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Geomorphic indicators of Holocene winds in Egypt’s

Western Desert

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 Western Desert 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 30jN to 20jN, 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, alsoindicates that W and NW winds were early Holocene ‘palaeowesterlies’. Some evidence also indicates that they extended

eastward to at least 30jE, perhaps to the Red Sea. 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; Sahara; Holocene; Aeolian geomorphology; Palaeoclimate; Palaeowinds

1. Introduction

Palaeoclimatic reconstructions in NE Africa for the

period of the last glacial to the present have been based

on evidence from (i) lacustrine and aeolian sediments

and their physical and chemical properties, (ii) pollen

spectra and other palaeobiological indicators within

these sediments, and (iii) archaeological remains.

Chronology has been supported by radiocarbon and

optically stimulated luminescence (OSL) ages (e.g.,

Hassan, 1986; Brookes, 1989a; Haynes et al., 1989;

Neumann, 1989; Kropelin, 1993; Street-Perrott and

Perrott, 1993; Pachur and Wunnemann, 1996; Stokes

et al., 1998; Gasse, 2000, 2002; Hassan et al., 2001;

Swezey, 2001). Interpretations converge on a cold, dry,

windy last glacial maximum (15–20 14C kyr BP),

changing through an erratic transition to a multiphase,

perhaps still cool, wetter, early Holocene (10–5 kyr

BP), with pronounced arid intervals, the ‘‘African

Humid Period’’ of DeMenocal et al. (2000), then to a

drier and windier later Holocene (5 kyr BP to present).

These empirical studies have spawned theoretical

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

* Tel.: +1-416-265-8318.

E-mail address: [email protected] (I.A. Brookes).

www.elsevier.com/locate/geomorph

Geomorphology 56 (2003) 155–166

on modelling of atmospheric and surface responses to

orbitally forced insolation. The purpose has been to

simulate atmospheric processes and surface feedbacks

responsible for the presence and character of Late

Pleistocene and Holocene lakes and playas in the

now hyperarid region of the Sahara and its arid borders

(e.g., Kutzbach et al., 1993, 1996; Claussen and

Gayler, 1997; Kutzbach and Liu, 1997; Texier et al.,

1997; Ganapolski et al., 1998; de Noblet et al., 2000;

Doherty et al., 2000). Wind patterns, however, are not

usually considered in either empirical reconstructions

or simulation models, and then only as output rather

than input (e.g., Kutzbach et al., 1993). This paper

reports geomorphic evidence of wind patterns over an

area of Egypt encompassing 8j of latitude and longi-

tude, patterns that are, in part, of Holocene age, and

which provide input data to palaeoclimatic models.

2. Study area

The study area encompasses 70% of the Western

Desert, about half of Egypt’s area (Fig. 1). Physio-

graphically, the northern half of this desert is a low-

relief, cuesta-form plateau developed across north-

dipping Palaeogene limestones, sloping south to north

for 550 km from f 500 to f 200 m elevation. Pro-

longed wind erosion has formed fields of yardangs

over much of it (Brookes, 2001). Its southern boundary

is a 200- to 300-m-high escarpment which overlooks a

low-relief mosaic of plains, low cuestas, and isolated

hills at 100–250 m, developed on north-dipping

Mesozoic sandstones extending over 500 km south-

ward into Sudan. The geomorphic evolution of this

southern desert has been interpreted by Haynes (1982)

and Maxwell and Haynes (2001).

Climate in the study area is arid, with a large

hyperarid core area where the few meteorological

stations at oases record practically no rainfall (Fig.

1). Over the wider region rain does fall, however,

mainly from cyclonic winter storms, tracking SE

across the Mediterranean Sea to meet westerly waves

crossing the Sahara from the tropical Atlantic. A

concise meteorological perspective on winter rainfall

across North Africa is given by Geb (2000). Rare

summer rains mark exceptional northern excursions of

monsoonal convective systems, which are normally

restricted to south of 21jN. Evaporation in the hyper-

arid core of the Western Desert is estimated at between

2500 and 5000 mm (Shahin, 1985). Gasse (2002)

gives an excellent summary of North African clima-

tology with references.

