VOL. 84, NO. B14 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979

Eolian Features in the Western of and Some Applications to Mars

FAROUK EL-BAZ

National Air and Space Museum, SmithsonianInstitution, Washington,D.C. 20560

C. S. BREED, M. J. GROLIER, AND J. F. MCCAULEY

U.S. GeologicalSurvey, Flagstaff,Arizona 86001

Relations of landform types to wind regimes, bedrock composition, sediment supply, and topography are shown by field studiesand satellite photographsof the Western Desert of Egypt. This desert, which lies at the core of the largest hyperarid region on earth, provides analogs of Martian wind-formed fea- tures. These include sand , alternating light and dark streaks,knob 'shadows,'and yardangs. Sur- face particleshave been segregatedby wind into deposits(dunes, sand sheets,and light streaks)that can be differentiated by their grain size distributions, surface shapes, and colors. Throughgoing sand of mostly fine to medium grain size is migrating southward in longitudinal belts and barchan chains whose long axes lie parallel to the prevailing northerly winds, but topographicvariations such as scarps and depressionsstrongly influence the zones of deposition and dune morphology. Sand from the longitu- dinal duneson the plains is commonly redistributedinto barchansin the depressions.These barchansare generally simple crescentsthat are morphologicallysimilar to many of the dunesseen on Viking orbiter pictures of the north polar sand sea on mars. Light streaksare depositional features consistingof dune belts and elongate sheetsof coarseto medium sand and granules.Intervening dark streaksare erosional features consistingof strips of desert-varnishedbedrock and lag gravel surfacesexposed between the sand deposits.The shape of both light and dark streaksis controlled by wind flow around topographic highs. Dark zones (shadows)in the lee of mountains,hills, and knobs are erosionalproducts from the topographichighs; they changeshape only in responseto movement of the adjacent lighter-colored sand deposits.Streamlined yardangs carved in crystallinelimestone constitute one of the largestyardang fields on earth. Yardangs occur also in sandstoneof the Nubian Seriesand in lacustrinesediments. The vari- ables that affect the patternsof wind erosionand depositionin the Western Desert are topographiceffects on wind velocitiesand directions,resistance of the bedrock, sand supply, and climatic changewith time; vegetation is essentiallyabsent and is not a controlling factor.

INTRODUCTION summarized by Said [1962] and Smith [1963]. However, the In Egypt the Western Desert extends from the Libyan bor- work of the wind in hyperarid regions such as western Egypt der to the Nile River and from the Mediterranean Sea to the remains largely unrecognizedamong geologistswhose experi- ence has been limited to the semiarid of the American border of Sudan at latitude 22øN. The desertoccupies 681,000 km2, more than two thirds of the total area of Egypt. The re- southwest[McCauley et al., 1977a, p. 1.4]. Although the West- gion consistsof gently northward dipping sandstoneand lime- ern Desert of Egypt is still relatively unexplored, it has re- stone that have been eroded to an eolian peneplain and ve- cently been the subject of comparative planetology studies neered with windblown debris. The remarkably fiat terrain is through the use of earth orbital photographsin combination broken by numerous escarpmentsthat bound depressions,by with multidisciplina• field investigations[El-Baz et al., 1979]. scattered inselbergs,and in the southwestcorner, by the Gilf Despite recurrentepisodes of lesseraridity than the present, Kebir Plateau and the isolated, circular mountains centered as evidenced by Paleolithic and Neolithic artifacts found in around Uweinat (Figures 1 and 2). parts of the Western Desert [Murray, 1951;Haynes, 1978, also Much of the basic research on dune classification and sand oral communication, 1978], wind excavation of the surface movement by wind has been carried out in the Western may have begun as early as Pliocene or even post-middle- Desert of Egypt, where 'the free interplay of sand and wind Miocene time [Murray, 1951; Said, 1960, p. 14, 1962, pp. 43- has been allowed to continue for a vast period of time, and 44]. 'Although the frequency of rain must have been greater than today, rainfall of the Pleistocenefluvial phaseswas inef- [where] if anywhere,it shouldbe possiblein the future to dis- cover the laws of sand movement and growth of dunes' [Bag- fective in producing drainage lines or vegetativecover' [Said, nold, 1933, p. 121]. Early geomorphic descriptions include 1962, p. 43]. The Western Desert is a 'desertpeneplain' [Sand- ford, 1933] in which most of the geomorphicfeatures are pri- those of Beadnell [1910], King [1918], Ball [1927], Bagnold marily due to wind action [Said, 1960, p. 12]. Vegetation is es- [1933, 1939], Kdddr [1934], Sandford [1935], Peel [1939], and sentially absent. Fluvial processesin that part of Egypt no Mitwally [1953]. Bagnold's [1941] classictreatise on the phys- longer play an effective role in modifying the landscape. As ics of sand movement by wind is based largely on observa- on Mars, wind is the dominant agent of erosion, transporta- tions made in the Western Desert. Sand flow patterns in the ,including western Egypt, have been mapped by Wilson tion, and deposition. [1971, Figure 5] from the 'sand storm resultants' of Dubief The Landsat mosaicof Egypt (Figure 1) showsthe regional [1952] and from barchan dune orientationsas shown on topo- patterns of large-scaleeolian featuresin the Western Desert, graphic maps [Wilson, 1971, p. 187]. Recent work in Egypt is including fields of sand dunes, alternating light and dark streaks, dark knob 'shadows,' and yardang fields. Many of This paperis not subjectto U.S. copyright.Published in 1979by these features are morphologically similar to wind forms ob- the American GeophysicalUnion. servedon Mariner 9 and Viking orbiter picturesof Mars.

