,u4 ,s4 ,department of the ,interior _/ ,u4 ,s4 ,geological ,survey ,deserts3 ,geology and ,resources by ,a4 ,s4 ,walker ,illustration3 ,great ,sand ,dunes ,national ,monument1 ,colorado 7photograph by ,john ,keith74 ,beauty is before me ,and beauty behind me1 ,above and below me hovers the beautiful1 ,i am surrounded by it1 ,i am immersed in it4 ,in my youth ,i am aware of it1 ,and in old age ,i shall walk quietly ,the beautiful trail4 from a ,navajo benedictory chant describing the desert ,illustration3 ,cacti dominate the ,sonoran ,desert vegetation near ,tucson1 ,arizona 7photograph by ,peter ,kresan74 ,what ,is a ,desert8 ,approximately one-third of the ,earth's land surface is desert1 arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals4 ,deserts--stark1 sometimes mysterious worlds--have been portrayed as fascinating environments of adventure and exploration from narratives such as that of ,lawrence of ,arabia to movies such as 8,dune40 ,these arid regions are called deserts because they are dry4 ,they may be hot1 they may be cold4 ,they may be regions of sand or vast areas of rocks and gravel peppered with occasional plants4 ,but deserts are always dry4 ,illustration3 ,ripples on a dune in the ,gran ,desierto1 ,mexico 7photograph by ,peter ,kresan74 ,deserts are natural laboratories in which to study the interactions of wind and sometimes water on the arid surfaces of planets4 ,they contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion4 ,because deserts are dry1 they are ideal places for human artifacts and fossils to be preserved4 ,deserts are also fragile environments4 ,the misuse of these lands is a serious and growing problem in parts of our world4 ,illustration3 ,,distribution ,,of ,,non-,,polar ,,arid ,,land 7after ,meigs1 #aiec7 ,higher-resolution map ,there are almost as many definitions of deserts and classification systems as there are deserts in the world4 ,most classifications rely on some combination of the number of days of rainfall1 the total amount of annual rainfall1 temperature1 humidity1 or other factors4 ,in #aiec1 ,peveril ,meigs divided desert regions on ,earth into three categories according to the amount of precipitation they received4 ,in this now widely accepted system1 extremely arid lands have at least #ab consecutive months without rainfall1 arid lands have less than #bej millimeters of annual rainfall1 and semiarid lands have a mean annual precipitation of between #bej and #ejj millimeters4 ,arid and extremely arid land are deserts1 and semiarid grasslands generally are referred to as steppes4 ,illustration3 ,gran ,desierto of the ,sonoran ,desert1 ,mexico4 ,surrounding the dark #be-kilometer-long and #e-kilometer-wide ,sierra del ,rosario mountains 7upper right7 are dunes and sheets of sand4 ,how the ,atmosphere ,influences ,aridity ,we live at the bottom of a gaseous envelope--the atmosphere--that is bound gravitationally to the planet ,earth4 ,the circulation of our atmosphere is a complex process because of the ,earth's rotation and the tilt of its axis4 ,the ,earth's axis is inclined #bc from the ecliptic1 the plane of the ,earth's orbit around the ,sun4 ,due to this inclination1 vertical rays of the ,sun strike #bc ,n4 latitude1 the ,tropic of ,cancer1 at summer solstice in late ,june4 ,at winter solstice1 the vertical rays strike #bc ,s4 latitude1 the ,tropic of ,capricorn4 ,in the ,northern ,hemisphere1 the summer solstice day has the most daylight hours1 and the winter solstice has the fewest daylight hours each year4 ,the tilt of the axis allows differential heating of the ,earth's surface1 which causes seasonal changes in the global circulation4 ,illustration3 ,the circulation pattern of the ,earth's atmosphere4 ,most of the nonpolar deserts lie within the two trade winds belts4 ,on a planetary scale1 the circulation of air between the hot ,equator and the cold ,north and ,south ,poles creates pressure belts that influence weather4 ,air warmed by the ,sun rises at the ,equator1 cools as it moves toward the poles1 descends as cold air over the poles1 and warms again as it moves over the surface of the ,earth toward the ,equator4 ,this simple pattern of atmospheric convection1 however1 is complicated by the rotation of the ,earth1 which introduces the ,coriolis ,effect4 ,to appreciate the origin of this effect1 consider the following4 ,a stick placed vertically in the ground at the ,north ,pole would simply turn around as the ,earth rotates4 ,a stick at the ,equator would move in a large circle of almost #dj1jjj kilometers with the ,earth as it rotates4 ,the ,coriolis ,effect illustrates ,newton's first law of motion--a body in motion will maintain its speed and direction of motion unless acted on by some outside force4 ,thus1 a wind travelling north from the equator will maintain the velocity acquired at the equator while the ,earth under it is moving slower4 ,this effect accounts for the generally east-west direction of winds1 or streams of air1 on the ,earth's surface4 ,winds blow between areas of different atmospheric pressures4 ,the ,coriolis ,effect influences the circulation pattern of the ,earth's atmosphere4 ,in the zone between about #cj ,n4 and #cj ,s41 the surface air flows toward the ,equator and the flow aloft is poleward4 ,a low-pressure area of calm1 light variable winds near the equator is known to mariners as the doldrums4 ,around #cj ,n4 and ,s41 the poleward flowing air begins to descend toward the surface in subtropical high-pressure belts4 ,the sinking air is relatively dry because its moisture has already been released near the ,equator above the tropical rain forests4 ,near the center of this high-pressure zone of descending air1 called the 8,horse ,latitudes10 the winds at the surface are weak and variable4 ,the name for this area is believed to have been given by colonial sailors1 who1 becalmed sometimes at these latitudes while crossing the oceans with horses as cargo1 were forced to throw a few horses overboard to conserve water4 ,the surface air that flows from these subtropical high-pressure belts toward the ,equator is deflected toward the west in both hemispheres by the ,coriolis ,effect4 ,because winds are named for the direction from which the wind is blowing1 these winds are called the northeast trade winds in the ,northern ,hemisphere and the southeast trade winds in the ,southern ,hemisphere4 ,the trade winds meet at the doldrums4 ,surface winds known as 8westerlies0 flow from the ,horse ,latitudes toward the poles4 ,the 8westerlies0 meet 8easterlies0 from the polar highs at about #ej-fj ,n4 and ,s4 ,near the ground1 wind direction is affected by friction and by changes in topography4 ,winds may be seasonal1 sporadic1 or daily4 ,they range from gentle breezes to violent gusts at speeds greater than #cjj kilometers_/hour4 ,illustration3 ,these dunes in the ,algodones ,sand ,sea of southeastern ,california move as much as #e meters per year4 ,the dunes in this photograph1 looking south1 move toward the east 7left7 7photograph by ,peter ,kresan74 ,illustration3 ,searles ,lake1 ,california 7photograph courtesy of ,kerr-,mc,gee1 ,inc474 ,where ,deserts ,form ,dry areas created by global circulation patterns contain most of the deserts on the ,earth4 ,the deserts of our world are not restricted by latitude1 longitude1 or elevation4 ,they occur from areas close to the poles down to areas near the ,equator4 ,the ,people's ,republic