Modern wind patterns in the Western Desert com-

prise two fields. A northern zone of variable winds

extends south from the Mediterranean coast, more

westerly and stronger in winter (November to April),

bringing rain in cyclonic storms to about 25jN. Insummer (May to October), weaker westerly winds

reach only to about 30jN and yield no rainfall. Over

a more extensive southern zone of the Western Desert

between 30jN and 20jN, northerly winds dominate

and bring no rainfall at either season. They strengthen

in winter, often meeting westerly winds along rain-

bearing fronts which can affect any part of the desert at

this season. Southward, these northerly winds veer

northeasterly around the eastern limb of the subtropical

anticyclone.

3. Methods

The study area is covered by a set of 24 LANDSAT-

MSS images acquired from 1972 to 1976. These are

false-colour composites of bands 4, 5, and 7, processed

at 1:250,000 scale by Earth Satellite Corporation

(ESC) and held at the National Air and Space Museum

(both in Washington, DC). Images were enhanced

using GEOPIC, developed at ESC, a procedure which

emphasized subtle tonal variations of surface materials

and which revealed linear features, such as roads and

dunes, below the 79-m pixel resolution (El-Baz,

Centre for Remote Sensing, Boston University, written

communication, 2001). Some sense of resolution of

the images can be gained from the visibility of roads

roughly 20 m wide (including bordering disturbed

ground), and from visible barchans of comparable

width that were visited in the field.

Aeolian erosional landforms mapped from these

images and recorded in the field for directional infor-

mation are discussed in the following.

Aeolian erosional lineations (AELs): These are

parallel, unstreamlined, ridge-trough sets, as well as

fields of streamlined forms (yardangs). Those AELs

capable of resolution on the LANDSAT images are all

in bedrock, whereas smaller ones in unconsolidated

sediments were recorded in the field. Because of the

I.A. Brookes / Geomorphology 56 (2003) 155–166156

Fig. 1. Egypt showing major physiographic features—plateaus, scarps (toothed lines), oasis depressions, dune 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.

I.A. Brookes / Geomorphology 56 (2003) 155–166 157

time required to form these features in bedrock, bed-

rock AELs are interpreted as of mainly pre-Holocene

age (Brookes, 2001). Within sets of bedrock AELs,

however, features such as faceting of upwind faces can

reflect more recent wind erosion. Also, bedrock AELs

are sometimes parallel to lineations in unconsolidated

Holocene sediments and can then be argued to reflect

at least some Holocene wind erosion.

Aeolian scour zones: These are swaths of bedrock

or surficial deposits, < 10 to hundreds of kilometres

long and < 1–5 km wide, where varnish and other

patina has been erased or prevented from forming, and

which therefore appear on the images lighter in colour

than adjacent terrain, with sharp boundaries against it.

In the lee of cliffs, scour zones reflect wind accelerated

through ravines, but, where not topographically local-

ized, they reflect regional wind structure. These scour

zones, too, probably record a longer period of wind

erosion than the last 10 kyr; but their parallelism with

mobile features such as small dunes, and with yard-

angs in Holocene sediments, indicates that, in part,

they reflect Holocene wind directions. If Holocene

wind directions were different, this would be reflected

in these scour zones, which are probably quick to

respond to such changes.

Depositional aeolian landforms mapped from the

images for directional information are the following.

Transverse dunes: These are sets of 5–20 sinuous,

subparallel sand ridges, each V 10 km long, spaced at

f 500–700 m. They are oriented perpendicular to

formative winds.

