Geomorphology 56 (2003) 155–166 www.elsevier.com/locate/geomorph

Geomorphic indicators of Holocene winds in ’s Western

Ian A. Brookes*

Department of Geography, York University, Toronto, ON, Canada M3J 1P3

Received 1 July 2002; received in revised form 6 January 2003; accepted 14 January 2003

Abstract

Geomorphic mapping of Egypt’s from LANDSAT-MSS images reveals oriented aeolian landforms that record, in part, Holocene winds. Wind directions reconstructed from these landforms indicate the dominance of N–S airflow from 30jNto20jN, turning clockwise southward to NE–SW, conformable with modern circulation. A second direction appears over western Egypt, W between 30jN and 26jN, NW between 26jN and 20jN. Cross-cutting aeolian landforms show that W/ NW winds are older than the N/NE winds. Geomorphic evidence, abundant south to 26jN and less abundant to 20jN, also indicates that W and NW winds were early Holocene ‘palaeowesterlies’. Some evidence also indicates that they extended eastward to at least 30jE, perhaps to the . These winds steered moist Atlantic/Mediterranean air masses to Egypt, sustaining early Holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20jN. Upon aridification, beginning after 5 kyr BP, yardangs oriented west to east were eroded in early Holocene basinal sediments in western Egypt, indicating that these winds continued there for 1–2 kyr, until 3–4 kyr BP. Optically stimulated luminescence (OSL) ages of surface sand sheet in southern Egypt indicate that the present north–south winds were established ca. 3–4 kyr BP, at the same time as the northern savanna boundary was stabilized at its present position. D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Egypt; ; Holocene; Aeolian geomorphology; Palaeoclimate; Palaeowinds

1. Introduction Hassan, 1986; Brookes, 1989a; Haynes et al., 1989; Neumann, 1989; Kro¨pelin, 1993; Street-Perrott and Palaeoclimatic reconstructions in NE Africa for the Perrott, 1993; Pachur and Wu¨nnemann, 1996; Stokes period of the last glacial to the present have been based et al., 1998; Gasse, 2000, 2002; Hassan et al., 2001; on evidence from (i) lacustrine and aeolian sediments Swezey, 2001). Interpretations converge on a cold, dry, and their physical and chemical properties, (ii) pollen windy last glacial maximum (15–20 14C kyr BP), spectra and other palaeobiological indicators within changing through an erratic transition to a multiphase, these sediments, and (iii) archaeological remains. perhaps still cool, wetter, early Holocene (10–5 kyr Chronology has been supported by radiocarbon and BP), with pronounced arid intervals, the ‘‘African optically stimulated luminescence (OSL) ages (e.g., Humid Period’’ of DeMenocal et al. (2000), then to a drier and windier later Holocene (5 kyr BP to present). * Tel.: +1-416-265-8318. These empirical studies have spawned theoretical E-mail address: [email protected] (I.A. Brookes). research into climate change in North Africa, focussed

0169-555X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(03)00076-X 156 I.A. Brookes / Geomorphology 56 (2003) 155–166 on modelling of atmospheric and surface responses to arid core of the Western Desert is estimated at between orbitally forced insolation. The purpose has been to 2500 and 5000 mm (Shahin, 1985). Gasse (2002) simulate atmospheric processes and surface feedbacks gives an excellent summary of North African clima- responsible for the presence and character of Late tology with references. Pleistocene and Holocene lakes and playas in the Modern wind patterns in the Western Desert com- now hyperarid region of the Sahara and its arid borders prise two fields. A northern zone of variable winds (e.g., Kutzbach et al., 1993, 1996; Claussen and extends south from the Mediterranean coast, more Gayler, 1997; Kutzbach and Liu, 1997; Texier et al., westerly and stronger in winter (November to April), 1997; Ganapolski et al., 1998; de Noblet et al., 2000; bringing rain in cyclonic storms to about 25jN. In Doherty et al., 2000). Wind patterns, however, are not summer (May to October), weaker westerly winds usually considered in either