Geomorphological Evidence and Pleistocene Refugia in Africa
Author(s): Janet E. Nichol
Reviewed work(s):
Source: The Geographical Journal, Vol. 165, No. 1 (Mar., 1999), pp. 79-89
Published by: Blackwell Publishing on behalf of The Royal Geographical Society (with the Institute of
British Geographers)
Stable URL: http://www.jstor.org/stable/3060512 .
Accessed: 11/11/2011 03:23
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .
http://www.jstor.org/page/info/about/policies/terms.jsp
JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of
content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms
of scholarship. For more information about JSTOR, please contact [email protected].
Blackwell Publishing and The Royal Geographical Society (with the Institute of British Geographers) are
collaborating with JSTOR to digitize, preserve and extend access to The Geographical Journal.
http://www.jstor.org
7heGeographicalXounlal,
Vol. 165, No. 1, March 1999, pp. 7>89
Geomorphological evidence
P
renlgla
*
*
ln
and
*
P
aIrlca
Pleistocene
P
JANET E. NICHOL
Department
of Geography,
UniversitCollegeCork,Ireland
E-mail:jn@,ucc.ie
Ehispaperwas acceptedfor
publicationinune 1998
New geomorphologicalevidence from satellite images is interpretedin the light of
recentworkon Quaternarydesertadvancein West and CentralAfricato suggestthat
regionscurrentlysupportingtropicallowlandrain forestwere duringthe Quaternary
period, coveredby windblownsand in the form of linear desertdunes. The evidence
suggeststhat the last glacial advance at approximately18kyrBP was not the most
extensiveand that the Sahara,like the Kalahari,extendedto withinone or two degrees
of the Equatorduringthe Quaternaryperiod.Thus the southernlimit of fossildunesat
the WestAfricancoastis not a climaticlimit. Since desertdunesindicatethe minimum
formerextentof desertconditions,the findingshave implicationsfor Pleistocenerefugia
in Africa and may explain both the relativepaucity and high degree of endemism
observedamongAfricanbiota.
KEY
WORDS:
CentralAfrica,WestAfrica,Quaternary,
fossildunes,rainforest,refugia.
THE
SUCCESS OF THE PLEISTOCENE (1971) in Australia,and Lancaster(1981) in South
REFUGIAtheory (Haffer,1969) is based on Africa.
its ability to explain the unparalleledbiotic
Such estimatesof desert advance are consistent
diversityof the Neotropicalrain forests.Its applica- with the Pleistocene refugia hypothesis,since the
tion to Africamust contend with firstly,a relatively retreatof otherclimaticand vegetationzones accomimpoverished biota (Richards, 1952; Hamilton, panying desert advancewould confine any remain1976;Livingstone,1982)and, secondly,a paucityof ing lowland forest to fragmentsin a sea of open
plant fossil and other evidence of climatic change savanna.The absence of large mountainranges in
(Hamilton,1976; Sowunmi,1986).Thus it has been West Africa means that this retreat would be less
difficultto locate refugia(core areas)where humid complicatedby altitudinaleXectsthan in EastAfrica.
