Wind erosion in deserts

A. S. Goudie

GOUDIE, A. S. 1989. Wind erosion in deserts. Proc. Geol. Ass., 100(1), 83-92. Wind is animportant erosive force in deserts, and in recent years there has been a re-appraisal of itspower, notably with respect to the development of yardangs, inverted relief, desert depressionsand dust storms. This paper provides a review of each of these phenomena.

School of Geography, University of Oxford, Mansfield Road, Oxford, OX1 3TB

1. INTRODUCfION

As recently as two decades ago the erosive action ofwind in the desert environment was accorded a limitedrole, and desert landscapes were seen to be largelyinactive under present climatic regimes. This view wastypified by Walton's (1969) analysis of the factorsshaping desert morphology (pp. 48-9):

"True aridity mummifies a landscape by slowing down therates of change just as the dry sand and desiccating airpreserved Egyptians and Peruvians for the museums of thetwentieth century. As the mummies once had vitality, growthand change so had the arid zone landscapes in the past, withbut few exceptions. Such landscapes betray the result ofhigher precipitation and reduced rates of evaporation duringthe 'pluvials'."

Such a view had not always been the norm, for atthe turn of the century there was a phase of what hasbeen termed extravagant aeolation (Cooke & Warren,1973). This had its roots in the work undertaken inAfrica by French and German geomorphologists (e.g.Walther, 1900; Passarge, 1904), but was put forwardin its most exuberant form in the U.S.A. by Keyes(1913), who believed that material weakened bythermoclasty (insolation weathering) would be eva­cuated by wind and deposited as dust sheets on desertmargins. He argued that the end result of such activitywould be the formation of great plains, mountainranges without foothills, and towering eminences.

The reasons for the decline of aeolianist views weremany. Firstly, the great pediment landscapes of theAmerican deserts were seen, following the work ofMcGee (1897) and others, as being attributable toplanation by sheetflood activity. Secondly, manydesert landscapes were thought to have been mouldedby fluvial processes that had been more powerful andwidespread during the pluvial phases that were held tobe a feature of the Pleistocene. Current aridconditions were widely thought to be a relativelyrecent and short-lived phenomenon caused bypost-glacial progressive desiccation (Goudie, 1972).Thirdly, doubt was expressed about the power ofthermoclasty as a process capable of preparing desertsurfaces for subsequent aeolian attack. Such doubtarose because of the experimental work of Black-

83

welder (1933) and Griggs (1936) (see Goudie, inpress, for a review). Fourthly, it was widely held thatlag gravels (stone pavements) and salt and clay crustswould limit the extent to which aeolian processescould cause lowering of surfaces, and that it wasimpossible for aeolian processes to cause excavationof surfaces below the water-table (Cooke & Warren,1973, p. 251). Fifthly, it became apparent that manyof the world's great dust sheets, in North America,China, and the U.S.S.R., were the product ofdeflation from glacial areas rather than from deserts.Glacial grinding was thought to be the most efficientway of producing silt-sized quartz particles (Smalley &Krinsley, 1978). Sixthly, it was recognised that not alldeserts had either adequate supplies of abrasive sandor of frequent high-velocity winds for wind erosion tobe achieved with any degree of facility. Finally,features that were conceded to have an aeolian origin(e.g. yardangs, ventifacts and pedestal rocks) werethought to be but minor, bizarre embellishments ofotherwise fluvial environments, whilst other possiblyaeolian features (notably stone pavements and closeddepressions) were also explicable by other means.Stone pavements, for example, could be the productof the removal of fine sediments by sheetflood activityor they could result from vertical sorting processesassociated with wetting and drying, salt hydration orfreezing and thawing (Cooke, 1970). Deflationalremoval of fines to leave a lag was just one possibleformative mechanism. In the same way, closeddepressions could be attributed to wind excavation,but might also be explained by tectonic, solutional orzoogenic processes (Goudie & Thomas, 1985).

Many of these objections to widespread aeolationare cogent and sound, but nonetheless in the last twodecades the power of wind to erode desert landscapeshas been reassessed and it has been shown to have aconsiderable significance in moulding rock surfaces toproduce yardangs, in causing relief inversion, inexcavating desert depressions (pans) and in causingdeflation (as evidenced by dust storms). Two of themajor stimuli to such a reassessment have been thesearch for analogues for Martian features, and theincreasing availability of remote sensing images fromaircraft and from space vehicles.