empirical reconstructions reach only to about 30jN and yield no rainfall. Over or simulation models, and then only as output rather a more extensive southern zone of the Western Desert than input (e.g., Kutzbach et al., 1993). This paper between 30jN and 20jN, northerly winds dominate reports geomorphic evidence of wind patterns over an and bring no rainfall at either season. They strengthen area of Egypt encompassing 8j of latitude and longi- in winter, often meeting westerly winds along rain- tude, patterns that are, in part, of Holocene age, and bearing fronts which can affect any part of the desert at which provide input data to palaeoclimatic models. this season. Southward, these northerly winds veer northeasterly around the eastern limb of the subtropical anticyclone. 2. Study area

The study area encompasses 70% of the Western 3. Methods Desert, about half of Egypt’s area (Fig. 1). Physio- graphically, the northern half of this desert is a low- The study area is covered by a set of 24 LANDSAT- relief, cuesta-form plateau developed across north- MSS images acquired from 1972 to 1976. These are dipping Palaeogene limestones, sloping south to north false-colour composites of bands 4, 5, and 7, processed for 550 km from f 500 to f 200 m elevation. Pro- at 1:250,000 scale by Earth Satellite Corporation longed wind erosion has formed fields of yardangs (ESC) and held at the National Air and Space Museum over much of it (Brookes, 2001). Its southern boundary (both in Washington, DC). Images were enhanced is a 200- to 300-m-high escarpment which overlooks a using GEOPIC, developed at ESC, a procedure which low-relief mosaic of plains, low cuestas, and isolated emphasized subtle tonal variations of surface materials hills at 100–250 m, developed on north-dipping and which revealed linear features, such as roads and Mesozoic sandstones extending over 500 km south- , below the 79-m pixel resolution (El-Baz, ward into Sudan. The geomorphic evolution of this Centre for Remote Sensing, Boston University, written southern desert has been interpreted by Haynes (1982) communication, 2001). Some sense of resolution of and Maxwell and Haynes (2001). the images can be gained from the visibility of roads Climate in the study area is arid, with a large roughly 20 m wide (including bordering disturbed hyperarid core area where the few meteorological ground), and from visible barchans of comparable stations at oases record practically no rainfall (Fig. width that were visited in the field. 1). Over the wider region rain does fall, however, Aeolian erosional landforms mapped from these mainly from cyclonic winter storms, tracking SE images and recorded in the field for directional infor- across the to meet westerly waves mation are discussed in the following. crossing the Sahara from the tropical Atlantic. A Aeolian erosional lineations (AELs):Theseare concise meteorological perspective on winter rainfall parallel, unstreamlined, ridge-trough sets, as well as across North Africa is given by Geb (2000). Rare fields of streamlined forms (yardangs). Those AELs summer rains mark exceptional northern excursions of capable of resolution on the LANDSAT images are all monsoonal convective systems, which are normally in bedrock, whereas smaller ones in unconsolidated restricted to south of 21jN. Evaporation in the hyper- sediments were recorded in the field. Because of the I.A. Brookes / Geomorphology 56 (2003) 155–166 157

Fig. 1. Egypt showing major physiographic features—plateaus, scarps (toothed lines), oasis depressions, fields (stippled), from El-Baz and Wolfe (1982); mean annual isohyets (mm) after Haynes (1987); rectangles (a–e) represent sample areas shown in Fig. 2a–e; latitude/longitude shown at 5j intervals; scale bar 200 km. 158 I.A. Brookes / Geomorphology 56 (2003) 155–166 time required to form these features in bedrock, bed- taken either from simple linear dunes, or from com- rock AELs are interpreted as of mainly pre-Holocene pound linear dunes only where they are parallel to age (Brookes, 2001). Within sets of bedrock AELs, nearby simple ones, which therefore allow only one