of ,china has both the highest desert1 the ,qaidam ,depression that is #b1fjj meters above sea level1 and one of the lowest deserts1 the ,turpan ,depression that is #aej meters below sea level4 ,deserts are not confined to ,earth4 ,the atmospheric circulation patterns of other terrestrial planets with gaseous envelopes also depend on the rotation of those planets1 the tilts of their axes1 their distances from the ,sun1 and the composition and density of their atmospheres4 ,except for the poles1 the entire surface of ,mars is a desert4 ,venus also may support deserts4 ,illustration3 ,the ,garlock fault1 near the bottom of this ,landsat image1 is generally considered to be the geologic border between the ,mojave ,desert in the south and the ,great ,basin ,desert in the north4 ,the ,great ,basin contains more than #aej discrete desert basins separated by more than #afj mountain ranges4 ,types of ,deserts ,deserts are classified by their geographical location and dominant weather pattern as trade wind1 midlatitude1 rain shadow1 coastal1 monsoon1 or polar deserts4 ,former desert areas presently in nonarid environments are paleodeserts1 and extraterrestrial deserts exist on other planets4 ,trade wind deserts ,the trade winds in two belts on the equatorial sides of the ,horse ,latitudes heat up as they move toward the ,equator4 ,these dry winds dissipate cloud cover1 allowing more sunlight to heat the land4 ,most of the major deserts of the world lie in areas crossed by the trade winds4 ,the world's largest desert1 the ,sahara of ,north ,africa1 which has experienced temperatures as high as #eg ,c1 is a trade wind desert4 ,midlatitude deserts ,midlatitude deserts occur between #cj and #ej ,n4 and ,s41 poleward of the subtropical high-pressure zones4 ,these deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures4 ,the ,sonoran ,desert of southwestern ,north ,america is a typical midlatitude desert4 ,rain shadow deserts ,rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee1 or protected side1 of the range4 ,as air rises over the mountain1 water is precipitated and the air loses its moisture content4 ,a desert is formed in the leeside 8shadow0 of the range4 ,illustration3 ,the ,sahara of ,africa is the world's largest desert4 ,it contains complex linear dunes that are separated by almost #f kilometers 7,skylab photograph74 ,illustration3 ,a rare rain in the ,tengger1 a midlatitude desert of ,china1 exposes ripples and a small blowout on the left4 ,winds will shortly cover or remove these features4 ,illustration3 ,this ,landsat image shows the ,turpan ,depression in the rain shadow desert of the ,tian ,shan of ,china4 ,a sand sea is in the lower center on the right1 but desert pavement1 gray in color1 dominates this desert4 ,the few oases in the desert and the vegetation in the mountains at the top are in red4 ,a blanket of snow separates the vegetation in the ,tian ,shan from the rain shadow desert4 ,coastal deserts ,coastal deserts generally are found on the western edges of continents near the ,tropics of ,cancer and ,capricorn4 ,they are affected by cold ocean currents that parallel the coast4 ,because local wind systems dominate the trade winds1 these deserts are less stable than other deserts4 ,winter fogs1 produced by upwelling cold currents1 frequently blanket coastal deserts and block solar radiation4 ,coastal deserts are relatively complex because they are at the juncture of terrestrial1 oceanic1 and atmospheric systems4 ,a coastal desert1 the ,atacama of ,south ,america1 is the ,earth's driest desert4 ,in the ,atacama1 measurable rainfall--#a millimeter or more of rain--may occur as infrequently as once every #e-bj years4 ,illustration3 ,crescent-shaped dunes are common in coastal deserts such as the ,namib1 ,africa1 with prevailing onshore winds4 ,low clouds cover parts of the ,namib in this space shuttle photo4 ,illustration3 ,high dunes of the ,namib ,desert near ,sossus ,vlei 7photograph by ,georg ,gerster74 ,illustration3 ,morning fog moistens the dunes of the ,namib coastal desert 7photograph by ,georg ,gerster74 ,monsoon deserts 8,monsoon10 derived from an ,arabic word for 8season10 refers to a wind system with pronounced seasonal reversal4 ,monsoons develop in response to temperature variations between continents and oceans4 ,the southeast trade winds of the ,indian ,ocean1 for example1 provide heavy summer rains in ,india as they move onshore4 ,as the monsoon crosses ,india1 it loses moisture on the eastern slopes of the ,aravalli ,range4 ,the ,rajasthan ,desert of ,india and the ,thar ,desert of ,pakistan are parts of a monsoon desert region west of the range4 ,illustration3 ,the ,indus ,river floodplain1 lower left1 is the western border of the ,thar ,desert4 ,this ,landsat image of the monsoon desert shows small patches of sand sheets in the upper right1 with three types of dunes2 some dunes are almost #c kilometers long4 ,polar deserts ,polar deserts are areas with annual precipitation less than #bej millimeters and a mean temperature during the warmest month of less than #aj ,c4 ,polar deserts on the ,earth cover nearly #e million square kilometers and are mostly bedrock or gravel plains4 ,sand dunes are not prominent features in these deserts1 but snow dunes occur commonly in areas where precipitation is locally more abundant4 ,temperature changes in polar deserts frequently cross the freezing point of water4 ,this 8freeze-thaw0 alternation forms patterned textures on the ground1 as much as #e meters in diameter4 ,illustration3 ,the ,dry ,valleys of ,antarctica have been ice-free for thousands of years 7courtesy of ,,usgs ,image ,processing ,facility1 ,flagstaff1 ,arizona74 ,paleodeserts ,illustration3 ,this aerial photograph of the ,nebraska ,sand ,hills paleodesert shows a well-preserved crescent-shaped dune 7or barchan7 about #fj to #ge meters high 7photograph by ,thomas ,s4 ,ahlbrandt74 ,data on ancient sand seas 7vast regions of sand dunes71 changing lake basins1 archaeology1 and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the ,earth in the recent geologic past4 ,during the last #ab1ejj years1 for example1 parts of the deserts were more arid than they are today4 ,about #aj percent of the land between #cj ,n4 and #cj ,s4 is covered now by sand seas4 ,nearly #ah1jjj years ago1 sand seas in two vast belts occupied almost #ej percent of this land area4 ,as is the case today1 tropical rain forests and savannahs were between the two belts4 ,illustration3 ,a dry community of vegetation grows among the dunes of the ,nebraska ,sand ,hills 7photograph by ,n4 ,h4 ,darton74 ,fossil desert sediments that are as much as #ejj million years old have been found in many parts of the world4 ,sand dune-like patterns have been recognized in presently nonarid environments4 ,many such relict dunes now receive from #hj to #aej millimeters of rain each year4 ,some ancient dunes are in areas now occupied by tropical rain forests4 ,the ,nebraska ,sand ,hills is an inactive #eg1jjj-square kilometer dune field in central ,nebraska4 ,the largest sand sea in the ,western ,hemisphere1 it is now stabilized by vegetation and receives about #ejj millimeters of rain each year4 ,dunes in the ,sand ,hills are up to #abj meters high4 ,extraterrestrial deserts ,mars is the only other planet on which we have identified wind-shaped 7eolian7 features4 ,although its surface atmospheric pressure is only about one-hundredth