Linear dunes: These occur both in fields and iso-

lated, as a result of differences in sand supply. In the

Great Sand Sea of western Egypt, where they occur as

fields, they comprise a stable, basal ‘plinth’, f 3 km

wide (therefore more correctly a ‘draa’), topped by a

narrow, sharp-crested, sinuous, mobile dune, the

whole 10–50 km long, and spaced at 2–5 km. In

central and eastern Egypt, on the other hand, where

sand supply is more limited, most linear dunes are

simple, shorter, forms not built on a ‘plinth’. The

relationship of linear dune orientation to wind direc-

tions is complicated, but in central and eastern Egypt,

simple linear dunes are parallel to prevailing northerly

winds. In western Egypt, the construction of com-

pound linear dunes may have been influenced by

northerly and by westerly/northwesterly winds, but

in this area, wind directional information has been

taken either from simple linear dunes, or from com-

pound linear dunes only where they are parallel to

nearby simple ones, which therefore allow only one

wind direction to be inferred.

In western Egypt, where geomorphic features indi-

cate two palaeowind directions (W/NW and N/NE),

linear and transverse dunes are combined in fields

where transverse dunes perpendicular to the W/NW

palaeowinds occur as ‘barbs’ attached to the flanks of

linear dune ‘shafts’ lying parallel to N/NE palaeowinds

(Sections 4.1.2–4.1.4 below). On the other hand, in

the central and eastern parts of the Western Desert,

where only one palaeowind direction is indicated, this

compound dune form is absent and the two types,

transverse and linear, are less common and occur

separately.

As with AELs, dunes (possibly) and ‘draa’ (defi-

nitely) represent a longer geomorphic history than the

Holocene (Embabi, 1998). However, where smaller,

more mobile, linear dunes and the mobile superstruc-

tures of larger forms are parallel to other mobile

directional indicators, such as sand drifts (see below)

and yardangs in Holocene sediments, or are perpen-

dicular to transverse dunes, they can again be related to

Holocene winds.

Sand drifts: These appear on the images as multi-

plumed, diffusely bounded, thin spreads of light-col-

oured sand over darker terrain, resembling ‘mares

tails’ in cirrus clouds. Being diffuse and difficult to

outline, for mapping purposes, their medial long axes

were chosen to provide directional information. These

axes, tens of kilometres long, usually occur in ‘fields’,

numerous and adjacent, spaced at 5–10 km intervals,

and are restricted over the image coverage to western

Egypt (again, largely because of sand supply from the

Great Sand Sea).

Cross-cutting relationships between landform sets

are readily apparent on the images, so that the restric-

tion of the two sets of palaeowind directions to western

Egypt is immediately apparent. From the interpreted

geomorphic maps of each of the 24 LANDSAT

images, directional information was first generalized

at image scale over each map quadrant. Generalization

was not interpretive, merely simplifying by reducing

the number of flow lines. The generalized pattern was

transferred to a small-scale map (4 cells� 24 maps),

then smoothed visually into the regional pattern shown

in Fig. 3.


4. Results

4.1. Interpretation of sample areas

Sample geomorphic maps showing directional aeo-

lian landforms are shown in Fig. 2a–e. Plateaus,

scarps, mesas, valleys, and deflation basins have been

added to indicate terrain influences on wind.

4.1.1. Bahariya Oasis

The area shown in Fig. 2a straddles the scarped

eastern edge of the Bahariya Oasis depression (Fig. 1).

Two sets of AELs are present, one oriented at f 110–

130j, and another at 165–180j. A dune complex 5–8

km wide, oriented at 165j (Ghard Abu Muhariq),

contains simple linear dunes and transverse dunes

oriented perpendicularly to them. Linear dunes occur

east and west of this complex at f 165j.

4.1.2. Farafra Oasis depression north

The area shown in Fig. 2b contains Farafra Oasis

(stippled) at the western edge of a depression between

limestone scarps. AELs occur in two sets: over a

narrow western plateau, a strong set at 110j; and over

the northern plateau and cuestas, a more variable but

still consistent set at 160–190j. Aeolian scour zones

occur below the western plateau, oriented parallel to a

set of AELs at 110j. Transverse dunes occur as small

and large fields oriented at f 180–190j and perpen-

dicular to the AELs and scour zones at 110j. Trans-verse dunes also occur as ‘barbs’ on the western flanks

of linear dunes.