84 A. S. GOUDIE

2. YARDANGS AND RELATED FEATURES

Yardangs, which are wind-produced streamlinederosional features, so long regarded as bizarreembellishments or mere curiosities, developed prima­rily in soft strata, have recently been shown to havegreat areal extent on a wide range of rock types(Table 1).

Of very considerable importance in this reassess­ment has been the utilisation of air photography andsatellite imagery of areas such as the central Sahara(Mainguet, 1972), the interior deserts of China,including the Tarim basin, and the ignimbrite sheetsof the High Andes. Another stimulus to yardangresearch on Earth has been the discovery of extensiveyardang fields on Mars (Greeley & Iversen, 1985). Anexcellent discussion of terrestrial examples is providedby McCauley, Grolier & Breed (1977), who makesome pertinent general observations (p. 162):

"Yardangs, regardless of size, can be recognised by theirparallelism and similarity to inverted ship hulls, oriented intothe direction of the prevailing wind. They may have concavedownward tapering bows or the upwind ends may be convexand bulbous. The highest and widest part of the structure isgenerally about one third of the way between the bow andthe stern in a well streamlined yardang .... The downwind

ends of yardangs are characterised by gently taperingbedrock surfaces or elongate sand tails. Yardangs often occurin closely packed arrays or fleets separated from one anotherby either "U"-shaped troughs or flat-bottomed troughs."

Yardangs are probably produced by a combinationof abrasion and deflation. The same is probably trueof areas of aligned drainage, though in this case theremay be a degree of inheritance from a previous coverof linear dunes. Wells (1983), building on the earlierwork of Russell (1927), Flint (1955) and Crandell(1958) has shown how extensive the aligned ~rainage

systems of the United States of Amenca are,especially in the northern portions of The GreatPlains. They are aligned approximately NW-SEreflecting the directions of Pleistocene winds andformer Pleistocene dune-fields.

Other fluvial features which show the imprint ofaeolian excavation, in this case primarily by deflation,are the extensive spreads of inverted relief associatedwith palaeochannels. Notable examples of thesefeatures have recently been identified in Oman(Maizels, 1986), where braiding and m~anderi.ng

spreads of river gravels, of several generatIOns, nsesome metres above the surrounding plains. Suchpalaeochannels, the "suspendritic drainage lines" ofMiller (1937), have been produced through

TABLE 1. The locations and lithologies of major yardang fields

Location

Cerro GalanHigh Andes, Argentina

Taklimakan, China

Lut, IranKhash, AfghanistanSinai, EgyptSaudi ArabiaBahrainEgypt

South-Central Algeria

Borkou, Chad

Jaisalmer, IndiaNamib, Namibia

Rogers Lake, CaliforniaNorthern Peru

South-Central Peru

Wahiba Sands, OmanColorado Plateau, U.S.A.

Lithology

Ignimbrite sheets

Pleistocene fluvial andlacustrine sediments

Pleistocene clays, silts and sandsClayNubian sandstoneCalcrete, limestoneDolomite, aeolianiteEocene limestone, lake beds,

Nubian sandstoneCambrian claystones,

Cretaceous claysPalaeozoic and lower Mesozoic

sandstones and shalesEocene limestonesPrecambrian dolomites, granites

and gneissesDune sands and lake bedsUpper Eocene to Palaeocene shales

and sandstonesUpper Oligocene to Miocene

siltstonesAeolianiteNavajo sandstone and Mesozoic

claystones and siltstones

WIND EROSION IN DESERTS 85

differential deflation of fine-grained, poorly cementedinterfluve sediments, while coarse-grained, wellcemented channel sediments have remained resistantto deflation and hence have been preserved asupstanding ridges. Because of dating uncertainties it isdifficult to estimate the speed at which this deflationhas occurred, but the maximum rates may have beenof the order of 2 m 1000 y-t (Beydoun, 1980).