however, features such as faceting of upwind faces can wind direction to be inferred. reflect more recent wind erosion. Also, bedrock AELs In western Egypt, where geomorphic features indi- are sometimes parallel to lineations in unconsolidated cate two palaeowind directions (W/NW and N/NE), Holocene sediments and can then be argued to reflect linear and transverse dunes are combined in fields at least some Holocene wind erosion. where transverse dunes perpendicular to the W/NW Aeolian scour zones: These are swaths of bedrock palaeowinds occur as ‘barbs’ attached to the flanks of or surficial deposits, < 10 to hundreds of kilometres linear dune ‘shafts’ lying parallel to N/NE palaeowinds long and < 1–5 km wide, where varnish and other (Sections 4.1.2–4.1.4 below). On the other hand, in patina has been erased or prevented from forming, and the central and eastern parts of the Western Desert, which therefore appear on the images lighter in colour where only one palaeowind direction is indicated, this than adjacent terrain, with sharp boundaries against it. compound dune form is absent and the two types, In the lee of cliffs, scour zones reflect wind accelerated transverse and linear, are less common and occur through ravines, but, where not topographically local- separately. ized, they reflect regional wind structure. These scour As with AELs, dunes (possibly) and ‘draa’ (defi- zones, too, probably record a longer period of wind nitely) represent a longer geomorphic history than the erosion than the last 10 kyr; but their parallelism with Holocene (Embabi, 1998). However, where smaller, mobile features such as small dunes, and with yard- more mobile, linear dunes and the mobile superstruc- angs in Holocene sediments, indicates that, in part, tures of larger forms are parallel to other mobile they reflect Holocene wind directions. If Holocene directional indicators, such as sand drifts (see below) wind directions were different, this would be reflected and yardangs in Holocene sediments, or are perpen- in these scour zones, which are probably quick to dicular to transverse dunes, they can again be related to respond to such changes. Holocene winds. Depositional aeolian landforms mapped from the Sand drifts: These appear on the images as multi- images for directional information are the following. plumed, diffusely bounded, thin spreads of light-col- Transverse dunes: These are sets of 5–20 sinuous, oured sand over darker terrain, resembling ‘mares subparallel sand ridges, each V 10 km long, spaced at tails’ in cirrus clouds. Being diffuse and difficult to f 500–700 m. They are oriented perpendicular to outline, for mapping purposes, their medial long axes formative winds. were chosen to provide directional information. These Linear dunes: These occur both in fields and iso- axes, tens of kilometres long, usually occur in ‘fields’, lated, as a result of differences in sand supply. In the numerous and adjacent, spaced at 5–10 km intervals, of western Egypt, where they occur as and are restricted over the image coverage to western fields, they comprise a stable, basal ‘plinth’, f 3km Egypt (again, largely because of sand supply from the wide (therefore more correctly a ‘draa’), topped by a Great Sand Sea). narrow, sharp-crested, sinuous, mobile dune, the Cross-cutting relationships between landform sets whole 10–50 km long, and spaced at 2–5 km. In are readily apparent on the images, so that the restric- central and eastern Egypt, on the other hand, where tion of the two sets of palaeowind directions to western sand supply is more limited, most linear dunes are Egypt is immediately apparent. From the interpreted simple, shorter, forms not built on a ‘plinth’. The geomorphic maps of each of the 24 LANDSAT relationship of linear dune orientation to wind direc- images, directional information was first generalized tions is complicated, but in central and eastern Egypt, at image scale over each map quadrant. Generalization simple linear dunes are parallel to prevailing northerly was not interpretive, merely simplifying by reducing winds. In western Egypt, the construction of com- the number of flow lines. The generalized pattern was pound linear dunes may have been influenced by transferred to a small-scale map (4 cells  24 maps), northerly and by westerly/northwesterly winds, but then smoothed visually into the regional pattern shown in this area, wind directional information has been in Fig. 3. I.A. Brookes / Geomorphology 56 (2003) 155–166 159