4.1.3. Farafra Oasis depression south

The area shown in Fig. 2c lies at the southern end of

the depression containing Farafra Oasis at its northern

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-

verse dune ‘barbs’ oriented at f 200j. In the bottom

centre of the area, a ‘feathery’ dune association con-

tains transverse dunes oriented perpendicular to 165j,which curve to become short linear dunes parallel to

165j.

4.1.4. Abu Ballas west

The area shown in Fig. 2d is crossed by two sand-

stone scarps trending generally E–W. AELs occur

in two directions, at f 110j and 170j. Linear dunesoccur parallel to 170j, whereas scour zones are

at f 135j. Transverse dunes are perpendicular to

f 135j, in separate fields and as ‘barbs’ on linear

dune ‘shafts’.

4.1.5. Gebel Uweinat east

The area shown in Fig. 2e lies just south of the

Egypt/Sudan border, and contains a low rocky ‘ham-

ada’ scored by many AELs oriented at f 250j and a

few at f 160j. A ‘scour shadow’ is oriented at 250j,as are several linear dune crests and a small dune

complex entering from the northern edge.

4.2. Regional palaeowind fields

The regional pattern of winds reconstructed from

geomorphic indicators lies south of the modern west-

erly wind belt. It is dominated by N–S winds, with a

pronounced turn toward SW in SWEgypt (solid arrows

in Fig. 3). This is the dominant annual pattern today,

driven by return of air subsided in the subtropical high-

pressure cell toward the equatorial low. Geomorphic

evidence (discussed below) indicates that this pattern

was initiated in the late Holocene. Also revealed is a W

and NW flow over western Egypt, more westerly

between 30jN and 26jN, veering to NW between

26jN and 20jN (dashed arrows in Fig. 3).

Also noteworthy is the fact that in the north of the

area, as far east as 30jE, several indicators of older

westerly flows are disjunct to younger N–S stream-

lines. Lastly, a few short flowlines oriented NE–SW in

the centre and north of the area (dotted arrows in Fig.

3) are topographically influenced by promontories and

embayments in the high scarp dividing the Western

Desert and thus do not enter the regional interpretation.

5. Interpretation

5.1. Wind fields

The reconstructed regional airflow pattern over the

north and central Western Desert is dominated by N

winds, and in the south by NE winds. A second wind

field, however, emerges—W over NW Egypt, veering

to NW over SW Egypt. The N/NE pattern is similar to

the present-day circulation. As argued in the following


section, this pattern ‘switched on’ in the last few

millennia, but has also dominated much of the last

2.5 myr (Brookes, 2001). The W/NW pattern over

western Egypt is argued below to represent early

Holocene ‘palaeowesterlies’.

5.2. Chronology

Over western Egypt, aeolian landforms indicative

of N/NE flow cross-cut those indicating W/NW flow,

and therefore formed earlier. Examples are noted in the

captions of Fig. 2 and in Sections 4.1.1–4.1.5, while

the overall pattern is shown in Fig. 3. In the Farafra

Oasis depression (Fig. 1), Donner et al. (1999)

reported cross-cut, wind-abraded surfaces on lime-

stone bedrock that record the same sequence of air-

flows (W/NW then N/NE). These two wind directions

are well shown in Fig. 2b (described in Section 4.1.2).

Significantly for the argument developed here, these

authors also reported yardangs oriented W–E formed

in playa sediments with radiocarbon dates of 9–6 14C

kyr BP, indicating that the W/NW pattern at least partly

postdates the sediments. These authors concluded that

‘‘. . .observations. . . show a pattern with an older


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 LANDSAT and 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).