conditionswere maintainedduring cool dry glacial Thus, at the last glacialmaximum(LGM)at approxiphases. In this case geomorphologicalevidence can mately 18kyrBP (whichis thought to be the most
provide an importantcorrelationbetween available intense and extensive of Pleistocene cold/arid
data on the magnitudeof climaticchange.Estimates phases, Sarnthein, 1978), with desert postulatedas
of the spatial extent of arid conditions during extending as far south as 12-14°N (Grove, 1958;
Quaternaryglacialshave concentratedon presently Dupontand Hooghiemstra,1989),Maley(1991)possemi-aridregions owing to the greater visibilityof tulatedlowland rainforestrefugiain the uplandsof
fossil landformsin sparsely-vegetatedareas and to SierraLeone/Liberiain WestAfrica,Cameroonand
the greateravailabilityof cloud-freeremotelysensed EasternZaire.Hamilton(1976)had earliersuggested
images.Thus in WestAfrica,Grove(1958)has noted similar locations, though not confined to uplands,
that during Pleistoceneglacial phases, active sand and with an additionalsite in West Africain southdunes covered large areas of presently-cultivated ern IvoryCoast/Ghana(Fig. 1). The transitionfrom
landson desertmargins,extendingas far southas the desert to forest which today takes place over 12°
presentday 800-millimetrerainfallisohyetat approx- latitudewould then have been compressedto 7° latiimately 12°N and at a distance of 500 kilometres tude. This paper drawstogetherexistingknowledge
equatorwardsof the present desert. Dune building of formeraridityin the humid tropicsin the light of
today approximatesthe 100 mm mean annualrain- recent (Nichol, 1998) and newly-reportedobservafall isohyet.These observationsof the magnitudeof tions including satellite data of West and Central
climaticchange in semi-aridregions are confirmed Africa,and proposesa reinterpretation
of Pleistocene
by the workof Goudieet al. (1973)in India,Mabbutt refugiatheoriesfor Africa.
00 16-7398/99/000 1-0079J$00.20J0
(C)1999 The Royal GeographicalSociety
L,Chad
LI
-
Postulated Pleistocene refugia in West Africa(after Hamilton,1976)
"
refugiafor LGM
"
(after Maley, 1991 )
300 Km
l
-
-
5W
l
O
Fig. 1. Refugia,
present-day
isohyets
andthepresentandpastknownextentof desertin WestandCentral
Africa
PLEISTC3GENERENUGIAIN AFRIGA
81
humidforest
zones
eridencefirom
Palaeogeomorphological
Althoughexistingliteratureon the geomotphologyof
the humid forest regions of West Africa refers to
landformswhich are assumedto requirearid conditions for their formation;namely, inselbergs(Smyth
and Montgomery,1962; Thomas, 1962; Burkeand
Durotoye, 1971), layersof concretionarylaterite(de
Swardt, 1964; Thotp, 1970), gravel horizons and
stonelines(Bruckner,1956;Cooke, 1970)the degree
and timingof aridityis unspecified.The high stream
density and intense dissection observed today in
many areas is also untypical under forest cover
(Tricart,1974).A formersemi-arid,seasonalclimate
with open woodlandand grasslandthroughoutmost
of today's forest zone in West Africa has been
assumed for glacial phases (Burke et al., 1971;
Thomas and Thotp, 1992)thoughlittleprogresshas
been made in correlatingsuch geomotphic-based
assumptionswith other types of data. However, in
CentralAfrica,recentworkby Runge (1996)at 1°S
in easternZaire has relatedgeomotphicevidenceof
aridconditionsaroundthe LGMwith increasedsedimentationof the riverZaire Jansen et al., 1984)and
grasspollen found in MatupiCave, Mt Hori, Zaire
(Fig. 1) dated to the same period(Brooket al., 1990).
Thus, at least for the LGM, data for both West and
CentralAfricaappearto confirmthe existenceof a
semi-aridclimate with open savannavegetationin
presentlyforestedareas. Furthermore,Runge'sgeomotphic observationssuggest considerabledrying
out of the putativecore refugearea in easternZaire,
to the extent that forest would not have been supportedat datescorrespondingto the LGM.