that of ,earth1 global circulation patterns on ,mars have formed a circumpolar sand sea of more than five million square kilometers1 an area greater than the ,empty ,quarter of ,saudi ,arabia1 the largest sand sea on our planet4 ,martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north polar area4 ,smaller dune fields occupy the floors of many large craters in the polar regions4 ,illustration3 ,this ,viking spacecraft image of ,mars shows alternating layers of ice and windblown dust near the north polar cap4 ,annual and other periodic climatic changes due to orbit fluctuations may occur on ,mars 7courtesy of ,,usgs ,image ,processing ,facility1 ,flagstaff1 ,arizona74 ,illustration3 ,one of the first images taken at the ,viking #b landing site on ,mars shows the pink sky over ,utopia and the desert pavement on the ground 7courtesy of ,,nasa74 ,illustration3 ,some of the crescent-shaped dunes in this ,viking image of ,mars are more than a kilometer wide4 ,the dark material that streaks from the horn-shaped features may be dust recently blown from the dunes 7courtesy of ,,nasa74 ,illustration3 ,natural ,bridge1 ,arches ,national ,monument1 ,utah 7photograph by ,peter ,kresan74 ,desert ,features ,sand covers only about #bj percent of the ,earth's deserts4 ,most of the sand is in sand sheets and sand seas--vast regions of undulating dunes resembling ocean waves 8frozen0 in an instant of time4 ,nearly #ej percent of desert surfaces are plains where eolian deflation--removal of fine-grained material by the wind--has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles4 ,the remaining surfaces of arid lands are composed of exposed bedrock outcrops1 desert soils1 and fluvial deposits including alluvial fans1 playas1 desert lakes1 and oases4 ,bedrock outcrops commonly occur as small mountains surrounded by extensive erosional plains4 ,oases are vegetated areas moistened by springs1 wells1 or by irrigation4 ,many are artificial4 ,oases are often the only places in deserts that support crops and permanent habitation4 ,illustration3 ,underground channels carry water from nearby mountains into the ,turpan ,depression of ,china4 ,if the channels were not covered1 the water would evaporate quickly when it reached the hot1 dry desert land4 ,soils ,soils that form in arid climates are predominantly mineral soils with low organic content4 ,the repeated accumulation of water in some soils causes distinct salt layers to form4 ,calcium carbonate precipitated from solution may cement sand and gravel into hard layers called 8calcrete0 that form layers up to #ej meters thick4 ,caliche is a reddish-brown to white layer found in many desert soils4 ,caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction between water and carbon dioxide released by plant roots or by decaying organic material4 ,plants ,most desert plants are drought- or salt-tolerant4 ,some store water in their leaves1 roots1 and stems4 ,other desert plants have long tap roots that penetrate the water table1 anchor the soil1 and control erosion4 ,the stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion4 ,illustration3 ,sparse1 very dry1 single species vegetation in ,death ,valley1 ,california4 ,illustration3 ,vegetation amidst the desert pavement of the ,sonoran ,desert 7photograph by ,john ,olsen74 ,deserts typically have a plant cover that is sparse but enormously diverse4 ,the ,sonoran ,desert of the ,american ,southwest has the most complex desert vegetation on ,earth4 ,the giant saguaro cacti provide nests for desert birds and serve as 8trees0 of the desert4 ,saguaro grow slowly but may live #bjj years4 ,when #i years old1 they are about #ae centimeters high4 ,after about #ge years1 the cacti are tall and develop their first branches4 ,when fully grown1 saguaro are #ae meters tall and weigh as much as #aj tons4 ,they dot the ,sonoran and reinforce the general impression of deserts as cacti-rich land4 ,although cacti are often thought of as characteristic desert plants1 other types of plants have adapted well to the arid environment4 ,they include the pea family and sunflower family4 ,cold deserts have grasses and shrubs as dominant vegetation4 ,water ,rain does fall occasionally in deserts1 and desert storms are often violent4 ,a record #dd millimeters of rain once fell within #c hours in the ,sahara4 ,large ,saharan storms may deliver up to #a millimeter per minute4 ,normally dry stream channels1 called arroyos or wadis1 can quickly fill after heavy rains1 and flash floods make these channels dangerous4 ,more people drown in deserts than die of thirst4 ,though little rain falls in deserts1 deserts receive runoff from ephemeral1 or short-lived1 streams fed by rain and snow from adjacent highlands4 ,these streams fill the channel with a slurry of mud and commonly transport considerable quantities of sediment for a day or two4 ,although most deserts are in basins with closed1 or interior drainage1 a few deserts are crossed by #8exotic' rivers that derive their water from outside the desert4 ,such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts1 but their volumes are such that they maintain their continuity4 ,the ,nile1 the ,colorado1 and the ,yellow are exotic rivers that flow through deserts to deliver their sediments to the sea4 ,illustration3 ,the ,wei ,river in the ,loess ,plateau1 ,china 7photograph by ,i-,ming ,chou74 ,illustration3 ,running water created this canyon in arid ,big ,bend ,national ,park1 southwest ,texas4 ,lakes form where rainfall or meltwater in interior drainage basins is sufficient4 ,desert lakes are generally shallow1 temporary1 and salty4 ,because these lakes are shallow and have a low bottom gradient1 wind stress may cause the lake waters to move over many square kilometers4 ,when small lakes dry up1 they leave a salt crust or hardpan4 ,the flat area of clay1 silt1 or sand encrusted with salt that forms is known as a playa4 ,there are more than a hundred playas in ,north ,american deserts4 ,most are relics of large lakes that existed during the last ,ice ,age about #ab1jjj years ago4 ,lake ,bonneville was a #eb1jjj-square-kilometer lake almost #cjj meters deep in ,utah1 ,nevada1 and ,idaho during the ,ice ,age4 ,today the remnants of ,lake ,bonneville include ,utah's ,great ,salt ,lake1 ,utah ,lake1 and ,sevier ,lake4 ,because playas are arid land forms from a wetter past1 they contain useful clues to climatic change4 ,the flat terrains of hardpans and playas make them excellent race tracks and natural runways for airplanes and spacecraft4 ,ground-vehicle speed records are commonly established on ,bonneville ,speedway1 a race track on the ,great ,salt ,lake hardpan4 ,space shuttles land on ,rogers ,lake ,playa at ,edwards ,air ,force ,base1 ,california4 ,illustration3 ,the ,qaidam ,depression in ,china is the highest desert in the world4 ,this ,landsat image illustrates a salt lake and evaporite basins in the depression4 ,eolian ,processes ,eolian processes pertain to the activity of the winds4 ,winds may erode1 transport1 and deposit materials1 and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments4 ,although water is much more powerful than wind1 eolian processes are important in arid environments4 ,eolian erosion ,wind erodes the ,earth's surface by deflation1 the removal of loose1 fine-grained particles by the turbulent eddy action of the wind1 and by abrasion1 the wearing down of surfaces by the grinding