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 Wand 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.


effective wind direction from the west,. . . and with a

younger wind direction similar to that of the present

surface winds in the summer [north]. . .’’ (p. 82,

brackets added). This field evidence conforms with

widespread geomorphic evidence from the LANDSAT

images for the same sequence of wind directions over

much of western Egypt (Fig. 3). This local and

regional evidence further indicates a sequence of air-

flows, negating a seasonal explanation.

In Dakhla and Kharga oases (Fig. 1), yardangs of

comparable height to those at Farafra (f 4 m) have

formed in unconsolidated sediments accreted between

the second and seventh centuries C.E. (Brookes,

1989b). Relief generation in these cases is therefore

closely constrained to a minimum of z 2.2–3.0 m

kyr� 1. The W–E yardangs of Farafra could therefore

have formed in under 2 kyr, following 6 kyr BP. It

follows that the N/NE wind regime was established

since 4 kyr BP. Interestingly, therefore, Stokes et al.

(1998) reported OSL ages of 3–4 kyr for the surface

sand sheet in southernmost Egypt (Fig. 1). In that area,

surface aeolian features are aligned N–S (Brookes,

1999), so it can safely be concluded that N/NE winds

were established 3–4 kyr ago. Accounting for a lag

behind falling water table (Pachur and Hoelzmann,

2000), these winds and aridification probably set in

1–2 kyr earlier.

Studies of basins containing early Holocene sedi-

ments, from northern Sudan to northern Egypt, indicate

that palaeolakes existed between approximately 9 and

4.5 kyr BP. ‘‘Lake’’ in this context signifies either (i) a

perennial water body, as at Oyo and Selima in Sudan

(Ritchie et al., 1985; Haynes et al., 1989) and Faiyum in

Egypt (Hassan, 1986) (Fig. 1), at the humid southern

and northern extremities, respectively, of the Western

Desert, or (ii) an intermittent/ephemeral water body

(playa) (e.g., Brookes, 1989a; Hassan et al., 2001),

responsive to individual storms or stormy intervals,

which is the predominant basin type in the present

hyperarid core. The end of the ‘lacustrine’ interval

therefore appears to coincide with the onset of N–S

winds over the Western Desert.

5.3. Moisture source(s)

A European/Mediterranean source for early and

mid-Holocene precipitation over NE Africa was in-

ferred from stable isotopes in shallow groundwater

(Sonntag et al., 1979) and in organic carbonates

(land-snail shell, Goodfriend, 1991; archaeological

ostrich eggshell, Sonninen, in Donner et al., 1999).

Goodfriend inferred a NW trajectory for these rain-

bearing winds, coincident with the orientation of the

older set of aeolian landforms mapped here.

Early Holocene lakes and playas in NE Africa could

not all have been fed by the same air masses, either

solely from the winter temperate westerlies or from the

summer tropical monsoon. From geobotanical evi-

dence at NE African localities, Haynes (1987) inferred

a steep drop in early Holocene tropical monsoonal

rainfall spanning 21–20jN (vs. 15–14jN today).

Northward at f 24jN, early Holocene rainfall steeplyincreased again, reflecting southward intrusion of mid-

latitude rainfall. Between these rainfall clines, early

Holocene moisture was sufficient to eliminate hyper-

aridty, replacing it with mere aridity or semi-aridity.

This picture conforms with evidence from radio-

carbon-dated archaeological charcoal at Neolithic sites

on a N–S transect across the eastern Sahara (Neu-

mann, 1989). This showed that during the early

Holocene moisture maximum species of trees and

woody shrubs similar to modern ones were more

widespread, probably within a desert steppe, eliminat-

ing the barren desert core. It also showed that the treed

savanna receded southward in response to aridifica-

tion, reaching its present northern limit at 3.3 kyr BP

(3.5–3.6 cal kyr BP, Klein et al., 1982). This age

bisects the range of OSL ages given for the surface of

the Selima Sand Sheet by Stokes et al. (1998). A

probable lag behind falling water table should again

be noted here.

