at giza pyramid may 1972 wind erodes the earth s surface by deflation the removal of loose fine grained particles by the turbulent action of the wind and by abrasion the wearing down of surfaces by the grinding action and sandblasting by windborne particles once entrained in the wind collisions between particles further break them down a process called attrition 7 worldwide erosion by water is more important than erosion by wind but wind erosion is important in semiarid and arid regions 8 wind erosion is increased by some human activities such as the use of 4x4 vehicles 9 deflation edit deflation is the lifting and removal of loose material from the surface by wind turbulence 10 11 it takes place by three mechanisms traction surface creep saltation and suspension traction or surface creep is a process of larger grains sliding or rolling across the surface saltation refers to particles bouncing across the surface for short distances suspended particles are fully entrained in the wind which carries them for long distances 12 saltation likely accounts for 50 70 of deflation while suspension accounts for 30 40 and surface creep accounts for 5 25 13 regions which experience intense and sustained erosion are called deflation zones 14 most aeolian deflation zones are composed of desert pavement a sheet like surface of rock fragments that remains after wind and water have removed the fine particles the rock mantle in desert pavements protects the underlying material from further deflation areas of desert pavement form the regs or stony deserts of the sahara these are further divided into rocky areas called hamadas and areas of small rocks and gravel called serirs 7 desert pavement is extremely common in desert environments 15 blowouts are hollows formed by wind deflation blowouts are generally small but may be up to several kilometers in diameter the smallest are mere dimples 0 3 meters 1 ft deep and 3 meters 10 ft in diameter the largest include the blowout hollows of mongolia which can be 8 kilometers 5 mi across and 60 to 100 meters 200 to 400 ft deep big hollow in wyoming us extends 14 by 9 7 kilometers 9 by 6 mi and is up to 90 meters 300 ft deep 7 abrasion edit yardangs in the qaidam desert qinghai province china see also abrasion geology abrasion also sometimes called corrasion is the process of wind driven grains knocking or wearing material off of landforms it was once considered a major contributor to desert erosion but by the mid 20th century it had come to be considered much less important wind can normally lift sand only a short distance with most windborne sand remaining within 50 centimeters 20 in of the surface and practically none normally being carried above 2 meters 6 ft many desert features once attributed to wind abrasion including wind caves mushroom rocks and the honeycomb weathering called tafoni are now attributed to differential weathering rainwash deflation rather than abrasion or other processes 7 yardangs are one kind of desert feature that is widely attributed to wind abrasion these are rock ridges up to tens of meters high and kilometers long that have been streamlined by desert winds yardangs characteristically show elongated furrows or grooves aligned with the prevailing wind they form mostly in softer material such as silts 7 abrasion produces polishing and pitting grooving shaping and faceting of exposed surfaces these are widespread in arid environments but geologically insignificant polished or faceted surfaces called ventifacts are rare requiring abundant sand powerful winds and a lack of vegetation for their formation 7 in parts of antarctica wind blown snowflakes that are technically sediments have also caused abrasion of exposed rocks 16 attrition edit attrition is the wearing down by collisions of particles entrained in a moving fluid 17 18 it is effective at rounding sand grains and at giving them a distinctive frosted surface texture 19 collisions between windborne particles is a major source of dust in the size range of 2 5 microns most of this is produced by the removal of a weathered clay coating from the grains 18 transport edit see also aeolian dust dust storm approaching spearman texas 14 april 1935 dust storm in amarillo texas fsa photo by arthur rothstein 1936 a massive sand storm cloud is about to envelop a military camp as it rolls over al asad iraq just before nightfall on 27 april 2005 wind dominates the transport of sand and finer sediments in arid environments wind transport is also important in periglacial areas on river flood plains and in coastal areas coastal winds transport significant amounts of siliciclastic and carbonate sediments inland while wind storms and dust storms can carry clay and silt particles great distances wind transports much of the sediments deposited in deep ocean basins 12 in ergs desert sand seas wind is very effective at transporting grains of sand size and smaller 20 particles are transported by winds through suspension saltation skipping or bouncing and creeping rolling or sliding along the ground the minimum wind velocity to initiate transport is called the fluid threshold or static threshold and is the wind velocity required to begin dislodging grains from the surface once transport is initiated there is a cascade effect from grains tearing loose other grains so that transport continues until the wind velocity drops below the dynamic threshold or impact threshold which is usually less than the fluid threshold in other words there is hysteresis in the wind transport system 12 21 small particles may be held in the atmosphere in suspension turbulent air motion supports the weight of suspended particles and allows them to be transported for great distances wind is particularly effective at separating sediment grains under 0 05 mm in size from coarser grains as suspended particles 12 saltation is downwind movement of particles in a series of jumps or skips saltation is most important for grains of up to 2 mm in size a saltating grain may hit other grains that jump up to continue the saltation the grain may also hit larger grains over 2 mm in size that are too heavy to hop but that slowly creep forward as they are pushed by saltating grains 12 surface creep accounts for as much as 25 percent of grain movement in a desert 13 vegetation is effective at suppressing aeolian transport vegetation cover of as little as 15 is sufficient to eliminate most sand transport 22 23 the size of shore dunes is limited mostly by the amount of open space between vegetated areas 6 aeolian transport from deserts plays an important role in ecosystems globally for example wind transports minerals from the sahara to the amazon basin 24 saharan dust is also responsible for forming red clay soils in southern europe 25 dust storms edit main article dust storm dust storms are wind storms that have entrained enough dust to reduce visibility to less than 1 kilometer 0 6 mi 26 27 most occur on the synoptic regional scale due to strong winds along weather