action and sand blasting of windborne particles4 ,most eolian deflation zones are composed of desert pavement1 a sheetlike surface of rock fragments that remains after wind and water have removed the fine particles4 ,almost half of the ,earth's desert surfaces are stony deflation zones4 ,the rock mantle in desert pavements protects the underlying material from deflation4 ,illustration3 ,the sand and rock of ,china's ,turpan ,depression resemble closely those in the view of the ,martian surface on page #ba4 ,illustration3 ,the arrow points to shiny black desert varnish on these rocks of ,egypt's southwest desert 7photograph by ,carol ,breed74 ,a dark1 shiny stain1 called desert varnish or rock varnish1 is often found on surfaces of some desert rocks that have been exposed at the surface for a long period of time4 ,manganese1 iron oxides1 hydroxides1 and clay minerals form most varnishes and provide the shine4 ,deflation basins1 called blowouts1 are hollows formed by the removal of particles by wind4 ,blowouts are generally small1 but may be up to several kilometers in diameter4 ,wind-driven grains abrade landforms4 ,grinding by particles carried in the wind creates grooves or small depressions4 ,ventifacts are rocks which have been cut1 and sometimes polished1 by the abrasive action of wind4 ,sculpted landforms1 called yardangs1 are up to tens of meters high and kilometers long and are forms that have been streamlined by desert winds4 ,the famous sphinx at ,giza in ,egypt may be a modified yardang4 ,yardangs of the ,lut ,desert of ,iran4 ,these yardangs are among the largest on ,earth1 with almost #ajj meters of relief4 ,illustration3 ,view from ,landsat4 ,illustration3 ,view from high-altitude photograph 7photograph by ,u4,s4 ,air ,force74 ,illustration3 ,view from low-altitude photograph 7photograph by ,j4,t4 ,daniels74 ,eolian transportation ,particles are transported by winds through suspension1 saltation1 and creep4 ,small particles may be held in the atmosphere in suspension4 ,upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air4 ,typical winds near the ,earth's surface suspend particles less than #j4b millimeters in diameter and scatter them aloft as dust or haze4 ,saltation is downwind movement of particles in a series of jumps or skips4 ,saltation normally lifts sand-size particles no more than one centimeter above the ground1 and proceeds at one-half to one-third the speed of the wind4 ,a saltating grain may hit other grains that jump up to continue the saltation4 ,the grain may also hit larger grains that are too heavy to hop1 but that slowly creep forward as they are pushed by saltating grains4 ,surface creep accounts for as much as #be percent of grain movement in a desert4 ,eolian turbidity currents are better known as dust storms4 ,air over deserts is cooled significantly when rain passes through it4 ,this cooler and denser air sinks toward the desert surface4 ,when it reaches the ground1 the air is deflected forward and sweeps up surface debris in its turbulence as a dust storm4 ,illustration3 ,saltation moves small particles in the direction of the wind in a series of short hops or skips4 ,crops1 people1 villages1 and possibly even climates are affected by dust storms4 ,some dust storms are intercontinental1 a few may circle the globe1 and occasionally they may engulf entire planets4 ,when the ,mariner #i spacecraft arrived at ,mars in #aiga1 the entire planet was enshrouded in global dust4 ,illustration3 ,dust storm along the ,mohave ,river near ,daggett1 ,california1 ,october #bd1 #aiai 7photograph by ,d4 ,g4 ,thompson74 ,most of the dust carried by dust storms is in the form of silt-size particles4 ,deposits of this windblown silt are known as loess4 ,the thickest known deposit of loess1 #cce meters1 is on the ,loess ,plateau in ,china4 ,in ,europe and in the ,americas1 accumulations of loess are generally from #bj to #cj meters thick4 ,small whirlwinds1 called dust devils1 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 mass4 ,dust devils may be as much as one kilometer high4 ,eolian deposition ,wind-deposited materials hold clues to past as well as to present wind directions and intensities4 ,these features help us understand the present climate and the forces that molded it4 ,wind-deposited sand bodies occur as sand sheets1 ripples1 and dunes4 ,sand sheets are flat1 gently undulating sandy plots of sand surfaced by grains that may be too large for saltation4 ,they form approximately #dj percent of eolian depositional surfaces4 ,the ,selima ,sand ,sheet1 which occupies #fj1jjj square kilometers in southern ,egypt and northern ,sudan1 is one of the ,earth's largest sand sheets4 ,the ,selima is absolutely flat in some places2 in others1 active dunes move over its surface4 ,wind blowing on a sand surface ripples the surface into crests and troughs whose long axes are perpendicular to the wind direction4 ,the average length of jumps during saltation corresponds to the wavelength1 or distance between adjacent crests1 of the ripples4 ,in ripples1 the coarsest materials collect at the crests4 ,this distinguishes small ripples from dunes1 where the coarsest materials are generally in the troughs4 ,illustration3 ,wind-blown sand moves up the gentle upwind side of the dune by saltation or creep4 ,sand accumulates at the brink1 the top of the slipface4 ,when the buildup of sand at the brink exceeds the angle of repose1 a small avalanche of grains slides down the slipface4 ,grain by grain1 the dune moves downwind4 ,accumulations of sediment blown by the wind into a mound or ridge1 dunes have gentle upwind slopes on the wind-facing side4 ,the downwind portion of the dune1 the lee slope1 is commonly a steep avalanche slope referred to as a slipface4 ,dunes may have more than one slipface4 ,the minimum height of a slipface is about #cj centimeters4 ,sand grains move up the dune's gentle upwind slope by saltation and creep4 ,when particles at the brink of the dune exceed the angle of repose1 they spill over in a tiny landslide or avalanche that reforms the slipface4 ,as the avalanching continues1 the dune moves in the direction of the wind4 ,some of the most significant experimental measurements on eolian sand movement were performed by ,ralph ,bagnold1 a ,british engineer who worked in ,egypt prior to ,world ,war ,,ii4 ,bagnold investigated the physics of particles moving through the atmosphere and deposited by wind4 ,he recognized two basic dune types1 the crescentic dune1 which he called 8barchan10 and the linear dune1 which he called longitudinal or 8sief0 7,arabic for 8sword074 ,illustration3 ,sand dunes in ,death ,valley1 ,california 7photograph by ,richard ,frear74 ,illustration3 ,ripples on a dune in ,eureka ,valley1 ,california 7photograph by ,terrence ,moore74 ,types of ,dunes ,a worldwide inventory of deserts has been developed using images from the ,landsat satellites and from space and aerial photography4 ,it defines five basic types of dunes3 crescentic1 linear1 star1 dome1 and parabolic4 ,illustration3 ,these crescentic dunes of coastal ,peru are migrating toward the left 7photograph by ,john ,mc,cauley74 ,the most common dune form on ,earth and on ,mars is the crescentic4 ,crescent-shaped mounds generally are wider than long4 ,the slipface is on the dune's concave side4 ,these dunes form under winds that blow from one direction1 and they also are known as barchans1 or transverse