3. DESERT DEPRESSIONS (PANS)

In many parts of the world's dry zones there arenumerous topographic lows, a substantial proportionof which are closed basins. Such 'pans' may have acharacteristic morphology that has been likened to aclam, a kidney or a pork chop. As with yardangs,remote sensing techniques have revealed the wide-

,,II

II_I

Mean annual IsohYeta(mm)

Landsat fra"'es on whichaeolian panshave been Identified

SEA

N

f

YELLOW

..:..........

R

A

.•..; .... ;;;;;. Hong Kong

s

l

s

o

u

G

o

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,-",,-, "".-'...." \ " ~

-" \- - I '..... _- ...... ..... _---..-,

"" III,,

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Fig. 1. The distribution of pans in China as determined from Landsat images.

86 A. S. GOUDIE

spread occurrence of such features. Major areasinclude South Africa, Botswana, Namibia, Zimbabweand Zambia (Goudie & Thomas, 1985), western andSouthern Australia (Killigrew & Gilkes, 1974), theHigh Plains of the U.S.A. (Reeves, 1966; Colton,1978; Osterkamp & Wood, 1987), the Pampas,Pantanal and high plateaux of South America(Dangavs, 1979; Tricart, Pagney & Frescaut, 1984),Manchuria (Fig. 1) and the West Siberian steppes.

One of the prime controls of pan distribution is theavailability of susceptible surfaces. In South Africa,pans are preferentially developed on the Ecca shalesand Kalahari Beds; in the U.S.A. pans are especiallywell displayed on the calcrete caprock of the Ogallalaformation and on the Pierre, Carlile and Steele shalesand the Mesaverde and Fox Hills sandstones; and onthe Andean Altiplano pans have been excavated inignimbrite sheets. Examples are provided in Table 2and this highlights the importance of unconsolidatedsediments, shales and fine-grained sandstones inexplaining the distribution pattern.

Other pans occur in particular types of topographicsituation. For example, palaeolacustrine pans arewidespread in many of the desiccated pluvial lakebasins of the western United States, where deflationof the old lake sediments has been active (e.g. thefloor of pluvial lake Estancia in New Mexico has over60 excavated basins with associated lunette ridges).

Palaeodrainage pans occur where deflation hasdisrupted former river courses, either because of achange of climate towards increased aridity or becauseof tectonic deformation (Marshall, 1987). Notable

examples of this type include the large pans of theOrange Free State and Eastern Transvaal in SouthAfrica, those of the fossil drainage lines of westernAustralia (van der Graaff, Crowe, Bunting & Jackson,1977) (Fig. 2), and those of the southern High Plainsin Texas and New Mexico.

A further class of pan is the interdunal pan, formedby deflation of inter-dune swales and the noses ofparabolic dunes. Some of the aligned pans of thesouthern High Plains of the U.S.A. are the relictinterdunal basins of a formerly extensive PleistoceneSand ridge desert (e.g. near Eunice, New Mexico, andLamesa, Texas).

Finally, there are coastal surface pans caused bydeflation of coastal sediments. Possibly the mostcontroversial but classic examples are the CarolinaBays of the eastern seaboard of the U.S.A., but otherexamples are those found on the Pampa Deprimada inArgentina (Tricart, 1969), and the Cape Agulhas areaof South Africa (Tinley, 1985).

The characteristic shapes and alignments of manypans give a clue to their aeolian origins and this issupported by the widespread (though far fromuniversal) occurrence of lunette dunes on their leesides and of clay pellets in downwind dune systems(Hills, 1940; Lancaster, 1978; Coque, 1979; Holliday,1985; Benazzouz, 1986; Thomas, 1986) (Fig. 3).

4. A MODEL OF PAN DEVELOPMENT

Pans occur preferentially in areas of relatively lowprecipitation (Fig. 4) where vegetation cover is limited

TABLE 2. Favoured materials for pan development

Location

South Africa

Botswana

Zimbabwe, Zambia and ZaireNamibia (Aminuis)Tunisia

Yorke Peninsula, S. Australia

Egypt (S. W. desert)Stirling area, W. Australia

MongoliaAltiplano of Chile,Bolivia and ArgentinaHigh Plains, U.S.A.

Material

Ecca and Dwyka shales andsandstones

Kalahari beds(sands, calcretes, etc.)