4. Results in two directions, at f 110j and 170j. Linear dunes occur parallel to 170j, whereas scour zones are 4.1. Interpretation of sample areas at f 135j. Transverse dunes are perpendicular to f 135j, in separate fields and as ‘barbs’ on linear Sample geomorphic maps showing directional aeo- dune ‘shafts’. lian landforms are shown in Fig. 2a–e.Plateaus, scarps, mesas, valleys, and deflation basins have been 4.1.5. Gebel Uweinat east added to indicate terrain influences on wind. The area shown in Fig. 2e lies just south of the Egypt/Sudan border, and contains a low rocky ‘ham- 4.1.1. ada’ scored by many AELs oriented at f 250j and a The area shown in Fig. 2a straddles the scarped few at f 160j. A ‘scour shadow’ is oriented at 250j, eastern edge of the Bahariya Oasis depression (Fig. 1). as are several linear dune crests and a small dune Two sets of AELs are present, one oriented at f 110– complex entering from the northern edge. 130j, and another at 165–180j. A dune complex 5–8 km wide, oriented at 165j (Ghard Abu Muhariq), 4.2. Regional palaeowind fields contains simple linear dunes and transverse dunes oriented perpendicularly to them. Linear dunes occur The regional pattern of winds reconstructed from east and west of this complex at f 165j. geomorphic indicators lies south of the modern west- erly wind belt. It is dominated by N–S winds, with a 4.1.2. Farafra Oasis depression north pronounced turn toward SW in SW Egypt (solid arrows The area shown in Fig. 2b contains Farafra Oasis in Fig. 3). This is the dominant annual pattern today, (stippled) at the western edge of a depression between driven by return of air subsided in the subtropical high- limestone scarps. AELs occur in two sets: over a pressure cell toward the equatorial low. Geomorphic narrow western plateau, a strong set at 110j; and over evidence (discussed below) indicates that this pattern the northern plateau and cuestas, a more variable but was initiated in the late Holocene. Also revealed is a W still consistent set at 160–190j. Aeolian scour zones and NW flow over western Egypt, more westerly occur below the western plateau, oriented parallel to a between 30jNand26jN, veering to NW between set of AELs at 110j. Transverse dunes occur as small 26jN and 20jN (dashed arrows in Fig. 3). and large fields oriented at f 180–190j and perpen- Also noteworthy is the fact that in the north of the dicular to the AELs and scour zones at 110j. Trans- area, as far east as 30jE, several indicators of older verse dunes also occur as ‘barbs’ on the western flanks westerly flows are disjunct to younger N–S stream- of linear dunes. lines. Lastly, a few short flowlines oriented NE–SW in the centre and north of the area (dotted arrows in Fig. 4.1.3. Farafra Oasis depression south 3) are topographically influenced by promontories and The area shown in Fig. 2c lies at the southern end of embayments in the high scarp dividing the Western the depression containing Farafra Oasis at its northern Desert and thus do not enter the regional interpretation. end. AELs occur in two sets: at 070j and at 165j. Scour zones and a sand drift axis occur at 070j. A field of linear dunes oriented at f 165j possesses trans- 5. Interpretation verse dune ‘barbs’ oriented at f 200j. In the bottom centre of the area, a ‘feathery’ dune association con- 5.1. Wind fields tains transverse dunes oriented perpendicular to 165j, which curve to become short linear dunes parallel to The reconstructed regional airflow pattern over the 165j. north and central Western Desert is dominated by N winds, and in the south by NE winds. A second wind 4.1.4. Abu Ballas west field, however, emerges—W over NW Egypt, veering The area shown in Fig. 2d is crossed by two sand- to NW over SW Egypt. The