Paper number 9B 1264. 8205 8206 EL-BAZ ET AL.' SECONDMARS COLLOQUIUM

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Fig. 1. Mosaic of 65 Landsat imagesof Egypt, showingthe distinct differenceof the landforms in the Western Desert from those in the Eastern Desert (east of the Nile, Figure 2). Light-colored eolian depositsin the form of dunes, sand sheets, and streaks cover most of the Western Desert. In contrast to the Eastern Desert, few traces of fluvial processesre- main in the Western Desert [Haynes, 1978], and the landscapeis dominantly wind formed.

SAND DUNES basin [Ball, 1927; Said, 1962], the largestdepression in Egypt, occupiesan area of approximately19,500 km: below sealevel. One of the largest accumulationsof dune sand in North The depression'snorthern and western margins are marked Africa is the Great Sand Sea of Libya and Egypt. In Egypt by a steep scarp capped by limestone of the Mamarican For- the sand sea occupiesthe west central part of the Western mation of Miocene age, which are underlain by the t¾iable Desert(Figure 2), beginningjust southof the Siwa Depression sand and silt of the Moghra Formation of Miocene age [Said, and continuing uninterruptedfor 600 km to the Gilf Kebir 1960].The southernand easternmargins of the depressionrise Plateau. The main sand massconsists of subparallel arrays of gradually into the fiat plain that stretchesunbroken to the Ba- enormouscompound linear dunes.These are broad whale- hariya Depression(Figure 2). Nearly 5800 km: of the floor of backs ('plinths') surmounted by sharp-crestedlongitudinal the Qattara Depressionis coveredby sebkha (water-saturated ('seif') dunes [Bagnold,1941, pp. 230-232, Plate 13; EI-Baz, salt that is thinly mantied with sand). The remainder consists 1976]. The whalebacks are relatively static, whereas the longi- of exposedbedrock and depositsof sand,pebbles, and clay. tudinal dunes are actively migrating southwardunder the pre- Enormous volumes of sedimentary rock debris have been vailing winds. removed from the Qattara Depression by deflation [Said, East of the Great Sand Sea, numerous other belts of longi- 1960; Squyresand Bradley, 1964].Winds have transportedthe tudinal dunes are migrating southward across the Western sand-sizedparticles southward in belts of long, compound, Desert. The largest array of dunes extends south-southeast longitudinal sand dunes. As depicted in the Apollo-Soyuz from the Qattara Depression(Figure 2). This wind-excavated photographs(Figure 3), the dunes south of Qattara are char- EL-BAZET AL.: SECONDMARS COLLOQUIUM 8207

MEDITERRANEAN SEA

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Fig. 2. Map of the WesternDesert of Egyptshowing the distributionof majorsand deposits in relationto the numerous scarpsthat boundthe major depressions(modified from Giffordet al. [1979]). acterizedby diffusenorthern endsand tapering southernends, effectsof the east-westQattara scarpon the prevailing north- and they join togetherdownwind, producing a 'spearhead'ap- northwesterlywinds; as the scarpincreases in height to the pearance. west, it forms a barrier to the prevailing winds that may en- South of Qattara the dunesincrease in number to the west, hance the relative effectivenessof winds from other directions. and on orbital photographsit is difficult to identify individual This effectis also suggestedby patternson aerial photographs dunes within these westernmost belts. This variation in dune showing crescenticforms concentratedat the northern ends of densityhas been attributed [Giffordet al., 1979]to topographic the westernmostdunes. Slip face orientations on the crescentic 8208 EL-BAZET AL.: SECONDMARS COLLOQUIUM

tantly, agricultural fields [E1-Baz, 1978], thus creating signifi- cant problems becauseless than 4% of the land area of Egypt is presently used for agriculture. The Kharga Depression is bounded on the north by a 200- m-high scarp capped by limestone of Eocene age, but it is open to the south and southwest.Oases and villages lie in its center, along a north trending fault that provides accessto