Kalahari sandsKaroo sedimentsNeogene clays and sands,Cretaceous marlsPermian clays and sands,

coastal sedimentsNubian sandstoneTertiary marine sediments,

Middle Proterozoic sedimentariesSandstones, coarse granitesIgnimbrite sheets and

volcanoclastic sedimentsOgallala sediments (including

calcrete), shales (Pierre, Carlileand Steele) and sandstone(Mesaverde and Fox Hills).

WIND EROSION IN DESERTS 87

Information derived from 1:100,000Topo sheels, 2531, 2533, 2632, and2633

32'30'8

Lake Grace N

Chlnocup

I I

o

11S'E

N

iArea of Indistinct Iwampdrainage with many smallpans

Pan

o 1;

'----'km

Fig. 2. The distribution of pans in a portion of Western Australia, showing the preferential developmentalong drainage lines.

88 A. S. GOUDIE

Fig. 3. The characteristic shape of pans from the south west Kalahari, showing associated sand ridgesand lunettes.

WIND EROSION IN DESERTS 89

ANIMAL PRESSURESON LIMITED WATER

RESOURCES

LIMITED SAND

ACCUMULATION

AND INFILLING

LOCATION IN

AEOLIAN

TRANSPORT REGIME

SUSCEPTIBLESURFACES AND

LITHOLOGIES

DOLERITEINTRUSIONS

TECTONICDISTURBANCE

ANDVOLCANIC DOMING

EPISODICDESICCAnON

LOW SLOPES

Fig. 4. A model of pan development extended from that of Goudie & Thomas (1985).

and where deflation can therefore occur. Further­more, aridity encourages salt accumulation, whichboth retards the growth of vegetation and can causesalt weathering of susceptible rocks and sediments,thereby producing fine-grained debris for aeolianevacuation. As initial depressions become larger theybecome increasingly attractive in wet seasons and wetyears to large game populations, who trample andover-graze the surface (making it more susceptible towind attack). They may also physically removematerial in and on their bodies. The positive feedbackrole of salt and animals means that in arid areas initialdepressions caused by such processes as tectonicsubsidence (e.g. AlIenby, 1988), drainage tilting anddismemberment by tectonic warping (Marshall, 1987),or solutional enlargement of joints (Osterkamp &Wood, 1987) may become expanded by deflational

activity. That deflation does excavate pan surfaces ismade clear by many decades of ground observations(Woodward 1897; Du Toit, 1906; Haynes, 1982) andby recent analysis of space shuttle photography(Middleton, Goudie & Wells, 1986) which shows theimportance of pans as a source of deflational materialfor dust storms.

A model of continued salt attack and deflation cantherefore be proposed (Goudie & Thomas, 1985), andits action has been eloquently described for theEgyptian Desert by Haynes (1982, p. 104):

"From observations in many depressions of The WesternDesert, from the Quattara in the north to Merga in theSouth, I am of the opinion that most if not all are the resultof eolian corrasion and deflation of beds weakened byleaching of cement and salt efflorescence.... The mechan­ism is self-enhancing. once started by the development of an

90 A. S. GOUDIE

inilial area of internal drainage. Each wetting allows newsalts to form upon drying, and crystallisation andrecrystallisation of expanding salts weakens the cementing ofthe sedimentary matrix and loosens grains for their pluckingby the wind. Once the water table is approached by the basinfloor, efftorescence by evaporation from the capillary fringewould further aid deflation as long as evaporation exceededrecharge. "

However, it is not simply the power of wind actionthat leads to pan development in areas of suitablelithology or relief. It is also necessary thatdepressions, once formed, are not obliterated by theaction of integrated fluvial systems. Factors that maycontribute to such non-obliteration may include theblocking of drainage by dunes, the presence ofigneous intrusions such as dolerite dykes (de Bruiyn,1971), broad-scale tectonic disruption of drainage(Mayer, 1973), the disorganisation of drainage byclimatic deterioration, the presence of highly perme­able sandy soils that do not permit much surfacerunoff, and the existence of large areas (e.g. oldplanation surfaces) with low angle slopes. Finally,pans may be obliterated in areas where there isexcessive sand deposition.