Additionally,some referencesto extreme aridity
have been made for humid tropicalareas in Africa
and the Neotropics.Tricart(1974)refersto an extensive NE-SW trending dune system of Upper
Pleistoceneage, between 5 and 6°N in the Orinoco
lowlands of Venezuela. On the Congo Plateau in
Zaire at 3° S, de Ploey (1965)correlatesdune sands
and fossilblowoutswith radio-carbondata and artefacts, indicatingan arid period between 50000 and
30 000 BP, when the Kalahari encroached northwards.The particlesizes cited, 80 per cent between
150 and 300 plm,typifydune sandswhich accumulate by saltationratherthan by fluvialactivityor in
suspensionas loess. Moreau (1963) statesthat these
could have been laid downwhen a steppe-typevegetationprevailedand the Congo forestwas reducedto
a strip along the coast north of the mouth of the
Congo river(1-5°S).Livingstone(1982)mentionsde
dunes as extendingnorthPloey's'Kalahari-derived'
wardsacrossthe Equatorinto the Zaireancuvette(03°N). The assumedderivationfromKalaharisandsis
reasonablesince the Kalahari,not the Sahara,was
the nearest known continuousarea of fossil dunes
(Sarnthein 1978; Lancaster, 1981). However, the
new observationsfrom AVHRk images (Figs2 and
3a) indicate that these dunes extendingnorthwards
acrossthe Equatorare longitudinaldunes, oriented
ENE-WSW, identical to the 'ancient erg' of the
Saharaat 14°N and alignedwith the northernhemispheredry seasonwinds.Suchlongitudinaldunesare
usuallyformedwhere winds from one major direction predominate,such as those outblowingfrom
anticyclone cells, and over large unobstructed
expansesof sand (Tsoar, 1989). Thus, their formation in the presence of steppe-typevegetation as
suggestedby Moreauis unlikely.Furthernew observations show similarlyorientedfossil dune features
fartherwest at the same latitude(1-4°N)betweenthe
Sanagaand Rio Benitoriversin southernCameroon
and Gabon (Figs 2 and 3b). Even furtherwest, at
5°30'N near Abidjan,Ivory Coast, a sirnilardune
pattern appears (Fig. 3c). Recently, Nichol (1998)
using geomotphologicalcriteria, has related fossil
dune landformsat 7°N in southernNigeriawith the
knownfossildune systemsof the semi-aridzone further north (Grove, 1958; Nichol, 1991).It is, therefore, suggestedthat the Sahara,like the Kalahari(de
Ploey, 1965)has at some time duringthe Quaternary
period,extendedwell into the presentlyhumidtropical zone and moreoverthat the southernlimit of fossil dune forms at the West African coast is not a
.
cllmatlc
.
.
.
llmlt.
of duneformsbehreensemi-aridandhumidareas
Correlation
in WestA0iza
In the semi-aridclimateof north-eastNigeriawhere
mean annual rainfall is approximately600 millimetres,dune buildingduringa late-Pleistocenearid
phase correspondingto the LGM (Sarnthein,1978;
Rognon, 1987) took place near the sourcearea, the
Chad Basin.These dunes,firstrecognizedby Grove
(1958)as the 'AncientErg of Hausaland'are visible
today as linear featureswith distinctrelief and are
found on the Chad Basin sedimentaryrocks to the
north and east of Kano. The dunes have been preserved as elevated red sandy ridges oriented
ENE-WSW and one to two kilometresapart, with
streams and wet depressionsoccupying the interdunes.Nichol (1991),usingsatelliteimages,extended
observationsof fossil dune landformsto the more
humid climate of the southern Sudan zone (mean
annualrainfall800-1000 mm) south-westof Kano at
12°N where dunes are not visiblein the field or on
air photos.Here7althoughthe topographyof the former duneshas not been preserved,some remnantsof
the dune soils and drainagepattern remain.Thus,
red sandysoils alternatewith grey gleys corresponding to formerdune and interdunerespectively,and
lower orderstreams,at a distanceof one to two kilometres apart, are still oriented ENE-WSW, in the
same directionas the formerdune lineations,though
owing to landscape inversion, they are currently
being displaced to occupy the (now lower lying)
+*Nismey
L.Chad
II
Approximateextent of recent observations
I of
fossil dune locations (Nichol,1998)
\
New satelliteobservations(thisstudy).Areas
correspond
to Figures3a,b,c and 4
Fig.