'%.. groundwater. Downwind extensionsof large compound longi- tudinal dunes enter the depressionfrom the limestone plateau to the north. The largest of these is the Ghard Abu Muharik, which beginsjust east of the Bahariya Depressionand extends downwind to the Kharga Depressionmore than 300 km to the south-southeast(Figure 2). This dune reaches the edge of the depressionnorth-northeast of Kharga, and as sandfrom this dune descendsthe escarpment,it is redistributed into fields of barchans and barchanoid ridges. The transition in dune type is apparently due to the effect of the abrupt change in slope on the oncoming sand-moving winds. Vegetation is virtually ab- sent except in oasesand villages in the center of the depres- sion, along a north trending fault that provides access to groundwater, and thus is not associatedwith dunes of either longitudinal or crescentictype. The idea that the presenceor absence of vegetation controls dune morphology, first pro- posed to explain the distribution of dune types in semiarid desertsof North America [Hack, 1941], does not seem to ap- ply to the development of dune types in the hyperarid Egyp- tian desert. The barchans in the Kharga Depression (Figure 4) are simple crescenticforms that range in length from 30 to 650 m, in width from 25 to 540 m, and in height from 0.5 m to 25 m [Embabi, 1970-1971]. Their rate of movement (20-100 m per year) is roughly inversely proportional to their height (N. S. Embabi, oral communication, 1976). Barchans of similar ap- pearance are seen on Viking orbiter picturesof the north po- lar sand sea on Mars, for example, at latitude 71 øN, longitude 50 ø [McCauley et al., 1979; Breed et al., 1979a]. The Martian barchans grade poleward into fields of closely spaced, large crescenticdunes ridges that are morphologically very similar to the megabarchanoid or transverse variety of crescentic dunes that typically occur in topographically closed basins in terrestrial deserts[Breed et al., 1979a]. Crescentic dunes of this variety are also common in the crater floor dune fields on Mars [Breed, 1977]. In the closed basins of the Sahara, oncom- ing sand-laden winds deposit their load in typically massive crescenticdune ridges that act as sand traps; such dunes may, Fig. 3. Compound longitudinal dunesjust south of the Qattara Depressionin the northernpart of the WesternDesert of Egypt (part as in the Marzfiq Basin of Libya, accumulate sand to depths of Apollo-Soyuz photographAST- 16-1253). of 300 m or more [Glennie, 1970, Enclosure 4]. In the Egyptian depressionsthat are open at their downwind ends, however, dune sand is not trapped but rather continues its southward forms indicate transversewinds normal and oblique to the di- migration in the form of throughgoing barchan chains. Differ- rection of the prevailing north-northwestwind. encesin growth habits between these two common varieties of Crescentic dunes (barchans and barchanoid ridges) occur crescentic dunes in the Sahara thus seems to be related, at mostly in the seven depressionsthat lie 100-200 m below the least in part, to topographic controls on the migration versus averageelevation of the Western Desert. Where belts of longi- the accumulation, in place, of windblown sand. Variations in tudinal dunes crossescarpments and spill into the depressions, the shapesof dunes on Mars may also reflect similar topo- they commonly break up into fields of crescenticdunes. These graphic controls on the transportation and deposition of parti- dunes tend to run southward in chains whose long axes lie cles capable of saltation. parallel to the long, bounding north-southscarps on the east Numerous workers have studied the variables of atmo- side of the Kharga Depressionand just south of the Faiyum spheric pressure,wind velocity, particle size and composition, Depression.The barchansmove rapidly through the depres- and the nature of 'sand' sources on Mars that would account sions, inundating roads, telephone lines, and more impor- for the presencethere of dunes that are morphologically simi- EL-BAZ ET AL..' SECONDMARS COLLOQUIUM 8209 8210 EL-BAZ ET AL.' SECONDMARS COLLOQUIUM

Mode

Mean

dune sand ß ß ß sand sheet deposit

25

Mode

ß ß ß ß ß ß ß ß Mode ß ß

ß ß

ß ß ß

2.83 1.00 0.50 0.25 0.125 GranuleI Verycoarse sand I Coarsesand I Mediumsand I Finesand I Very f ine sand Grain Size in Millimeters

Fig. 5. Frequencycurves showing differences in grain sizedistribution between dune sand and sandsheet deposits on the 'sandplain' southof the Abu Tartur Plateau at about latitude 23øN, longitude30øE in the WesternDesert of Egypt. The sandsheet deposit forms the flat surfaceof the eoliansand plain, whichextends for hundredsot squarekilometers in the central part of the desert.In the sampled area, most of the fine to medium sand is concentratedin dunes.The dune sandshown on the graphis froman active,150-km 2 barchanfield that is migratingsouthward across the sandplain. The coarsestparticles on the graph are quartz lag granulesthat armor the surfaceof the sandsheet deposit against deflation. This lag surfaceis stablein windsof at least32 km/h, which is abovethe thresholdvelocity observed in the field for move- ment of the dune sandby saltation[Breed et al., 1979a].