5. DUST STORMS

Possibly the most important reason for a reassessmentof the power of wind action in deserts has been causedby the increasing appreciation of the significance ofdust storms (Morales, 1979; Pewe, 1981; Goudie,1983; Pye, 1987). Three developments have contrib­uted. Firstly, the analysis of meteorological recordsfor many different parts of the word includingAustralia (McTainsh & Pitblado, 1987), the MiddleEast (Middleton, 1986a), south West Asia (Mid­dleton, 1986b), and Arizona (Brazel & Nickling,1986) shows that dust storms occur with considerablefrequency. Secondly, the analysis of satellite imageryhas shown the main source regions, the great arealextent of some dust raising systems, and the largedistances over which dust transport occurs. Thirdly,the analysis of deep sea cores has revealed that desertshave been delivering aeolian dust to the oceans duringmuch of the Pleistocene, (Kolla & Biscaye, 1977;Thiede, 1979) that the rates of delivery duringinterpluvials have often exceeded those which havetaken place in the Holocene, (Sarnthein & Koopman,1980) that Pleistocene wind velocities may sometimeshave been greater than those of today (Parkin &Shackleton, 1973), and that in some deserts (e.g. theSahara) the export of dust may have occurred forsome tens of millions of years (Leinen & Heath, 1981;Lever & McCave, 1983).

The Sahara is the world's largest area of dust stormactivity and probably produces as much as half of theglobal atmospheric load of soil dust (Coude-Gaussen,1982). Major zones within this area include southern

Morocco, south-west Algeria, southern Mauritaniaand northern Mali, the Chad basin, eastern Algeriaand northern Libya and north-east Sudan (Middleton,1986c). The Horn of Africa appears to be animportant source of dust transported over the ArabianSea. In the Middle East major dust sources includethe alluvial plains of Mesopotamia and the deserts ofSyria and northern Saudi Arabia. Further east muchdust is derived from the Seistan Basin, from the plainsof Afghan Turkestan, from the alluvial plains ofRajasthan, from the southern U.S.S.R. and interiorChina. Other important source areas are centralAustralia, the Puna de Atacama, the Great Plains ofNorth America, and the basins of Mexico.

Among the desert terrain types that producesubstantial quantities of dust are playas (Young &Evans, 1986), alluvial fans (Goudie & Day, 1980),floodplains (Khalaf, Al-Kadi & AI-Saleh, 1985),abraded bedrock surfaces (Bucher 1986), and areas ofloess. Among the landforms that are created are someof those discussed in this paper: yardangs, invertedpalaeochannels, and pans. Dust deposition is alsobeing recognised as a major geomorphological processin arid lands, contributing much to the evolution ofmany <lesert surfaces. This is particularly the case withrespect to stone pavements which, paradoxically inview of the case made for the importance of winderosion in this paper, may only owe a little to theaction of deflational lowering. The most modemmodel that has been proposed for their formation isthat of McFadden, Wells & Jercinovich (1987), whoinvoke the contribution made to their development bysyndepositional lifting of surface clasts. They envisagesubsurface incorporation of aeolian silts and clays viavertical crack systems during dry periods, alternatingwith crack closure and compressional doming duringwetter periods. The net effect is an upwarddisplacement of coarse particles to give the stonepavement of coarse lag materials at the surface.

6. CONCLUSION

The importance of dust storms seems set to increase incoming decades as more and more pressures areimposed by humans on desert surfaces. An upwardtrend in the incidence of dust storms in the Sahel hasbeen identified by Middleton (1985), and deflation ofsediments from the artifically desiccated surfaces ofCalifornian lakes is creating an increased dust stormrisk (Saint-Amand , Mathews, Gaines & Reinking,1986) in the Owens and Mono Valleys. Likewise theextension of centre-pivot irrigation techniques in theHigh Plains of the U.S.A. is allowing new areas to bebrought under the plough, subjecting them to thethreat of aeolian attack. This applies particularly topreviously uncultivated areas of relict Pleistocenedunes, for they are composed of particularlysusceptible, poorly-structured sandy soils.

WIND EROSION IN DESERTS 91

ACKNOWLEDGEMENTS

In the preparation of this paper I should like toacknowledge the assistance given by a team of my

pupils, especially David Thomas, Ken Pye, NickMiddleton and Gordon Wells.

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