dunes4 ,some types of crescentic dunes move faster over desert surfaces than any other type of dune4 ,a group of dunes moved more than #ajj meters per year between #aied and #aiei in ,china's ,ningxia ,province2 similar rates have been recorded in the ,western ,desert of ,egypt4 ,the largest crescentic dunes on ,earth1 with mean crest-to-crest widths of more than #c kilometers1 are in ,china's ,taklimakan ,desert4 ,straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes4 ,they may be more than #afj kilometers long4 ,linear dunes may occur as isolated ridges1 but they generally form sets of parallel ridges separated by miles of sand1 gravel1 or rocky interdune corridors4 ,some linear dunes merge to form ,y-shaped compound dunes4 ,many form in bidirectional wind regimes4 ,the long axes of these dunes extend in the resultant direction of sand movement4 ,illustration3 ,star dunes1 such as these of the ,namib1 indicate the winds that formed them blew from many directions 7photograph by ,georg ,gerster74 ,radially symmetrical1 star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound4 ,they tend to accumulate in areas with multidirectional wind regimes4 ,star dunes grow upward rather than laterally4 ,they dominate the ,grand ,erg ,oriental of the ,sahara4 ,in other deserts1 they occur around the margins of the sand seas1 particularly near topographic barriers4 ,in the southeast ,badain ,jaran ,desert of ,china1 the star dunes are up to #ejj meters tall and may be the tallest dunes on ,earth4 ,illustration3 ,linear dunes advance on small playas east of ,lake ,eyre in the ,simpson ,desert of central ,australia 7photograph by ,c4 ,twidale74 ,illustration3 ,linear dunes in the ,western ,desert of ,egypt 7photograph by ,carol ,breed74 ,oval or circular mounds that generally lack a slipface1 dome dunes are rare and occur at the far upwind margins of sand seas4 ,u-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes4 ,sometimes these dunes are called ,u-shaped1 blowout1 or hairpin dunes1 and they are well known in coastal deserts4 ,unlike crescentic dunes1 their crests point upwind4 ,the elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation1 while the bulk of the sand in the dune migrates forward4 ,the longest known parabolic dune has a trailing arm #ab kilometers long4 ,illustration3 ,small crescentic dunes occur on the crests of these complex dome dunes of ,saudi ,arabia's ,empty ,quarter 7photograph by ,elwood ,friesen74 ,illustration3 ,ripples and horns of this crescentic dune in ,egypt indicate that the dune is moving right to left 7photograph by ,john ,olsen74 ,occurring wherever winds periodically reverse direction1 reversing dunes are varieties of any of the above types4 ,these dunes typically have major and minor slipfaces oriented in opposite directions4 ,all these dune types may occur in three forms3 simple1 compound1 and complex4 ,simple dunes are basic forms with a minimum number of slipfaces that define the geometric type4 ,compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed1 and complex dunes are combinations of two or more dune types4 ,a crescentic dune with a star dune superimposed on its crest is the most common complex dune4 ,simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune1 while compound and complex dunes suggest that the intensity and direction of the wind has changed4 ,illustration3 ,the northern ,mojave ,desert4 ,the ,landsat ,thematic ,mapper 7,,tm7 acquires data in seven bands of the electromagnetic spectrum4 ,on this image1 white and yellow colors indicate rocks rich in clay minerals and limonite in rocks of red and yellow hues4 ,the large concentrations of limonite and clays may indicate mineral deposits exposed at the surface or buried up to several thousand feet below it 7photograph courtesy of ,melvin ,podwysocki74 ,remote ,sensing of ,arid ,lands ,the world's deserts are generally remote1 inaccessible1 and inhospitable4 ,hidden among them1 however1 are hydrocarbon reservoirs1 evaporites1 and other mineral deposits1 as well as human artifacts preserved for centuries by the arid climate4 ,in these harsh environments1 the information and perspective required to increase our understanding of arid-land geology and resources often depends on remote-sensing methods4 ,remote sensing is the collection of information about an object without being in direct physical contact with it4 ,remote-sensing instruments in ,earth-orbit satellites measure radar1 visible light1 and infrared radiation4 ,radar imaging systems provide their own source of electromagnetic energy1 so they can operate at any time of day or night4 ,additionally1 clouds and all but the most severe storms are transparent to radar4 ,the first ,shuttle ,imaging ,radar ,system 7,,sir-,a71 flown on the ,u4,s4 space shuttle ,columbia in #aiha1 recorded images that show buried fluvial topography1 faults1 and intrusive bodies otherwise concealed beneath sand sheets and dunes of the ,western ,desert in ,egypt and the ,sudan4 ,most of these features are not visible from the ground4 ,the radar signal penetrated loose dry sands and returned images of buried river channels not visible at the surface4 ,these images helped find new archeologic sites and sources of potable water in the desert4 ,these 8radar rivers0 are the remnants of a now vanished major river system that flowed across ,africa some #bj million years before the development of the ,nile ,river system4 ,radar imagery also is a powerful tool for exploring for placer mineral deposits in arid lands4 ,in #aigb1 the ,united ,states launched the first of a group of unmanned satellites collectively known as ,landsat4 ,landsat satellites carry sensors that record 8light10 or portions of the electromagnetic spectrum1 as it reflects off the ,earth4 ,landsat acquires digital data that are converted into an image4 ,illustration3 ,the ,landsat simulated true color mosaic 7left7 shows the ,selima ,sand ,sheet covering all but rocky areas of the ,sahara ,desert in ,sudan4 ,on the right1 a #ej-kilometer-wide strip of ,shuttle ,imaging ,radar1 ,,sir-,a1 is placed over the ,landsat mosaic to reveal old stream channels and geologic structures like these4 ,structures that are otherwise invisible under the surface sands are potential sources of water1 placer minerals1 ancient artifacts1 and information on changes of climate in arid areas 7courtesy of ,,usgs ,image ,processing ,facility1 ,flagstaff74 ,the scarcity of vegetation makes spectral remote sensing especially effective in arid lands4 ,rocks containing limonite1 a hydrous iron oxide1 may be identified readily from ,landsat ,multispectral ,scanner data4 ,the ,landsat ,thematic ,mapper 7,,tm7 has increased our ability to detect and map the distribution of minerals in volcanic rocks and related mineral deposits in arid and semiarid lands4 ,more than a million images of ,earth have been acquired by the ,landsat satellites4 ,a ,landsat image may be viewed as a single band in black-and-white1 or as a combination represented by three colors1 called a color composite4 ,the most widely used ,landsat color image is called a false-color composite because it reproduces the infrared band 7invisible to the naked eye7 as red1 the red band as green1 and the green band as blue4 ,healthy vegetation in a false-color composite is red4 ,desert studies still are hampered in many regions by lack of accurate climate data4 ,most desert weather