2. Recentdatashowing
theapproximak
Ouakrnayexkntof desert
dSssnbed
inJfchol(1998), arzdthenewgeomorphic
databasedonJfOAAA VHRRsakllikim
REFUGS IN CA
PLEISTOCENE
former dunes. These fossil dune forms noted by
Nichol are as yet undated.They are visibleon satellite images across the semi-aridnorthernregion of
WestAfricain the formof ENE-WSWorientedstriations on NOAA AVHRR imagesand soil colourdifferenceson LANDSAT images (Nichol, 1991). The
same striationsare visible on dry season AVHRR
imagescontinuinginto the humidforestzone of West
Africaat 7°N aroundOwo and Ibadanin southern
Nigeria (Nichol, 1998) (Fig. 2). Here, mean annual
rainfallis 1500-2000 rnillimetresand no evidenceof
the originaldune topographyor dune soils remains.
The featuresare interpretedas the remnantsof a former dune landscape,fossilizedby concretionarylateritesformedduringalternatingQuaternaryaridand
humid phases. Landscapeinversionhas taken place
and the lower orderstreamsnow occupythe former
dunes, though still flow in an ENE-WSW direction
and are spaced between one and two ldlometres
apart.The ENE-WSWtrendingdrainagepatternon
the BasementComplexin southernNigeriahas previouslybeen attributedto bedrockjointing(Smythand
Montgomery,1962;Burkeand Durotoye,1971;Jeje,
1974). However, closer examination from topographicmaps and field observationsusuallyreveals
numerousstream capturesof the ENE-WSW oriented streamsby largerriverstrendingN-S according to the regionalslope (Fig.4), thus indicatingthe
presentobsolescenceof the originalfactorcontrolling
the drainagepattern.The featuresalso extend across
a varietyof bedrocktypes.
In spite of the recognizedsimilarityin direction
and spacingto dunesfurthernorthwhich have been
datedto 40kyrBP and 20kyrBP (Albertet al., 1997;
Stokes and Horrocks,1998) the high relativerelief
e.g. over 100 metres, of ENE-WSW aligned ridges
in the Owo-Akurearea (7°N)(Nichol,1998)makesit
likelythat the fossil dune formsin southernNigeria
belong to an early arid advance.A weatheringrate
of 100 metresin 100000 to 1 millionyears is often
cited, and since in Nigeriarockmay alreadybe partially decomposed to 6>90 metres below ground
(Thomas,1962;Burkeand Durotoye,1971)lowering
of the landscapeby the strippingaway of already
weatheredmaterialmay be at the shorterend of this
time scale. Thus an arid advance of around
12F90kyr BP, 190- 155kyrBP (VanceHaynesJr. et
al., 1997), or 250kyrBP (Dupont, 1993) is possible.
The lineationsare especially,but not exclusively,visible downwindof and alignedwith largerivervalleys
such as the lower Benue where a large fossil dunefield can be observed(Fig. 3d). They extend 1000
kilometressouthwardsof the previousknownextent
of Quaternarydunes (Fig. 1) and include the headwaters of the Logone and Sanaga river at 7°N in
Cameroon,southernGhanaaroundLakeVolta (Figs
2 and 4) and the lowlands of Ivory Coast at 6°N
(Figs2 and 3c). The featuresare also observableon
83
topographicmaps where they representrepetitive
patternsof dune-aligneddrainagenetworksspaced
one to two kilometresapart, as well as distinctively
shapedwaterbodiese.g. the shape of Ghana'sLake
Volta is influencedby the ENE-WSW trend of its
drownedtributaries,as are smallerlakessuch as Oba
Lake, Universityof Ibadan, Nigeria (Fig. 5). This
trend correspondswith the present dominantwind
directionfor the Novemberto Februarydry season
at this latitude and also aiigns approximatelywith
the ITCZ at this time of year (Fig. 2). On satellite
imagesthe lineartonal differencesrepresentdifferential vegetationand land cover statusbetweenformer
dune and interduneareas. For example, lineations
extending downwindof Lake Volta (Fig. 4) correspond to vegetated areas gight) and non-vegetated
areas (dark),the latter representingcleared agricultural land i.e. bare soil or senescentvegetationon
this dry seasonimage. These observationsdemanda
revisedapproachto models of Quaternaryclimatic
change in Africa and of the fate of forest during
glacialmaxima.