In addition to the conclusions of Goodfriend

(1991), Sultan et al. (1997), from 2H depletion in

fossil Saharan groundwaters and 18O depletion in

freshwater tufas, identified a ‘palaeowesterly’ trajec-

tory for winds across the NE Sahara, north of 24.5jN,as well as an Atlantic monsoonal one for the southern

and central Sahara. Although these groundwaters and

tufas are of several ages, and much older than Hol-

ocene, the similarity of the reconstructed trajectories to

wind directions mapped in this paper supports the

interpretation that westerly winds reconstructed here,

north of monsoonal influence, reflect a ‘palaeowes-

terly’ flow.

Thus, while a mid-latitude source for early Holo-

cene rainfall in Egypt south to 26jN is identified


confidently, the earlier inference herein that the NW

flows in southwest Egypt were also of mid-latitude

origin requires substantiation. The geomorphic indica-

tors of this flow, particularly the erosional ones, would

not have been expected to have formed in the rainy

winter season, when mid-latitude winds would be

drawn equatorward. The landforms more likely were

produced in the dry summer season, so that winds must

have been from the NW over SW Egypt (20–26jN)immediately north of the early Holocene Intertropical

Convergence.

Some authors have called for glacial-age westerlies

(20–10 kyr BP) to extend southward over the northern

Sahara (e.g., Nicholson and Flohn, 1980). A cold and

wet ‘last glacial’ in NW Africa (Maghreb) has been

explained thus by Rognon (1987), but evidence from

farther east is scarce, even contrary (Fontes and Gasse,

1991). The climate simulations of Kutzbach et al.

(1993) do not predict glacial-age penetration of west-

erly rain-bearing winds into NE Africa. Moreover,

within the Selima Sand Sheet of southern Egypt

(22–23jN), Stokes et al. (1998) reported OSL ages

between 15 and 20 kyr for a subsurface sand sheet

inferred to have formed under an arid/hyperarid cli-

mate. The age range implies that rain-bearing west-

erlies did not reach the northern tropic during the last

glacial (however, see Cooke et al., 1993, p. 395 and

Lancaster, 1995, pp. 165–166 on sand sheet genesis

and climate).

These arguments against ‘glacial’ westerlies, how-

ever, refer to elevations near sea level. Referring to

altitudes above 1000 m, Rognon (in Messerli and

Winiger, 1980), Street and Gasse (1981), and Maley

(2000) argued in favour of glacial-age runoff from

highlands in the North African desert belt, from

Hoggar (10jE) to the Red Sea Hills (f 35jE). Maley

(2000) reported two periods of highland humidity

(20–15 and 15–12 kyr BP). Gasse (2002, p. 758),

however, referring to the area west of 15jE, interpretsevidence of wetter climate to mean that ‘‘the monsoon

reactivation occurred in two steps, at 14.5 [cal] ka

[12.3 14C] and 11.5–11 [cal] ka [9.9–9.6 14C] sepa-

rated by a return to drier conditions related to the

Younger Dryas (YD) cold spell defined in higher

northern latitudes ca. 12.6–11.6 [cal] ka [10.7–9.814C]’’ (conversions in brackets added). From this, it

appears that at least the later of the two intervals

recognized by Maley (2000) refers to monsoonal rain-

fall, not intrusion/interception of mid-latitude air

masses. As for the 15–20 kyr BP wet interval recog-

nized by Maley (2000), no ‘Last Glacial Maximum’

ages have been reported from the plains fed by drain-

age from these highlands. Moreover, the similar ages

for a subsurface unit of the Selima Sand Sheet reported

by Stokes et al. (1998) point to lowland aridity (at

least) in this interval.