lar to common varieties of crescenticdunes on Earth [e.g., were taken, both from the northern (upwind) part of the Greeley, 1968, 1979; Greeleyet al., 1977b;Sagan et al., 1977; desert, near Bahariya Oasis, and from the southern part, or Arvidsonet al., 1978;Pollack, 1978; White et al., 1979;Krinsley Nubian Desert, between the Kharga Depressionand the Gilf and Leach, 1979; Krinsley et al., 1979]. Interpretations differ, Kebir. Directions of potential sand migration were calculated, particularly as to the compositionand sourceof particlessub- accordingto the method devisedby Fryberger[1978], for each ject to saltationon Mars and as to possibleimplications of cli- station where winds have been recorded [Wolfe and El-Baz, mate change, but most agree that within the constraintsof the 1979]. present Martian atmospheric conditions the particles most Mechanical analysesof the dune sandsshow that in most of easily moved by saltation are of 0.16-mm size. For com- the sampled localities, grain sizes within the dunes are uni- parison the sand collected from dunes throughout the West- modally distributed and have peak diameters in the fine to ern Desert of Egypt has a mean size of 0.25 mm (fine to me- medium sand categories(0.125-0.50 mm). Nearly all of the dium sand). dune sands are well to moderately well sorted, with two ex- Dunes of the Western Desert consistmostly of rounded to ceptions.Sand from a dome dune (incipient barchan) east of well-rounded grains of detrital quartz derived from primary the Gilf Kebir [Breed et al., 1979b] is poorly sorted, with sources,including sedimentaryrocks such as the Tertiary for- modes at 0.50 mm (medium to coarse sand) and 0.0625 mm mationsexposed in the Qattara Depressionto the north [Said, (very fine sand to coarse silt). Sand from the Ghard Abu 1960, p. 42] and the Nubian Series of Mesozoic age to the Moharik, near Bahariya Oasis, is poorly sorted and bimodal, south,and from secondarysources such as sedimentsin aban- with peak diameters at 0.50 (coarse to medium sand) and doned wadis. In order to relate dune depositsto sourceareas 0.088 mm (very fine sand). better, and to define patternsof wind erosion,transportation, The grain size distribution,composition, and color of sand and depositionin the Western Desert,numerous sand samples in dunessampled in the northern part of the desertdiffer from EL-B^z ET ^L: SECOND M^RS COLLOQUIUM 821 I 8212 EL-BAZ ET AL.: SECOND MARS COLLOQUIUM

Fig. 6b. Viking orbiter picture of the Cerberusregion of Mars (latitude 10.5øN, longitude 202ø) showinglight and dark streaks in the lee of craters and hills.

those farther downwind. Sand from the Ghard Abu Moharik more stable and older dunes of the central Western Desert and other longitudinal dunes near Bahariya Oasis consist of [EI-Baz, 1978]. Previous workers in deserts of the south- both quartz and carbonate grains with a relatively large ad- western United States [Norris, 1969], southern Africa [Logan, mixture of heavy minerals. Thin iron oxide coatings are vis- 1960], North Africa [Alimen, 1957], and Australia [Twidale, ible, with a hand lens, on lessthan half of the quartz grains. 1972] have suggestedthat relative rednessof sand increases These dunes are yellow (10 YR 8/6) on the Munsell Soil with age of the dunes. The progressivereddening of desert Color Charts. Sand from longitudinal dunes near the Gilf Ke- sands,due to the weathering of heavy mineral grains and fer- bir are well to moderately well sorted and highly unimodal, ruginous clay particles, first to yellow limonite and then to red with peak diameters of 0.25 mm (fine to medium sand). They hematite, has been describedby Folk [1976]. consist almost entirely of quartz grains, with very few heavy The reddest dunes observed by the 1979 expedition to the minerals. Iron oxide coatings are developed on most of the Western Desert [EI-Baz et al., 1979] are the climbing and fall- quartz particles and are readily visible to the naked eye. These ing dunes that are trapped in the lee of wadi cliffs in the Gilf dunes are reddish-yellow (5YR 6/8) on the Munsell Soil Kebir Plateau. Scanning electron microscopeanalysis of sand Color Charts. from one of these dunes revealedcoatings, 2-5 /•m thick, of Analysis of Landsat, Skylab, and Apollo-Soyuz color pic- hematite mixed with a clay mineral, on the quartz grains [Pre- tures of major terrestrial deserts,verified by field work in cen- stel and EI-Baz, 1979]. The deep yellowish-red color of the tral Australia [Breed and Breed, 1979] and Western Egypt [El- sandsin the topographically trapped dunes (hue 5YR 5/8 on Baz, 1978] have shown that many of the color zonesvisible on the Munsell Soil Color Chart) contrasts markedly with the the satellite pictures are related to the abundance of desert- much yellower hue (7.5 YR 7/6) of sandsin the active bar- varnished pebbles armoring the surface, the composition and chans about 170 km to the east, on the sand plain. grain sizesof the surface materials, and the thicknessof iron Marked differences in grain size distributions were found oxide coatings on individual sand grains. Recent studies between windblown sediments in the dunes and in the wide- [Breed et al., 1979a] indicate that in areas of uniform climate spread,essentially flat sand sheetdeposits in the southernpart and sand source materials, relative redness can be a useful cri- of the Western Desert. A typical comparison(Figure 5) shows terion for distinguishingyounger, active dunes from relatively that the dune sand is well sorted and rather narrowly distrib- older, less active forms. Field investigationsshow that in the uted within the very fine to medium grain size categories, Western Desert, sand dunes actively encroaching on arable whereas the sand sheet depositsare very poorly sorted and bi- land of the Nile Valley, south of Faiyum, are lessred than the modal and include particles with a wide range of sizes.These EL-BAz ET AL.: SECOND MARS COLLOQUIUM 8213