stations are in oases surrounded by trees and buildings and have been subjected to many location and elevation changes throughout the life of the station4 ,data from oases do not reflect conditions from the surrounding desert4 ,a wide variety of instruments has been used to record measurements over varying lengths of time and in different formats1 making data difficult to interpret and compare4 ,to overcome some of these problems in deserts of the ,american ,southwest1 the ,u4,s4 ,geological ,survey 7,,usgs7 established its ,desert ,winds ,project to measure in a standard format several key meteorologic characteristics of arid lands4 ,project scientists have successfully established instrument stations to measure wind-speed1 including peak gusts1 which alter the landforms the most4 ,a station recorded a windstorm near ,vicksburg1 ,arizona1 for example1 with peak gusts of almost #aej kilometers per hour4 ,using low-maintenance1 automatic1 solar-powered sensors1 the stations also measure wind direction1 precipitation1 humidity1 soil and air temperatures1 and barometric pressure at specific heights above the surface4 ,data are sampled at #f-minute intervals and transmitted every #cj minutes to a ,geostationary ,operational ,environmental ,satellite 7,,goes74 ,from ,,goes1 the data are transmitted to the ,,usgs laboratory in ,flagstaff1 ,arizona4 ,the ,desert ,winds ,project's investigators combine analyses of data with detailed geologic field studies and repetitive remote-sensing coverage in order to investigate and understand the long-term changes produced by wind in deserts of differing geologic and climatic types4 ,illustration3 ,this geometeorologic station of the ,desert ,winds ,project measures wind speed and direction1 soil and air temperature1 and precipitation and humidity in the ,great ,basin ,desert 7photograph by ,carol ,breed74 ,illustration3 ,nitrate workings in a broad valley in the ,atacama ,desert of northern ,chile1 where saline-cemented surficial deposits formed near a playa lake 7photograph by ,george ,ericksen74 ,mineral ,resources in ,deserts ,some mineral deposits are formed1 improved1 or preserved by geologic processes that occur in arid lands as a consequence of climate4 ,ground water leaches ore minerals and redeposits them in zones near the water table4 ,this leaching process concentrates these minerals as ore that can be mined4 ,of the #ae major types of mineral deposits in the ,western ,hemisphere formed by action of ground water1 #ac occur in deserts4 ,evaporation in arid lands enriches mineral accumulation in their lakes4 ,playas may be sources of mineral deposits formed by evaporation4 ,water evaporating in closed basins precipitates minerals such as gypsum1 salts 7including sodium nitrate and sodium chloride71 and borates4 ,the minerals formed in these evaporite deposits depend on the composition and temperature of the saline waters at the time of deposition4 ,significant evaporite resources occur in the ,great ,basin ,desert of the ,united ,states1 mineral deposits made forever famous by the 8#bj-mule teams0 that once hauled borax-laden wagons from ,death ,valley to the railroad4 ,boron1 from borax and borate evaporites1 is an essential ingredient in the manufacture of glass1 ceramics1 enamel1 agricultural chemicals1 water softeners1 and pharmaceuticals4 ,borates are mined from evaporite deposits at ,searles ,lake1 ,california1 and other desert locations4 ,the total value of chemicals that have been produced from ,searles ,lake substantially exceeds #a billion4 ,the ,atacama ,desert of ,south ,america is unique among the deserts of the world in its great abundance of saline minerals4 ,sodium nitrate has been mined for explosives and fertilizer in the ,atacama since the middle of the #aith century4 ,nearly #c million metric tons were mined during ,world ,war ,i4 ,valuable minerals located in arid lands include copper in the ,united ,states1 ,chile1 ,peru1 and ,iran2 iron and lead-zinc ore in ,australia2 chromite in ,turkey2 and gold1 silver1 and uranium deposits in ,australia and the ,united ,states4 ,nonmetallic mineral resources and rocks such as beryllium1 mica1 lithium1 clays1 pumice1 and scoria also occur in arid regions4 ,sodium carbonate1 sulfate1 borate1 nitrate1 lithium1 bromine1 iodine1 calcium1 and strontium compounds come from sediments and nearsurface brines formed by evaporation of inland bodies of water1 often during geologically recent times4 ,illustration3 ,this open-pit mine in the ,sonoran ,desert near ,ajo1 ,arizona1 has exposed an elliptical copper deposit about #a1jjj meters long and #gej meters wide4 ,the copper ore mined here is in a bed that averages #aej meters in thickness 7photograph by ,peter ,kresan74 ,the ,green ,river ,formation of ,colorado1 ,wyoming1 and ,utah contains alluvial fan deposits and playa evaporites created in a huge lake whose level fluctuated for millions of years4 ,economically significant deposits of trona1 a major source of sodium compounds1 and thick layers of oil shale were created in the arid environment4 ,illustration3 ,trona mine at ,searles ,lake1 ,california 7photograph by ,john ,keith74 ,some of the more productive petroleum areas on ,earth are found in arid and semiarid regions of ,africa and the ,mideast1 although the oil reservoirs were originally formed in shallow marine environments4 ,recent climate change has placed these reservoirs in an arid environment4 ,other oil reservoirs1 however1 are presumed to be eolian in origin and are presently found in humid environments4 ,the ,rotliegendes1 a hydrocarbon reservoir in the ,north ,sea1 is associated with extensive evaporite deposits4 ,many of the major ,u4,s4 hydrocarbon resources may come from eolian sands4 ,ancient alluvial fan sequences may also be hydrocarbon reservoirs4 ,illustration3 ,the ,sahelian drought that began in #aifh was responsible for the deaths of between #ajj1jjj and #bej1jjj people1 the disruption of millions of lives1 and the collapse of the agricultural bases of five countries 7photograph by ,daniel ,stiles1 ,,unep74 ,desertification ,the world's great deserts were formed by natural processes interacting over long intervals of time4 ,during most of these times1 deserts have grown and shrunk independent of human activities4 ,paleodeserts1 large sand seas now inactive because they are stabilized by vegetation1 extend well beyond the present margins of core deserts1 such as the ,sahara4 ,in some regions1 deserts are separated sharply from surrounding1 less arid areas by mountains and other contrasting landforms that reflect basic structural differences in the regional geology4 ,in other areas1 desert fringes form a gradual transition from a dry to a more humid environment1 making it more difficult to define the desert border4 ,illustration3 ,overgrazing has made the ,rio ,puerco ,basin of central ,new ,mexico one of the most eroded river basins of the ,american ,west and has increased the high sediment content of the river 7photograph by ,terrence ,moore74 ,illustration3 ,linear dunes of the ,sahara ,desert encroach on ,nouakchott1 the capital of ,mauritania4 ,the dunes border a mosque at left 7photograph by ,georg ,gerster74 ,these transition zones have very fragile1 delicately balanced ecosystems4 ,desert fringes often are a mosaic of microclimates4 ,small hollows support vegetation that picks up heat from the hot winds and protects the land from the