N/NE pattern is similar to stone scarps trending generally E–W. AELs occur the present-day circulation. As argued in the following 160 I.A. Brookes / Geomorphology 56 (2003) 155–166 section, this pattern ‘switched on’ in the last few the overall pattern is shown in Fig. 3. In the Farafra millennia, but has also dominated much of the last Oasis depression (Fig. 1), Donner et al. (1999) 2.5 myr (Brookes, 2001). The W/NW pattern over reported cross-cut, wind-abraded surfaces on lime- western Egypt is argued below to represent early stone bedrock that record the same sequence of air- Holocene ‘palaeowesterlies’. flows (W/NW then N/NE). These two wind directions are well shown in Fig. 2b (described in Section 4.1.2). 5.2. Chronology Significantly for the argument developed here, these authors also reported yardangs oriented W–E formed Over western Egypt, aeolian landforms indicative in playa sediments with radiocarbon dates of 9–6 14C of N/NE flow cross-cut those indicating W/NW flow, kyr BP, indicating that the W/NW pattern at least partly and therefore formed earlier. Examples are noted in the postdates the sediments. These authors concluded that captions of Fig. 2 and in Sections 4.1.1–4.1.5, while ‘‘...observations... show a pattern with an older I.A. Brookes / Geomorphology 56 (2003) 155–166 161

Fig. 2. Sample areas in Egypt’s Western Desert (locations in Fig. 1) showing aeolian erosional and depositional landforms on LANDSAT image extracts and map extracts from Brookes (1999) used to determine their relative age and directions of formative winds. Margins of extracts are N–S/W–E; all scale bars are 10 km. (a) Extract of geomorphic map, bounded by 28–28.5jN, 29–29.5jE (loc. 2a, Fig. 1). East rim of Bahariya Oasis depression with aeolian erosional lineations (AELs) to WNW–ESE and N–S, NNW–SSE; dune complex containing transverse dunes normal to (and longitudinal dunes parallel to) NNW winds. (b) Extract of LANDSATand geomorphic map of northern Farafra Oasis depression, bounded by 27–27.5jN, 27.25–28.6jE (loc. 2b, Fig. 1), showing AELs N–S/NNW–SSE and W–E/WNW–ESE; transverse dunes normal to and scour zones parallel to W/WNW winds; linear dunes parallel to N/NNW wind. (c) Extracts of LANDSAT and geomorphic map of south Farafra Oasis depression, bounded by 26.5–27jN, 26–26.5jE (loc. 2c, Fig. 1), showing transverse dunes normal to (and short linear dunes parallel to) NW/WNW winds; long linear dunes parallel to NNW winds, with transverse dune ‘barbs’ perpendicular to WNW winds; axis of sand drift and scour zones parallel to W winds; AELs (top) parallel to W winds, and (bottom) parallel to NNW winds. (d) Extract of LANDSAT and geomorphic map of area bounded by 25–25.5jN, 27.5–28jE (loc. 2d, Fig. 1), showing low, S-facing sandstone scarps, AELs parallel to NNW winds (two at bottom parallel to WNW winds); sand drift axes parallel to NW winds; linear dunes parallel to NNW winds; transverse dunes (top) normal to WNW winds. (e) Extract of geomorphic map of area bounded by 21–21.5jN, 25–25.5jE (loc. 2e, Fig. 1), showing sand plains surrounding higher sandstone bedrock plain, with AELs parallel to NE and NNW winds; linear dune crests parallel to NE winds; scour zone and elongate dune complex with small barchans (top) parallel to NE winds. Legend: escarpment—toothed line; geological structure—dash/ dot line; wadi—dotted line; linear dune crest—unornamented straight line; long axis of sand drift—curved line with diamond; margin of aeolian scour zone—curved line with tooth (tooth on scoured side); aeolian erosional lineation (AEL, e.g., yardang)—short line with central dot; transverse dunes—short sinuous lines (in groups). 162 I.A. Brookes / Geomorphology 56 (2003) 155–166