Fig. 7. Landsat view of the dark, spindle-shapedstreak in the lee of Uweinat Mountain at the bordersbetween Egypt, Libya, and Sudan.The light streaksare longitudinal(seif) dunes.

analysesindicate that the wind has selectivelysorted the surf- LIGHT AND DARK STREAKS icial debris into distinctly different depositsthat can be distin- The Landsat mosaicof the Western Desert of Egypt (Figure guishednot only by differencesin topographicexpression but 1) shows numerous alternating light and dark streaks. On also by their different grain size populations, although the Mars, orientations of streaks associatedwith craters have been compositionof the dunesand sand sheets(mainly quartz par- used to map global wind circulation patterns [Sagan et at., ticles) is the same [Breed et al., 1979a]. Breed et al. [1979b] 1973; Ward, 1978], and streaks have been successfullymod- suggestthat a similar segregationof particles by grain size eled in wind tunnel experiments[Greeley et at., 1974]. may have occurred on Mars and that such grain size differ- On the basis of Mariner 9 data, calculationsof the density enceswithin the Martian resistatesmay account for some of of the Martian atmospheresuggest that much higher wind ve- the differencesin surface characteristicson that planet, such locities and a smaller mean sediment size than on earth would as those observed by Kieffer and Palluconi [1978]. In contrast be the most important contraintson the eolian regime of Mars to the abundance of detrital quartz particles in Egypt, how- [,,lrvidson,1972]. It was also suggested[Sagan et at., 1973; Ar- ever, they suggestthat the sandderived from the mechanical vidson, 1974; Mutch et at., 1976; D',,llti, 1977] that whereas disintegrationof basaltic rockson Mars may be sparse. light streaks are most probably areas of deposition of fine- Interpretations of the relations of wind regimesto grain size grained material, the dark streaks may be areas of non- distributions, composition, and color variations among eolian deposition, erosion, or bedrock surfaces.These hypotheses depositsin the Western Desert are basedon work that is still have been tested in wind tunnel experiments[Greeley et at., in progress.However, the early findings indicate that wind 1974]. depositionalpatterns are more complex than those described Although particle entrainment size and wind speeds on by Bagnold [1933] and depicted on the generalized sandflow earth may be substantially different from those on Mars, the map by Wilson[1971, p. 5]. The mobility of particlesin areas southwesternpart of the Western Desert contains a range of subject to the same regional wind pattern is strongly depen- grain sizes, wind orientations, and natural topographic bar- dent on local topographiccontrols on the sand supply and on riers of circular shape that simulate conditionsof the Martian the directions of sand-moving winds. Topographic effects in- surface probably better than elsewhere on earth. The high- clude deflectionof the winds by hills and scarpsand the chan- quality Viking orbiter images allow detailed studies of the nelling of sand-laden winds through depressionsand local Martian eolian regime on both a local [Greeley et at., 1977a] wind gaps. Erosion of the bedrock outcropsalso adds locally and a regional [Veverka et at., 1977] basis. However, the rela- derived particles to the sandflow. tions of large-scalestreak patternsto topographyand wind re- 8214 EI•-BAz ET AL.: SECOND MARS COLLOQUIUM

•*,,:tt..... EL-BAZET AL.: SECONDMARS COLLOQUIUM 8215

...,.: .;•d.,•

' •½4•;;*.•......

... '.:

Fig. 9. (a) Small yardang (30 m long, 3 m high, and 1.5 m wide) in bedded and cross-beddedQuaternaw deposits about 9 • noah of Kharga in the Western Dese•. Blunt head and tapering proboscisgive the •11 a sph•like appear- ance facing into the northerly wind. The blocks that li• alongsidewere loosenedfrom the side slopesby deflation. (b) Yardangshundreds of meterslong eroded in the Nubian Sandstoneof Cretaceousage exposedin the floor of the Kharga Depression.Noah is to the left. The variationsof streamlinedform are controlledby rock type and structure.These fea- turesstrongly resemble some of the yardangsin the Ica Valley, P•B [McCauley et at, 1977a,Figures 64, 65].

gimes in terrestrial desertshave not yet been studied in detail. Recent studiesof satellite images of the Sahara [Breed et al., In particular, the southwesterndesert of Egypt, which in- 1979b] show that light streaksare typically elongate sand cludes both light and dark streaks accessibleto field studies, sheet depositsin which bedforms (dunes) generally cannot be providesa natural laboratory for the study of features similar discernedat the resolutionof spaceimages. Such light streaks in scale to those on Mars. are characteristicfeatures of regionsof high wind energy such 8216 EL-BAZET AL.: SECONDMARS COLLOQUIUM