of climaticchange
Otherevidence
Owing to the scarcityof suitablesitesfor fossilization
there is little plant evidence of climatic change in
humid West Africa. However, pollen data at Lake
Bosumtwi in lowland Ghana at 7°N (Talbot and
Delibrias,1977)indicatethat duringthe LGM,vegetationstructurallyresembledthat of today'sSahelian
grassland,with the complete disappearanceof rain
forest.Maley(1991),citingdatafroma pollencore at
Lake BarombiMbo, at 300 metres altitudein the
Cameroonmountainsmaintainsthat forestsurvived
duringthe LGM, though the remainingtree pollen
was more representativeof montaneforesttypes.He
thus hypothesizesforest refugiaat that time on the
moist windwardedges of upland sites in West and
Central Africa (Fig. 1). This appearsincompatible
with synchronousdune buildingat the same latitude
but may suggestthe greaterantiquityof the observed
dune landforms.It also suggeststhat the last glacial
advance was not the most extensiveof Pleistocene
glacialphases. This conclusionis also borne out by
the geomorphologicalevidencealone,with regardto
the rate of landscapeloweringaroundlateriteresiduals. The similarityin directionand spacingobserved
on AVHRR images to fossil dunes in northern
Nigeria,dated at 20 and 40kyrBP is attributableto
the constancy of the west-south-westerlyblowing
trade winds for differing posiiions of the ITCZ
between the currentlyhypothesizedarid phase and
subsequentones. The more north-southorientation
of those observedin southernCameroon (Fig. 3b)
may be due to the re-orientationof the ITCZ to a
north-south direction down the Central African
coast (Fig. 2). During arid phases this would take a
positionthantoday.
more south-westerly
PLEISTOCENE
REFUGIAIN AFRICA
84
N
lOOKm
g
l
a)
N
L
I UU Km
b)
|
Fig.3. Forlegend
seepage85.
...1 | | @ | ! e fi .......
| |>;yst-4 jN
_l
>:
yA
.x
r«.S,<v;
.
t,i.
:l S gi SR
?e-..i.<-X
>-<8.
:i.4
.ki
:':j;:,,,.>..j^ ^A:
...............................................
X. |
S
|
4
;F342>x9
il
|L
g
Y M
Bgo
_1
g
er
,;
vx
A''
...............................
,2
E
<+
g
W:
{
'
a;
i
=s
#.
>
i,
.@
'vx-i5v
{
C%A
<Evyve
-8
Xt
Y;
{|evyo,
i
.
.,{
V
N
@
PLEISTOCENE
REFUGIAIN AFRICA
lOOKm
|
85
l
c)
;
4''N'g.'S.WS>'^i,Yt,',.---;
@0@;iXg
1_
_
ii t
F.
t
XXa- (( xij -0 i
_;1
___
WS
v a9.s X,9
__
W
7g
_
X-u'-Y<5k'e
Kj
>'
-7N
_SKsS
t
100 Km
1 8E
9E
ldE
d)
Fig.3. J%OAA
A VHRRimagesof 21-30Xanuay, 1993. Arrowsindicate
thedirection
of lineations:
a) Vegetation
indeximageof themiddleCongo
valley,taire; b) Vegetation
indeximageof theregion
southof theSanagaRiverin southern
Cameroon
andnorthern
Gbon, c) Vegetation
indeximageof
theAbidjanregion,
Ivoy Coast,andd) 7he7znal
infra-red
imageof theMiddleBenuevalley,Xgeriashowingalternating
wanner(light)andcooler(dark)
lineations
paralleltotheriverchannel
REFUGIAIN AFR1CA
PLEISTOCENE
86
j
j
[
EW!