Fig. 3. Egypt (base as in Fig. 1, with most names omitted) showing wind streamlines reconstructed from geomorphic features interpreted from LANDSAT images. Dashed lines are early Holocene W and NW flows (‘palaeowesterlies’); solid lines are late Holocene N and NE flows; dotted flowlines tributary to major N flows are topographically steered. Latitude/longitude shown at 5j intervals; scale bar (top) 200 km. I.A. Brookes / Geomorphology 56 (2003) 155–166 163 effective wind direction from the west,... and with a (Sonntag et al., 1979) and in organic carbonates younger wind direction similar to that of the present (land-snail shell, Goodfriend, 1991; archaeological surface winds in the summer [north]...’’ (p. 82, ostrich eggshell, Sonninen, in Donner et al., 1999). brackets added). This field evidence conforms with Goodfriend inferred a NW trajectory for these rain- widespread geomorphic evidence from the LANDSAT bearing winds, coincident with the orientation of the images for the same sequence of wind directions over older set of aeolian landforms mapped here. much of western Egypt (Fig. 3). This local and Early Holocene lakes and playas in NE Africa could regional evidence further indicates a sequence of air- not all have been fed by the same air masses, either flows, negating a seasonal explanation. solely from the winter temperate westerlies or from the In Dakhla and Kharga oases (Fig. 1), yardangs of summer tropical monsoon. From geobotanical evi- comparable height to those at Farafra ( f 4 m) have dence at NE African localities, Haynes (1987) inferred formed in unconsolidated sediments accreted between a steep drop in early Holocene tropical monsoonal the second and seventh centuries C.E. (Brookes, rainfall spanning 21–20jN (vs. 15–14jN today). 1989b). Relief generation in these cases is therefore Northward at f 24jN, early Holocene rainfall steeply closely constrained to a minimum of z 2.2–3.0 m increased again, reflecting southward intrusion of mid- kyrÀ 1. The W–E yardangs of Farafra could therefore latitude rainfall. Between these rainfall clines, early have formed in under 2 kyr, following 6 kyr BP. It Holocene moisture was sufficient to eliminate hyper- follows that the N/NE wind regime was established aridty, replacing it with mere aridity or semi-aridity. since 4 kyr BP. Interestingly, therefore, Stokes et al. This picture conforms with evidence from radio- (1998) reported OSL ages of 3–4 kyr for the surface carbon-dated archaeological charcoal at Neolithic sites sand sheet in southernmost Egypt (Fig. 1). In that area, on a N–S transect across the eastern Sahara (Neu- surface aeolian features are aligned N–S (Brookes, mann, 1989). This showed that during the early 1999), so it can safely be concluded that N/NE winds Holocene moisture maximum species of trees and were established 3–4 kyr ago. Accounting for a lag woody shrubs similar to modern ones were more behind falling water table (Pachur and Hoelzmann, widespread, probably within a desert steppe, eliminat- 2000), these winds and aridification probably set in ing the barren desert core. It also showed that the treed 1–2 kyr earlier. savanna receded southward in response to aridifica- Studies of basins containing early Holocene sedi- tion, reaching its present northern limit at 3.3 kyr BP ments, from northern Sudan to northern Egypt, indicate (3.5–3.6 cal kyr BP, Klein et al., 1982). This age that palaeolakes existed between approximately 9 and bisects the range of OSL ages given for the surface of 4.5 kyr BP. ‘‘Lake’’ in this context signifies either (i) a the Selima Sand Sheet by Stokes et al. (1998).A perennial water body, as at Oyo and Selima in Sudan probable lag behind falling water table should again (Ritchie et al., 1985; Haynes et al., 1989) and Faiyum in be noted here. Egypt (Hassan, 1986) (Fig. 1), at the humid southern In addition to the conclusions of Goodfriend and northern extremities, respectively, of the Western (1991), Sultan et al. (1997),from2H depletion in Desert, or (ii) an intermittent/ephemeral water body fossil Saharan groundwaters and 18O depletion in (playa) (e.g., Brookes, 1989a; Hassan et al., 2001), freshwater tufas, identified a ‘palaeowesterly’ trajec- responsive to individual storms or stormy intervals, tory for winds across the NE Sahara, north of 24.5jN, which is the predominant basin type in the present as well as an Atlantic monsoonal one for the southern hyperarid core. The end of the ‘lacustrine’ interval and central Sahara. Although these groundwaters and therefore appears to