Fig.10. (a)Oblique aerial view of the yardang field a fewkilometers north of the Kharga Depression, Egypt. Kilome- ter-scalestreamlined hills of silicifiedlimestone and ½ouloirs are aligned parallel to thenortherly wind (north is to the left front).Joints control the crosscutting alignment. Compare the blunt-nosed hill(arrow) with the similar feature indicated bythe arrow in Figure 10b. (b) Yardangs inthe Biblis Patera region near the Martian equator. The sun illuminates the northslopes of thestreamlined hills (Viking orbiter image 732A61). EL-BAZET AL.: SECONDMARS COLLOQUIUM 8217

as the border region between western Algeria, Chad, and tribution of local sources to dark streaks in southwestern Mauritania, where the streaksgrade into belts of longitudinal Egypt. dunes [Breed et al., 1979b]. Field observationsin the south- Veverka[1975] has cited temporal variationsof light and eastern Gilf Kebir area show that light streaks are formed dark streak patternsobserved during the 1971 dust storm on both by dunesand by elongate sheetsof coarse,highly reflec- Mars as evidencethat dark streakshave an erosionalorigin. tive quartz granuleswithout dune forms. The wind has con- The rapidlychanging form of dark streaksover a shortperiod centrated both the finer, reddish dune sand and the coarser, of time suggeststhat there is not a large quantity of material nearly white lag particlesinto parallel depositsthat appear as being redistributed[Veverka, 1975]. Figure 8 showsan ero- streakson photographs. sionaldark streakextending southward in the lee of a granite As first proposedby Bagnold[1933], the orientationof the knob (Q•ret el Maiyit) about halfway betweenKharga and light streaksin the Western Desert suggestsa clockwiserota- the Gilf Kebir. This inselbergis a remnant of basementrocks tion of the prevailingwind about a center near Kufra Oasisin that protrudesabout 15m abovethe surroundingeolian pene- southeasternLibya (latitude 24.5øN, longitude 23øE). Devia- plain. The streakis composedof reddishsoil veneeredby a tions from this pattern, particularly at the southwestcorner of lag pavementof 5- to 7-mm granitic pebblesand granules Egypt, are controlled by local topography. Here in the Uwei- eroded from the hill. This lag surfacehas been deflated to a nat region, changesin streak orientations and patterns over depth of about 2 m lower than the surroundingplain. It is the past few years have been studied by comparing an Apollo bordered on both the east and west by a light-colored sand 9 photograph (AS9-23-2533) taken in 1969 with four Apollo- sheet.Because the pebbleson the surfaceof the streak are not Soyuz photographs(AST-2-126, 127, 129, and 130) taken in likely to be shiftedabout by the frequentsand-moving winds, 1975 [Slezak and El-Baz, 1979].The averagechange in the po- the shape of the streak will probably changemainly in re- sitions of the sand streaks, particularly in relation to the sur- sponseto the shifting of the light-colored sand on both sides rounding slopes,was found to be 2.5 km over the 6-year pe- [El-Baz and Maxwell, 1979a]. riod. These and similar measurements provide data for comparisonwith temporal changesof Martian streaksfollow- YARDANGS ing dust storms. Mariner 9 and Viking orbiters photographed numerous The barren surface of the Western Desert is broken here streaks on Mars, particularly in the Cerberus and Elysium and there by hills, which include yardangs,inselbergs, and Mons regions(Figure 6b). The uplifted rims of impact craters volcanic cones.Yardangs and inselbergsare among the most on Mars play a major role in shaping the streak directions, spectacular erosional landforms in the Western Desert. and the turbulence in the lee of craters is most likely respon- Whereas inselbergsare outlierseroded from escarpmentsand sible for some streak patterns there. An arcuate vortex of high thus require the existenceof sufficient relief for their forma- surface stressin the lee of craters has been presented as one tion, yardangsare hills or hillocksleft standingabove troughs model responsiblefor the characteristicpatterns of erosion that are excavatedby the wind. Their shapediffers from that and deposition [Greeleyet al., 1974]. Clustersof volcanic cra- of inselbergsin that yardangsare streamlinedby the wind ters occur along with discontinuousscarps (Figure 6a) in and their lengthgreatly exceeds their width--by a ratio of 3: 1 southwest Egypt. Streaks there, as on Mars, emanate from or more,--whereas inselbergsare roughly equant. The occur- crater rims and isolated knobs (Figure 6b), and the knobby rence of yardangs does not require the presenceor former material protrudesthrough the surroundingterrain on a line presenceof an escarpment,and they are restrictedto the most that is oblique to the prevailing wind [El-Baz and Maxwell, arid parts of deserts,where wind erosion dominates in mold- 1979a, b]. ing the landscape[McCauley et al., 1977a,b]. Yardangsare erodedin both soft and hard rocksby a combinationof abra- DARK KNOB SHADOWS sion and deflation. Their most obvious prerequisitesare Circular granitic mountains abound in the extreme south- strong,unidirectional or reversingwinds and great fetch over western corner of Egypt near the borders with Libya and Su- barren continuous rock exposuresor over the surfaces of dan (Figures I and 2). The largest and highest (1893 m) of poorly consolidatedsediments. In the southernpart of the these mountains is Uweinat. East and west of it are several Western Desert these conditions are met on the limestone mountains and smaller hills that, like Uweinat, display dark plateau between Assiut and the Kharga Depression,on the areas resemblingshadows. In all casesthe dark zone occursin floor of the Kharga Depression, and wherever lacustrine or the lee of the topographicbarriers; the largest streak is south- sebkha depositsoccur. west of Uweinat itself (Figure 7). This mountain stands 1200 One of the largestfields of small yardangsin soft lacustrine m above the surrounding sandy plain. Under the prevailing depositsoccupies the floor of the Kharga Depressioneast of wind from the north-northeast it createsa flow pattern that is Kharga. It is easilyaccessible along the paved highway to As- similar to thosegenerated by laboratory experimentsto simu- siut, a few kilometersnorth of Kharga. The lacustrinedeposits late the flow of wind around Martian craters [Greeley et al., consistof slightly indurated, brownish sand and clay that un- 1974]. conformably overlie harder and older rocks. They are fur- As observedin the field, the northern part of the Uweinat rowed by wind and sand into a multitude of elongated hum- dark streak is strewn with irregular chips or flakes of dark mocks, which are 3-5 m high, a few meters wide, and tens of rock a few centimetersin size. They appear to be fragments meters long [Beadnell, 1909, p. 111]. The rate at which the la- from the Uweinat Mountain rocks. Smaller, lighter-colored custrine depositsare deflated varies from layer to layer ac- fragments and sand grains occur between the dark chips. cording to slight differencesin resistanceto weathering and However, the color of the larger fragments dominates, giving erosion:degree of cementationand bedding and jointing pat- the area a dark color both on the ground and on space pic- terns are among the controlling factors. The resulting land- tures. The derivation of the streak material confirms the con- form is known under a variety of evocative names such as 8218 EL-BAZ ET AL.' SECOND MARS COLLOQUIUM