fronts 28 or locally from downbursts from thunderstorms 29 30 crops people and possibly even climates are affected by dust storms on earth dust can cross entire oceans as occurs with dust from the sahara that reaches the amazon basin 30 dust storms on mars periodically engulf the entire planet 31 when the mariner 9 spacecraft entered its orbit around mars in 1971 a dust storm lasting one month covered the entire planet thus delaying the task of photo mapping the planet s surface 32 most of the dust carried by dust storms is in the form of silt size particles deposits of this windblown silt are known as loess the thickest known deposit of loess up to 350 meters 1 150 ft is on the loess plateau in china 33 this very same asian dust is blown for thousands of miles forming deep beds in places as far away as hawaii 34 the peoria loess of north america is up to 40 meters 130 ft thick in parts of western iowa 35 the soils developed on loess are generally highly productive for agriculture 36 small whirlwinds called dust devils are common in arid lands and are thought to be related to very intense local heating of the air that results in instabilities of the air mass dust devils may be as much as one kilometer high 37 dust devils on mars have been observed as high as 10 kilometers 6 2 mi though this is uncommon 38 deposition edit mesquite flat dunes in death valley looking toward the cottonwood mountains from the north west arm of star dune 2003 aeolian deposition near addeha kola tembien ethiopia 2019 see also aeolian landform wind is very effective at separating sand from silt and clay as a result there are distinct sandy erg and silty loess aeolian deposits with only limited interbedding between the two loess deposits are found further from the original source of sediments than ergs an example of this is the sand hills of nebraska us here vegetation stabilized sand dunes are found to the west and loess deposits to the east further from the original sediment source in the ogallala formation at the feet of the rocky mountains 6 some of the most significant experimental measurements on aeolian landforms were performed by ralph alger bagnold 39 a british army engineer who worked in egypt prior to world war ii bagnold investigated the physics of particles moving through the atmosphere and deposited by wind 40 he recognized two basic dune types the crescentic dune which he called barchan and the linear dune which he called longitudinal or seif arabic for sword bagnold developed a classification scheme that included small scale ripples and sand sheets as well as various types of dunes 6 bagnold s classification is most applicable in areas devoid of vegetation 6 in 1941 john tilton hack added parabolic dunes which are strongly influenced by vegetation to the list of dune types 41 the discovery of dunes on mars reinvigorated aeolian process research 42 which increasingly makes use of computer simulation 39 wind deposited materials hold clues to past as well as to present wind directions and intensities these features help us understand the present climate and the forces that molded it 6 for example vast inactive ergs in much of the modern world attest to late pleistocene trade wind belts being much expanded during the last glacial maximum ice cores show a tenfold increase in non volcanic dust during glacial maxima the highest dust peak in the vostok ice cores dates to 20 to 21 thousand years ago the abundant dust is attributed to a vigorous low latitude wind system plus more exposed continental shelf due to low sea levels 43 wind deposited sand bodies occur as ripples and other small scale features sand sheets and dunes ripples and other small scale features edit wind ripples on crescent shaped sand dunes barchans in southwest afghanistan sistan wind blowing on a sand surface ripples the surface into crests and troughs whose long axes are perpendicular to the wind direction the average length of jumps during saltation corresponds to the wavelength or distance between adjacent crests of the ripples in ripples the coarsest materials collect at the crests causing inverse grading this distinguishes small ripples from dunes where the coarsest materials are generally in the troughs this is also a distinguishing feature between water laid ripples and aeolian ripples 44 a sand shadow is an accumulation of sand on the downwind side of an obstruction such as a boulder or an isolated patch of vegetation here the sand builds up to the angle of repose the maximum stable slope angle about 34 degrees then begins sliding down the slip face of the patch a sandfall is a sand shadow of a cliff or escarpment 6 closely related to sand shadows are sand drifts these form downwind of a gap between obstructions due to the funneling effect of the obstructions on the wind 6 sand sheets edit main article sand sheet sand sheets are flat or gently undulating sandy deposits with only small surface ripples an example is the selima sand sheet in the eastern sahara desert which occupies 60 000 square kilometers 23 000 sq mi in southern egypt and northern sudan this consists of a few feet of sand resting on bedrock sand sheets are often remarkably flat and are sometimes described as desert peneplains 6 sand sheets are common in desert environments particularly on the margins of dune fields although they also occur within ergs conditions that favor the formation of sand sheets instead of dunes may include surface cementation a high water table the effects of vegetation periodic flooding or sediments rich in grains too coarse for effective saltation 45 dunes edit sand dunes of the empty quarter to the east of liwa oasis united arab emirates main article dune a dune is an accumulations of sediment blown by the wind into a mound or ridge they differ from sand shadows or sand drifts in that they are independent of any topographic obstacle 6 dunes have gentle upwind slopes on the windward side the downwind portion of the dune the lee slope is commonly a steep avalanche slope referred to as a slipface dunes may have more than one slipface the minimum height of a slipface is about 30 centimeters 46 wind blown sand moves up the gentle upwind side of the dune by saltation or creep sand accumulates at the brink the top of the slipface when the buildup of sand at the brink exceeds the angle of repose a small avalanche of grains slides down the slipface grain by grain the dune moves downwind 46 dunes take three general forms linear dunes also called longitudinal dunes or seifs are aligned in the direction of the prevailing winds transverse dunes which include crescent dunes barchans are aligned perpendicular to the prevailing winds more complex dunes such as star dunes form where the directions of the winds are highly variable additional dune types arise from various kinds of topographic forcing such as from isolated hills or escarpments 47 transverse dunes edit transverse dunes occur in areas dominated by a single direction of the prevailing wind in areas whe...