prevailing winds4 ,after rainfall the vegetated areas are distinctly cooler than the surroundings4 ,in these marginal areas1 human activity may stress the ecosystem beyond its tolerance limit1 resulting in degradation of the land4 ,by pounding the soil with their hooves1 livestock compact the substrate1 increase the proportion of fine material1 and reduce the percolation rate of the soil1 thus encouraging erosion by wind and water4 ,grazing and the collection of firewood reduces or eliminates plants that help to bind the soil4 ,this degradation of formerly productive land--desertification--is a complex process4 ,it involves multiple causes1 and it proceeds at varying rates in different climates4 ,desertification may intensify a general climatic trend toward greater aridity1 or it may initiate a change in local climate4 ,desertification does not occur in linear1 easily mappable patterns4 ,deserts advance erratically1 forming patches on their borders4 ,areas far from natural deserts can degrade quickly to barren soil1 rock1 or sand through poor land management4 ,the presence of a nearby desert has no direct relationship to desertification4 ,unfortunately1 an area undergoing desertification is brought to public attention only after the process is well underway4 ,often little or no data are available to indicate the previous state of the ecosystem or the rate of degradation4 ,scientists still question whether desertification1 as a process of global change1 is permanent or how and when it can be halted or reversed4 ,illustration3 ,camels and other animals trample the soil in the semiarid ,sahel of ,africa as they move to water holes such as this one in ,chad 7photograph courtesy of the ,u4,s4 ,agency for ,international ,development74 ,problem ,desertification became well known in the #aicj's1 when parts of the ,great ,plains in the ,united ,states turned into the 8,dust ,bowl0 as a result of drought and poor practices in farming1 although the term itself was not used until almost #aiej4 ,during the dust bowl period1 millions of people were forced to abandon their farms and livelihoods4 ,greatly improved methods of agriculture and land and water management in the ,great ,plains have prevented that disaster from recurring1 but desertification presently affects millions of people in almost every continent4 ,illustration3 ,off-road vehicles significantly increase soil loss in the delicate desert environment of the western ,united ,states4 ,in a few seconds1 soils that took hundreds of years to develop can be destroyed 7photograph by ,terrence ,moore74 ,increased population and livestock pressure on marginal lands has accelerated desertification4 ,in some areas1 nomads moving to less arid areas disrupt the local ecosystem and increase the rate of erosion of the land4 ,nomads are trying to escape the desert1 but because of their land-use practices1 they are bringing the desert with them4 ,it is a misconception that droughts cause desertification4 ,droughts are common in arid and semiarid lands4 ,well-managed lands can recover from drought when the rains return4 ,continued land abuse during droughts1 however1 increases land degradation4 ,by #aigc1 the drought that began in #aifh in the ,sahel of ,west ,africa and the land-use practices there had caused the deaths of more than #ajj1jjj people and #ab million cattle1 as well as the disruption of social organizations from villages to the national level4 ,while desertification has received tremendous publicity by the political and news media1 there are still many things that we don't know about the degradation of productive lands and the expansion of deserts4 ,in #aihh ,ridley ,nelson pointed out in an important scientific paper that the desertification problem and processes are not clearly defined4 ,there is no consensus among researchers as to the specific causes1 extent1 or degree of desertification4 ,contrary to many popular reports1 desertification is actually a subtle and complex process of deterioration that may often be reversible4 ,illustration3 ,goat seeks food in the sparsely vegetated ,sahel of ,africa 7photograph courtesy of the ,u4,s4 ,agency for ,international ,development74 ,global monitoring ,in the last #be years1 satellites have begun to provide the global monitoring necessary for improving our understanding of desertification4 ,landsat images of the same area1 taken several years apart but during the same point in the growing season1 may indicate changes in the susceptibility of land to desertification4 ,studies using ,landsat data help demonstrate the impact of people and animals on the ,earth4 ,however1 other types of remote-sensing systems1 land-monitoring networks1 and global data bases of field observations are needed before the process and problems of desertification will be completely understood4 ,local remedies ,at the local level1 individuals and governments can help to reclaim and protect their lands4 ,in areas of sand dunes1 covering the dunes with large boulders or petroleum will interrupt the wind regime near the face of the dunes and prevent the sand from moving4 ,sand fences are used throughout the ,middle ,east and the ,united ,states1 in the same way snow fences are used in the north4 ,placement of straw grids1 each up to a square meter in area1 will also decrease the surface wind velocity4 ,shrubs and trees planted within the grids are protected by the straw until they take root4 ,in areas where some water is available for irrigation1 shrubs planted on the lower one-third of a dune's windward side will stabilize the dune4 ,this vegetation decreases the wind velocity near the base of the dune and prevents much of the sand from moving4 ,higher velocity winds at the top of the dune level it off and trees can be planted atop these flattened surfaces4 ,illustration3 ,straw grids 7one of which is shown above7 and vegetation irrigated by water from the ,yellow ,river stabilize dunes in this part of ,china's ,tengger ,desert 7shown below7 and protect a nearby railroad from windblown sand4 ,illustration3 ,tengger ,desert ,oases and farmlands in windy regions can be protected by planting tree fences or grass belts4 ,sand that manages to pass through the grass belts can be caught in strips of trees planted as wind breaks #ej to #ajj meters apart adjacent to the belts4 ,small plots of trees may also be scattered inside oases to stabilize the area4 ,on a much larger scale1 a 8,green ,wall10 which will eventually stretch more than #e1gjj kilometers in length1 much longer than the famous ,great ,wall1 is being planted in northeastern ,china to protect 8sandy lands0--deserts believed to have been created by human activity4 ,more efficient use of existing water resources and control of salinization are other effective tools for improving arid lands4 ,new ways are being sought to use surface-water resources such as rain water harvesting or irrigating with seasonal runoff from adjacent highlands4 ,new ways are also being sought to find and tap groundwater resources and to develop more effective ways of irrigating arid and semiarid lands4 ,research on the reclamation of deserts also is focusing on discovering proper crop rotation to protect the fragile soil1 on understanding how sand-fixing plants can be adapted to local environments1 and on how grazing lands and water resources can be developed effectively without being overused4 ,illustration3 ,from wasteland to