P
P
Z
r
;
N
8N
6N
1
200
Km
I
vallegsas wellas
tributaty
imageof 21-30 jranuaty,1993 showingtheEJ%E WSWtrendofLakeVolta'sdrowned
A VHRRtharmal
Ftg.4. J%OAA
of lineations
thedirection
of thelake.Arrowsindicate
downwind
landscapefeatures
Recentworkby Dupont and Hooghiemstra(1989)
pollen
and Dupont(1993),studyingwind-transported
in marinecoresoffWestAfrica,and datedby oxygen
isotope stratigraphysuggeststhat while the maximum southwardextent of the Sahara over the last
0.7M years was approximately14°N, tropicalrain
forest was never completelyeliminatedfrom West
Africa,but contractedto small coastalfragmentsat
20, 160 and 250kyrBP respectively.In view of the
presently observed southward extension of the
Saharadesertto 6° latitudein West Africa,this may
benefitfromsomereconsideration.
refilgia
Pluistocene
Implicationsfor
Today's transition from desert to forest across the
intervening savanna zone occupies approximately 12
degrees of latitude in West Africa. Bearing in mind
that desert dunes indicate the miliimum former extent
of desert conditions and even allowing some degree
of compression of tropical climatic gradients during
Pleistocene arid phases (contrary to the opinion of
Tiedemann and Sarnthein [cf. Dupont and
Hooghiemstra, 1989]) the present asserbonof apparent
dune advance southward to 6°N in West Africa and
1°N in Cameroon/Gabon and Central Zaire, requires
a reconstruction of the Pleistocene refugia hypothesis
for Africa, at least for periods earlierthan the LGM.
PLEISTOCENE
REFUGIAIN AFRICA
87
Contours
Fig. 5. Thedrainage
patternandshapeof ObaL&e, UniversiyofIbadan,JE?geria.
EheEJVE WSWtributray
streams
aregradualybeingcaptured
bythosetrending
JV S in response
to theregional
slope.
Source:
Topographicmap at a scale of 1: 50 000
Distributions
of flora and fauna Pleistocene refugia
would be expected to correspondto areas of high
present diversity,possess a fairly small number of
endemicsdependingon the durationof isolationand
have a number of species in common with other
areasowingto the minglingof biotaduringphasesof
forest expansion. However, the supposedrepeated
advanceand retreatof forest has not producedthe
diversefloraand fauna,or the hybridzones expected
according to refugia hypotheses (Grubb, 1982;
Endler, 1982) since many groups of Africanforest
biota exhibit a higher degree of endemism, and
fewer range disjunctionsbetweenforestblocksthan
would be expected had once-continuousforest survived as fragmentsthroughoutthe Pleistocene.Of
the 41 speciesof the genus Diospyros
(mahogany)for
example, four are endemic to the Guinea forest
block west of the Dahomey Gap, 24 to the Congo
block east of the Dahomey Gap and only six are
foundin both (White,1962).