coincide with the onset of N–S tufas are of several ages, and much older than Hol- winds over the Western Desert. ocene, the similarity of the reconstructed trajectories to wind directions mapped in this paper supports the 5.3. Moisture source(s) interpretation that westerly winds reconstructed here, north of monsoonal influence, reflect a ‘palaeowes- A European/Mediterranean source for early and terly’ flow. mid-Holocene precipitation over NE Africa was in- Thus, while a mid-latitude source for early Holo- ferred from stable isotopes in shallow groundwater cene rainfall in Egypt south to 26jN is identified 164 I.A. Brookes / Geomorphology 56 (2003) 155–166 confidently, the earlier inference herein that the NW fall, not intrusion/interception of mid-latitude air flows in southwest Egypt were also of mid-latitude masses. As for the 15–20 kyr BP wet interval recog- origin requires substantiation. The geomorphic indica- nized by Maley (2000), no ‘Last Glacial Maximum’ tors of this flow, particularly the erosional ones, would ages have been reported from the plains fed by drain- not have been expected to have formed in the rainy age from these highlands. Moreover, the similar ages winter season, when mid-latitude winds would be for a subsurface unit of the Selima Sand Sheet reported drawn equatorward. The landforms more likely were by Stokes et al. (1998) point to lowland aridity (at produced in the dry summer season, so that winds must least) in this interval. have been from the NW over SW Egypt (20–26jN) Whereas the argument presented herein has so far immediately north of the early Holocene Intertropical pointed to an early Holocene age for the ‘palaeowes- Convergence. terlies’ indicated by geomorphic evidence in western Some authors have called for glacial-age westerlies Egypt, recognition farther west of a drier end-Pleisto- (20–10 kyr BP) to extend southward over the northern cene interval coeval with the Younger Dryas of Europe Sahara (e.g., Nicholson and Flohn, 1980). A cold and (Gasse, 2002) raises the possibility that these winds wet ‘last glacial’ in NW Africa (Maghreb) has been may have prevailed during that interval. Insufficient explained thus by Rognon (1987), but evidence from evidence is available here to decide this question, but it farther east is scarce, even contrary (Fontes and Gasse, must be kept in mind that in western Egypt, ‘palae- 1991). The climate simulations of Kutzbach et al. owesterlies’ eroded yardangs in playa sediments as (1993) do not predict glacial-age penetration of west- young as 6 kyr BP. Therefore, regardless of whether erly rain-bearing winds into NE Africa. Moreover, these winds existed during the ‘Younger Dryas’-equiv- within the Selima Sand Sheet of southern Egypt alent interval in the eastern Sahara, they certainly (22–23jN), Stokes et al. (1998) reported OSL ages existed between 6 and 4 kyr BP, before the northerly between 15 and 20 kyr for a subsurface sand sheet wind regime ‘switched on’. inferred to have formed under an arid/hyperarid cli- mate. The age range implies that rain-bearing west- 5.4. Longitudinal extent of palaeowesterlies erlies did not reach the northern tropic during the last glacial (however, see Cooke et al., 1993, p. 395 and As previously noted, Fig. 3 shows that over the Lancaster, 1995, pp. 165–166 on sand sheet genesis north-central part of the Western Desert, winds recon- and climate). structed from the W/NW are disjunct to N–S stream- These arguments against ‘glacial’ westerlies, how- lines. The pattern can perhaps be explained as the ever, refer to elevations near sea level. Referring to effect of erasure by N–S winds of much the evidence altitudes above 1000 m, Rognon (in Messerli and of ‘palaeowesterlies’ which previously extended far- Winiger, 1980), Street and Gasse (1981), and Maley ther east. The longitudinal gradient of stable isotopes (2000) argued in favour of glacial-age runoff from in Holocene precipitation across NE Africa indicates highlands in the North African desert belt, from penetration of ‘palaeowesterlies’ as far as 30jE (Sonn- Hoggar (10jE) to the Red Sea Hills ( f 35jE). Maley