vineyard4 ,ground water and underground channels help this vineyard flourish on land reclaimed from desert pavement in ,china's ,turpan ,depression4 ,if we are to stop and reverse the degradation of arid and semiarid lands1 we must understand how and why the rates of climate change1 population growth1 and food production adversely affect these environments4 ,the most effective intervention can come only from the wise use of the best earth-science information available4 ,selected ,readings ,bagnold1 ,r4 ,a41 #aida1 ,the physics of blown sand and desert dunes3 ,methuen1 ,london1 #bfe p4 7,a classic treatise concerning the origin and evolution of dunes47 ,breed1 ,c4 ,s41 and others1 #aigi1 ,regional studies of sand seas1 using ,landsat 7,,erts7 imagery3 in ,mc,kee1 ,e4 ,d41 ed41 ,a study of global sand seas3 ,u4,s4 ,geological ,survey ,professional ,paper #ajeb1 p4 #cje-cig4 7,a study of selected sand seas based on analysis of remote sensing images1 surface wind summaries1 and available literature47 ,cook1 ,r4 ,u41 and ,warren1 ,andrew1 #aigc1 ,geomorphology in deserts3 ,university of ,california ,press1 ,berkeley1 ,california1 #cgd p4 7,examines the nature of landforms1 soils1 and geomorphological processes in the world's deserts47 ,eigeland1 ,tor1 and others1 #aihb1 ,the desert realm3 ,national ,geographic ,society1 ,washington1 #cjd p4 7,a well illustrated discussion of deserts of ,america1 ,africa1 ,asia1 and ,australia47 ,ericksen1 ,g4 ,e41 #aihc1 ,the ,chilean nitrate deposits3 ,american ,scientist1 v4 #ga1 p4 #cff-cgd4 7,a discussion of the origin of the ,chilean nitrate deposits which has puzzled scientists for more than #ajj years47 ,gerster1 ,georg1 #aifj1 ,sahara-desert of destiny3 ,coward-,mc,cann1 ,new ,york1 #cjb p4 7,how plants1 animals1 and people survive in the ,sahara47 ,greeley1 ,ronald1 and ,iversen1 ,j4 ,d41 #aihe1 ,wind as a geological process on ,earth1 ,mars1 ,venus and ,titan3 ,cambridge ,university ,press1 ,new ,york1 #ccc p4 7,expands the classic work of ,bagnold to discuss eolian processes in a planetary context4 ,describes the processes on all moons and terrestrial planets with atmospheres47 ,hare1 ,f4 ,k41 #aihc1 ,climate on the desert fringe3 in ,gardner1 ,ritz1 and ,scoging1 ,helen1 eds41 ,mega-geomorphology3 ,clarendon ,press1 ,oxford1 p4 #acd-aea4 7,the margins of many deserts are affected by tension between society and environment4 ,this paper summarizes the climatology of arid zones47 ,mac,mahon1 ,james ,a41 #aihe1 ,deserts3 ,alfred ,a4 ,knopf1 ,inc41 ,new ,york1 #fdj p4 7,an ,audubon ,society ,nature ,guide to the deserts of the ,united ,states1 and their inhabitants47 ,mc,cauley1 ,j4 ,f41 and others1 #aihd1 ,remote monitoring of processes that shape desert surfaces3 ,the ,desert ,winds ,project3 ,u4,s4 ,geological ,survey ,bulletin #afcd1 #ai p4 7,describes a new study on collecting weather data from solar-powered data-collection platforms in deserts4 ,the data are relayed by a ,,goes satellite to the ,,usgs in ,flagstaff1 ,arizona1 and converted to graphic form47 ,meigs1 ,peveril1 #aiec1 ,world distribution of arid and semi-arid homoclimates3 in ,reviews of research on arid zone hydrology3 ,paris1 ,united ,nations ,educational1 ,scientific1 and ,cultural ,organization1 ,arid ,zone ,programme-#a1 p4 #bjc-bji4 7,classifies arid lands according to precipitation47 ,nelson1 ,r41 #aihh1 ,dryland management3 the desertification problem3 ,environmental ,department ,working ,paper ,no4 #h1 ,washington3 ,world ,bank1 #db p4 7,an excellent review of the present state of knowledge concerning desertification47 ,tolba1 ,m4 ,k41 #aihd1 ,desertification is stoppable3 ,arid ,lands ,newsletter ,no4 #ba1 p4 #b-i4 7,a discussion of the problems involved in preventing desertification and reclaiming arid lands47 ,walker1 ,a4,s41 #aihf1 ,eolian geomorphology3 in ,short1 ,n4,m41 and ,blair1 ,r4,w41 eds41 ,geomorphology from space3 a global overview of regional landforms3 ,,nasa ,,sp-#dhf1 p4 #ddg-ebj 7a brief review of desert processes74 ,warren1 ,a4 and ,agnew1 ,c41 #aihh1 ,an assessment of desertification and land degradation in arid and semi-arid areas3 ,international ,institute for ,environment and ,development1 ,drylands ,programme1 ,paper #b1 ,london3 ,,iied1 #ajc p4 7,an evaluation of land degradation problems47 ,the metric units used in this publication can be converted to ,english units by using the approximate conversions given below3 ,length #a kilometer #j4f of a mile #a meter #ci4cg inches #a centimeter #j4d inch #a millimeter #j4jd inch ,area #a sq4 kilometer #j4jd sq4 mile #a sq4 meter #a4b sq4 yards #a sq4 centimeter #j4aee sq4 inch ,temperature ,to convert ,celsius to ,fahrenheit1 multiply ,c by #a4h and add #cb4 ,to convert ,fahrenheit to ,celsius1 subtract #cb from ,f and divide the result by #a4h4 ,illustration3 ,landsat image shows complex linear and crescentic dunes in the northeastern ,taklimakan ,desert of ,china4 ,in this desert there are a great many evil spirits and also hot winds2 those who encounter them perish to a man4 ,there are neither birds above nor beasts below4 ,gazing on all sides as far as the eye can reach in order to mark the track1 no guidance is to be obtained save from the rotting bones of dead men1 which point the way4 ,explorer ,fa ,xian describing the ,taklimakan ,desert of ,china about #djj ,a4,d4 ,this publication is one of a series of general interest publications prepared by the ,u4,s4 ,geological ,survey to provide information about the earth sciences1 natural resources1 and the environment4 ,to obtain a catalog of additional titles in the series 8,general ,interest ,publications of the ,u4,s4 ,geological ,survey10 write3 ,u4,s4 ,geological ,survey ,branch of ,distribution ,p4,o4 ,box #bebhf ,denver1 ,,co #hjbbe ,u4 ,s4 ,,government ,,printing ,,office 3 #aiib #j-ccb-cbf ,,ql #b ,as the ,nation's principal conservation agency1 the ,department of the ,interior has responsibility for most of our nationally-owned public lands and natural and cultural resources4 ,this includes fostering wise use of our land and water resources1 protecting our fish and wildlife1 preserving the environmental and cultural values of our national parks and historical places1 and providing for the enjoyment of life through outdoor recreation4 ,the ,department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people4 ,the ,department also promotes the goals of the ,take ,pride in ,america campaign by encouraging stewardship and citizen responsibility for the public land and promoting citizen participation in their care4 ,the ,department also has a major responsibility for ,american ,indian reservation communities and for people who live in ,island ,territories under ,u4,s4 ,administration4 ,cover ,photographs3 ,illustration3 ,granite ,mountain in the ,great ,basin ,desert 7photograph by ,terrence ,moore74 ,illustration3 ,sonoran ,desert 7photograph by ,peter ,kresan74 ,illustration3 ,zabriskie ,point in ,death ,valley1 ,california 7photograph by ,peter ,kresan74 ,illustration3 ,artists ,point in ,monument ,valley 7photograph by ,peter ,kresan74 ,illustration3 ,death ,valley1 ,california 7photograph by ,cecil ,stoughton74 ,illustration3 ,cacti in the ,sonoran ,desert 7photograph by ,john ,olson74 ,illustration3 ,back ,cover ,transcriber's ,notes --,corrected a few typographical errors4 --,slightly displaced photographs and captions for better display on scrolling e,book viewers4