Whereas the argument presented herein has so far

pointed to an early Holocene age for the ‘palaeowes-

terlies’ indicated by geomorphic evidence in western

Egypt, recognition farther west of a drier end-Pleisto-

cene interval coeval with the Younger Dryas of Europe

(Gasse, 2002) raises the possibility that these winds

may have prevailed during that interval. Insufficient

evidence is available here to decide this question, but it

must be kept in mind that in western Egypt, ‘palae-

owesterlies’ eroded yardangs in playa sediments as

young as 6 kyr BP. Therefore, regardless of whether

these winds existed during the ‘Younger Dryas’-equiv-

alent interval in the eastern Sahara, they certainly

existed between 6 and 4 kyr BP, before the northerly

wind regime ‘switched on’.

5.4. Longitudinal extent of palaeowesterlies

As previously noted, Fig. 3 shows that over the

north-central part of the Western Desert, winds recon-

structed from the W/NW are disjunct to N–S stream-

lines. The pattern can perhaps be explained as the

effect of erasure by N–S winds of much the evidence

of ‘palaeowesterlies’ which previously extended far-

ther east. The longitudinal gradient of stable isotopes

in Holocene precipitation across NE Africa indicates

penetration of ‘palaeowesterlies’ as far as 30jE (Sonn-

tag et al., 1979), which is as far as the isotopic

evidence extends. Moreover, radiocarbon-dated cave

deposits at Gebel Umm Hamad, in the Red Sea Hills at

26jN, 34jE, inland of Quseir on the Red Sea coast,

indicate a wetter climate at f 8 and 6.6 kyr BP than

during the late glacial and the later Holocene (Moyer-

sons et al., 1999). Because early Holocene monsoon

rains, even from the Indian Ocean, likely did not reach

26jN, ‘palaeowesterlies’ are a plausible moisture

source. Of those mapped in this study, the closest

geomorphic indicators of ‘palaeowesterly’ flow to

Gebel Umm Hamad are near Kharga Oasis, at 25jN,30jE, 380 km to the west.


6. Conclusion

Geomorphic mapping of Egypt’s Western Desert

(20–30jN, 22–32jE) from LANDSAT-MSS images

reveals two consistent orientations of aeolian land-

forms, which indicate Holocene wind directions, one

from the W and NW and another from N and NE. The

W/NW set are in evidence mostly over western Egypt:

from the W over the northern part and from the NW

over the southern part. Fragmentary evidence in north-

central Egypt extends this westerly wind domain

farther east to 30jE. The N/NE set is more extensive

over central and eastern Egypt, veering from N over

the north and central parts to NE in the SW.

In western Egypt, the reconstructed W/NW flows

veer smoothly to merge with N/NE flowlines, but

cross-cutting landforms provide definite evidence that

the former are the older set. In the west, W/NW winds

formed yardangs in playa sediments dated as young as

6 kyr BP. They are related to mid-latitude palaeowes-

terlies penetrating the Sahara in the early to mid-

Holocene. In the south, their source and timing is less

clear, but, because evidence indicates that they do not

represent southerly extension of glacial-age westerlies,

they are argued to represent early to mid-Holocene

winds, veered to NW and penetrating farther south

than previously recognized. Additionally, some evi-

dence indicates that early Holocene ‘palaeowesterlies’

penetrated as far east as 30jE, perhaps even reaching

the Red Sea, and that their trace over upwind areas has

since been almost erased by younger N–S flows.

The N/NE flow, because it is younger than the W/

NW flow, marks later Holocene ( < 5 kyr BP) estab-

lishment of the modern circulation over the eastern

Sahara. This agrees with 3–4 kyr OSL ages of the

northerly derived surface sand sheet in southern Egypt

and with the establishment of the present savanna

boundary at 3.5–3.6 cal kyr BP, both of which may

have lagged falling water table by 1–2 kyr.

Acknowledgements

Thanks to T.A. Maxwell, Center for Earth and

Planetary Studies, U.S. National Air and Space

Museum, for making the LANDSAT images available;

to C.V. Haynes, Jr., University of Arizona, for an

informal review; to C. King of the Cartography Unit at

York University for the figures; to R.A. Marston for

editorial work; and to two journal reviewers.

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