f' ';il ..*.. - - -.* .,, . • :.• :..• ;•.•- .- .. . ,., .... . - - . . , ** 'B'- •.. --. .- ...... "...... '...... ---"• '*"'i.-i.' ..... -*'-.... ' ' "'•

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Fig. 11. (a) Ground view of typical kharafish terrain, about 20 km north of the Kharga Depression.Streamlined lime- stone hillocks, with fluted prows pointing into the direction of the northerly wind, rise a few centimetersto a few meters above sand-chokedcouloirs. Flutes are parallel to one another on the gently sloping limestone surfaces(foreground) and vertical side slopes.The limestone rubble on top of the yardangsindicates the upper limit to active wind abrasion. The orientation of sand and granule streamersis contxolledby irregularities along the yardangs.The bush at center right of the photographis about I m across.(b) Close-rapview of an 'artichokehead' or prow in the yardangfield shownin Figure 10. Flutes diverge upward and sidewaysfrom a 'node' locatedat couloir level, a few centimetersupwind from the prow. The prowpoints into the directionof northerlywind. Limestonerubble downwind from the prowis beingabraded and deflated along bedrockjoints normal to the direction of the northerly wind. Notebook at lower left for scale. EL-BAZ ET AL.: SECOND MARS COLLOQUIUM 8219