tag et al., 1979), which is as far as the isotopic (2000) reported two periods of highland humidity evidence extends. Moreover, radiocarbon-dated cave (20–15 and 15–12 kyr BP). Gasse (2002, p. 758), deposits at Gebel Umm Hamad, in the Red Sea Hills at however, referring to the area west of 15jE, interprets 26jN, 34jE, inland of Quseir on the Red Sea coast, evidence of wetter climate to mean that ‘‘the monsoon indicate a wetter climate at f 8 and 6.6 kyr BP than reactivation occurred in two steps, at 14.5 [cal] ka during the late glacial and the later Holocene (Moyer- [12.3 14C] and 11.5–11 [cal] ka [9.9–9.6 14C] sepa- sons et al., 1999). Because early Holocene monsoon rated by a return to drier conditions related to the rains, even from the Indian Ocean, likely did not reach Younger Dryas (YD) cold spell defined in higher 26jN, ‘palaeowesterlies’ are a plausible moisture northern latitudes ca. 12.6–11.6 [cal] ka [10.7–9.8 source. Of those mapped in this study, the closest 14C]’’ (conversions in brackets added). From this, it geomorphic indicators of ‘palaeowesterly’ flow to appears that at least the later of the two intervals Gebel Umm Hamad are near , at 25jN, recognized by Maley (2000) refers to monsoonal rain- 30jE, 380 km to the west. I.A. Brookes / Geomorphology 56 (2003) 155–166 165

6. Conclusion York University for the figures; to R.A. Marston for editorial work; and to two journal reviewers. Geomorphic mapping of Egypt’s Western Desert (20–30jN, 22–32jE) from LANDSAT-MSS images reveals two consistent orientations of aeolian land- References forms, which indicate Holocene wind directions, one from the W and NW and another from N and NE. The Brookes, I.A., 1989a. Early Holocene basinal sediments, Dakhleh W/NW set are in evidence mostly over western Egypt: Oasis region, south-central Egypt. Quat. Res. 32, 139–152. from the W over the northern part and from the NW Brookes, I.A., 1989b. Above the salt: sediment accretion and irri- over the southern part. Fragmentary evidence in north- gation agriculture in an Egyptian oasis. J. Arid Environ. 17, 335–348. central Egypt extends this westerly wind domain Brookes, I.A., 1999. Geomorphic Maps of Egypt’s Western Desert. farther east to 30jE. The N/NE set is more extensive XV Congr, Int. Assoc. Quat. Res. (INQUA), Durban, South over central and eastern Egypt, veering from N over Africa, p. 33. Abstr. the north and central parts to NE in the SW. Brookes, I.A., 2001. Aeolian erosional lineations of the Libyan In western Egypt, the reconstructed W/NW flows Desert, Dakhla region, Egypt. Geomorphology 39, 189–209. Claussen, M., Gayler, V., 1997. The greening of the Sahara during veer smoothly to merge with N/NE flowlines, but the mid-Holocene: results of an interactive atmosphere–biome cross-cutting landforms provide definite evidence that model. Glob. Ecol. Biogeogr. Lett. 6, 369–377. the former are the older set. In the west, W/NW winds Cooke, R.U., Warren, A., Goudie, A.S., 1993. Desert Geomorphol- formed yardangs in playa sediments dated as young as ogy. UCL Press, London. 6 kyr BP. They are related to mid-latitude palaeowes- DeMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of terlies penetrating the Sahara in the early to mid- the : rapid climatic responses to gradual Holocene. In the south, their source and timing is less insolation forcing. Quat. Sci. Rev. 19, 347–361. clear, but, because evidence indicates that they do not de Noblet, N., Claussen, M., Prentice, C., 2000. Mid-Holocene represent southerly extension of glacial-age westerlies, greening of the Sahara: first results of the GAIM 6000 years they are argued to represent early to mid-Holocene BP experiment with two asynchronously coupled atmosphere/ biome models. Clim. Dyn. 16, 643–659. winds, veered to NW and penetrating farther south Doherty,R.,Kutzbach,J.,Foley,J.,Pollard,D.,2000.Fully than previously recognized. Additionally, some evi- coupled climate/dynamical vegetation model simulations over dence indicates that early Holocene ‘palaeowesterlies’ northern Africa during the mid-Holocene. Clim. 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