Mammalsand birds of the Guinean forestblock
also appear different genetically from those of
the Congolianblock (Booth, 1954;Marchant,1954)
in spite of the fact that many of these groups are
eurytypic;able to penetratethe savannaalong river
valleysand forestoutliersup to the 1000-millimetre
isohyet and to cross the Dahomey Gap (Robbins
1978; Grubb, 1982). The Guinean forest block is
impoverishedin mammals,no endemic Congolian
subgenusbeing foundin the Guineanblock and vice
versa.At the specieslevel, Grubb(1982) found that
of Africa's175 largeforestmammals,63 per cent are
confined to a single region. Butterflydistributions
(cf. Hamilton, 1976) show an even higherdegree of
endemism than mammals, with distinct races in
Upper Guinea,Cameroonand Gabon.Accordingly,
Endler(1982) notes that speciationmay occur in a
few thousand generations,suggesting that where
genetic distinctivenessoccurs among fauna, this
could resultfrompresent-dayenvironmentalbarriers
to migrationand not from past climaticisolation
and in the case of West Africa, from the fact that
therehas neverbeen a wide forestcorridoralongthe
Guinea coast. Additionally, unlike in Amazonia
there appear to be few hybrid belts where species
previouslyisolatedin refugiacould have re-merged
(Grubb,1982;Endler, 1982).The abilityof many of
today's mammals to survive outside high forest
(Grubb, 1982) does not supportthe refuge theory,
and may result from their wide tolerance derived
88
PLEISTOCENE
REfIJGIAIN AF*ICA
from continuousevolutionbetween alternatingarid arid and montane genotypes since the presentlyreporteddesertadvance,accompaniedand followed
and humidperiods.
by evolutionin partialisolationduring subsequent,
less extensive arid phases such as the LGM when
SummaCy
It is, therefore,suggestedthat WestAfricanand pos- desertadvancedto 12°N in WestAfrica.There is an
sibly also CentralAfricanlowlandrain forestswere urgentneed to establishabsolutetime periodsfor the
destroyed,at least in their present foorm,during at observedgeomoxphologicalevidence of arid condileast one mid-Pleistocenearidphase which was both tions usingluminescencedating.This could be more
more intenseand extensivethan the last, and during productive than the dating of organic materials
which the world'sancient tree familiessuch as the which are neither formed nor preservedin large
Dipterocarpaceae,
typical of the Miocene rain forests quantltlesm desertenvlronments.
(Ashton, 1982) but now found only in the lowland
rain forestsof South East Asia, were lost from the Acknowledggrnent
continent. The present biota of the lowland rain The authorwouldlike to thankA.T. Grovefor assisforestsof WestAfricamay resultfromevolutionfrom tance and ideasin researchon this topic.
.
*
.
.
.
REFERENCES
Haffer,J. 1969 Speciation in Amazon forest birds. Science165:
Albert, K.D., Andres, W. and Lang, A. 1997 Palaeodunes in
131-7.
NE BurkinaFaso;pedo- and morphogenesisin a chronological frameworkprovidedby luminescencedating.t. Geomorph.Hamilton, A.C. 1976 The significanceof patterns of distribution shown by forest plants and animals in tropical Africa
41: 167-82.
for the reconstructionof upper Pleistocene palaeoenvironAshton, P.S. 1982 Dipterocarpaceae. Flora Malesiana9:
ments:a review.Palaeoecol.
Afr. 18: 283-304.
237-552.
Booth, A.H. 1954 The Dahomey Gap and the mammalian Jansen, J.H.F., van Weering, T.C.E., Gieles, R. and van
Iperen,J. 1984 Late Quaternaryoceanographyand climafauna of the West African forests. Rev.Zool.Bot.Afr. 1:
tology of the Zaire-Congo fan and the adjacent eastern
305-14.
Angola Basin.Aeth.j8. SeaRes.17: 201-49.
Brook, G., Burney, D.A. and Coward, J.B. 1990
Palaeoenvironmentaldata for Ituri, Zaire, from sedimentsin Jeje, L.K. 1974 Relief and drainagedevelopmentin the Idanre
hills of south-westNigeria.NgenanGeogr.
j. 172: 83-92.
Matupi Cave, Mt Hoyo. ErginiaMas.Aat. Hist.Mem.1:
49-70.
Lancaster,N. 1981 Palaeoenvironmentalimplicationsof fixed
Bruckner,W.D. 1956 On structuralirregularitiesin the mantle
dune systems in southern Africa. Palaeogeogr.,
Palaeoclimatol.
Palaeoecol.