mud lion, recumbent lion, sitting sphinx, and sphinx hill (Fig- commonly found on limestoneterrain in much of the Medi- ure 9a). terranean region. Terra rossaextends downward into cracks The yardangsare separatedby long and shallow troughs,or and irregular cavitieswithin the limestone.The upper part of 'couloirs' [Mainguet et al., 1974],which bear no morphological the Thebes Limestone--the part that is furrowed by the evidence of fluvial erosion. The long axes of both yardangs wind--is silicified, and it is unusually hard and resistant. Si- and couloirs are parallel to the prevailing northerly wind. liceousconcretions--the melonsof British geologistsand 'bat- Fields of well-developed mud lions also occur south of tikh' of Egyptian geologists--arecommon erosional remnants Kharga [Ernbabi, 1972]; in the southernpart of the Western on the windsweptsurface of the kharafish terrain. The chem- Desert, southwestof Kharga [Bagnold, 1933, p. 105, 1939, p. ical changesimplied by terra rossa,by silicification,and by so- 283]; and on the floorsof shallowdepressions along Bagnold's lution features such as interconnectingcavities suggestthat a 1932 and 1938 routes to the Gilf Kebir. One of these fields of wetter climate had prevailed over the region for a long time 'mud' depositswas illustratedby Shaw [1936, p. 198]. prior to the conditionsof extreme aridity that must have pre- Besidesmud lions, yardangs have also been carved out of vailed during the carving, by the wind, of the limestoneyard- the much more resistantNubian Sandstoneof Cretaceousage angs. A general outline of the Quaternary climatic fluctua- within the Kharga Depression(Figure 9b). Outside of the de- tionsin this regionis describedby Murray [1951]. pressiona very large field of yardangshas been erodedin the The smaller streamlined rock exposurestypical of the kha- dense, crystalline Thebes Limestone of early Eocene age, rafish terrain are intensely fluted into the wind, in the lee of which caps the plateau that extends from the Nile valley to the wind, and also on side slopes.They resemble artichoke the edge of the Kharga Depression(Figure lea). These lime- headsor the prows of heavy ships,with the stern or the prow stoneyardangs and thosein the Nubian Sandstonewithin the pointinginto the wind (Figure I lb). The wind-flutedand pol- depressionwere first sighted and photographed during our ished crystallinelimestone glistens in the sun in an interval of flight from Cairo [Grolieret al., 1979]. severalmeters high above the couloirs,which are choked with The Thebes Limestone was previously reported to be fur- ripples of abrading sand and granules.Unlike sharp-crested, rowed by the wind into a very rough surfacewhich was called keel-shapedyardangs of other desertsthese are commonly fiat 'kharafish' [Beadnell, 1909, p. 35]. Kharafish topography as topped and retain someof the weatheredsurface that predates originally describedconsists of innumerablesharp-ridged hill- wind erosion. Thus these yardangs seem lessmature than the ocks separated by troughs that are partly buried with sand. sharp-crestedvariety, and the tops may mark a surfacebelow Hillocks and troughslie parallel to the direction of prevailing which the depth of wind erosion can be estimated. The rela- winds. This type of terrain was considered by Gautier and tion of yardang development to rock types and climate Mayhew [1935, pp. 112-113] to be similar to that in western changeswill be an important componentof our future investi- , where yardangswere first describedby Hedin [1905], gations. The discoveryand preliminary descriptionof these but the general impressionleft by these early explorers was wind features in bedrock of the Kharga area was one of the that these wind erosion forms were small features. major resultsof our expedition. A detailed study of the ex- In the flights to and from Kharga we realized that the kha- traordinarily large yardang field in the Thebes Limestonemay rafish topography representsonly a small part of a far more lead to a better understandingof the extensiveyardang fields extensive field of large yardangs (Figure lea). This field of seen on Mars (Figure 10b), first on Mariner 9 images streamlined hills and couloirs is approximately 150 km long [McCauley, 1973, Figure 2c] and in more detail on the re- and at least tens of kilometers wide. Westward, it extends at cently acquiredViking orbiter images[Ward, 1978,this issue]. least as far as the Ghard Abu Moharik as observedby Bag- nold [1933, pp. 104-105] and Sandford[1933, p. 214]. Bagnold REFERENCES [1933] states:'We kept to the dune edge.... with the result that we became entrapped in a seriesof parallel wind-scored Alimen, M.-H., Sables Quaternaires du Sahara Nord-Occidental, Bull. Serv. Carte Geol.Alger., 15, 205 pp., 1957. corridors in the rock, which led straight for the brink of the Arvidson, R. E., Aeolian processeson Mars: Erosive velocities, set- precipitous cliffs of the depression.'Farther south, Hobbs tling velocitiesand yellow clouds,Geol. $oc. Amer. Bull., 83, 1503- [1917, p. 349] had observedthe wind-eroded, flat-topped hill- 1508, 1972. ocks and associatedsiliceous concretions ('melons') from the Arvidson, R. E., Wind-blown streaks,splotches and associatedcraters on Mars: Statisticalanalysis of Mariner 9 photographs,Icarus, 21, railroad track (now abandoned)leading from the Nile valley 12-27, 1974. to Kharga. Forty-five years ago, P. A. Clayton photographed Arvidson, R. E., C. Carlston, E. Guinness, K. Jones, D. Pidek, C. Sa- a field of similar siliceous concretions on the Darb el Arbain gan, and S. Wall, Constraintson aeolian phenomenaon Mars from road south of Assiut [Murray, 1951]. analysis of Viking Lander camera data (abstract), NASA Tech. The innumerable hillocks reported by Beadnell [1909] are Memo., TMo 78455, 3-7, 1978. Bagnold,R. A., A further journey throughthe Libyan Desert, Geogr. only the smallest of the limestone yardangs in the Kharga J., 82, 103-129, 1933. area. Countlesslarger yardangs,which occur isolated and in Bagnold, R. A., An expeditionto the Gilf Kebir and Uweinat, 1938, clusters,are hundredsof meterslong and tens of meters high 1, Narrative of the journey, Geogr.J., 93, 281-287, 1939. (Figure 1la), but like the yardangs of the Takla Makan in Bagnold, R. A., The Physicsof Blown Sand and Desert Dunes,265 pp., Methuen, London, 1941. western China and unlike the larger yardangs of the Lut Ball, J., Problemsof the Libyan Desert, Geogr.J., 70, 21-38, 105-128, Desert in Iran [McCauley et al., 1977a, b] they are detectable 209-224, 1927. only as faint and very fine groovesat Landsat images(Land- Beadnell, H. J. L., An Egyptian Oasis(Kharga), 248 pp., John Murray, sat image 1110-07561, November le, 1972). London, 1909. One day's ground reconnaissanceof thesefeatures showed Beadnell, H. J. L., The sand dunesof the Libyan Desert, Geogr.J., 35, 370-395, 1910. the irregular tops of the larger yardangson the plateau to be Breed, C. S., Terrestrial analogs of the Hellespontus dunes, Mars, mantled by a grayish-pink residual material described by Icarus, 30, 326-340, 1977. Haynes [1978] as terra rossa.It is similar to the terra rossaso Breed, C. S., and W. J. Breed, Dunes and other windforms of central 8220 EL-BAZ ET AL.: SECOND MARS COLLOQUIUM

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