33: 327-46.
of the Gold Coast.ti. W.Afr.Sci.Assoc.
2: 87-97.
Burke, K.C.A. and Durotoye, B. 1971 Geomorphology and Livingstone, D.A. 1982 Quaternary geography of Africa and
the refuge theory. In Prance, G.T. (ed.)Btological
dwers;0cation
superficialdeposits related to Quaternary climate variation
in South-WesternNigeria. t. Geomorph.
15:43F44.
in thetropics.
New York:Columbia Univ. Press:523-36.
Camb. Mass.:MIT Press.
Burke, K.C.A., Durotoye, B. and Whiteman, AJ. 1971 A dry Mabbutt,J.A. 1971 Desertlandforms.
phase south of the Sahara 20000 years ago. W. Afr.j.
Maley,J. 1991 The Africanrain forestvegetationand palaeoenArchaeol.
1: 1-9.
vironmentsduringlate Quaternary.Clim.Change
19: 79-98.
Cooke, R. 1970 Stone pavements in deserts. Ann.Ass.Am. Marchant, S. 1954 The relationshipof the southern Nigerian
Geogr.
60(3): 56-77
avifauna to those of the Upper and Lower Guinea. Ibis 96:
Dupont, L.M. 1993 Vegetation zones in northwestAfrica dur371-9.
ing the Bruhnes chron reconstructedfrom marine palyno- Moreau, R.R. 1963 Vicissitudesof the African biomes in the
logical data. Qyat.Sci.Re7).
12: 198-202.
late Pleistocene.Proc.Zool.Soc.Lond.141: 395-421.
Dupont, L.M. and H. Hooghiemstra, 1989 The Sahara- Nichol,J.E. 1991 The extent of desertdunesin northernNigeria
Sahelian boundaryduring the Bruhneschron.ActaBot.Aeerl.
as shown by image enhancement.Geogrlj.157(1): 13-24.
38(4): 405-15.
, 1998 Quaternary climate and landscape development in
Endler, J. 1982 Pleistocene forest refuges: fact or fancy? In
West Africa.t. Geomorph.
42(3): 329-47 .
Prance, G.T. (ed.) Biological
diversScation
in thetropics.
New
de Ploey,J. 1965 Position geomorphique, genese et chronoloYork:Columbia Univ. Press:641-57.
gie de certains depots superficiels au Congo occidental.
Goudie, A.S., Allchin, B. and Hegde, K.T.M. 1973 The fori. Quaternarta
7: 131-54.
mer extensions of the Indian sand desert. Geogrl
j. 139: Richards,P.W. 1952 Ehetropical
rainforest.
Cambridge:C.U.P.
Robbins, C.B. 1978 The Dahomey Gap - a re-evaluationof its
243-57.
Grove, A.T. 1958 The ancient erg of Hausaland and other
significanceas a faunal barrier to West African high forest
similarformationson the southern side of the Sahara. Geogrl
mammals.Bull. Carnegie
Mus.Aat. Hist. 6: 168-74.
Rognon, P. 1987 Les phases d'ariditedu PleistoceneSuperieur
i. 124(4):526-33.
Grubb, P. 1982 Refuges and dispersal in the speciation of
et de l'Holocene au Sahara; arguments sedimentologique.
African forest mammals. In Prance, G.T. (ed.) Biological Palaeoecol.
Afr. 18: 111-33.
diversScation
in thetropics.
New York: Columbia Univ. Press: Runge,J. 1996 Palaeoenvironmentalinterpretationof geomor537-53.
phological and pedological studiesin rain forest 'core areas'
PLEISTOCENE REfIJGIA IN AFRICA
j. 78(2): 91-7.
of eastern Zaire (CentralAfrica).S. Afr.Geogr.
Sarnthein,M. 1978 Sand deserts during glacial maximum and
climatic optimum.Aature272(2): 43-6.
Smyth, AJ. and Montgomery, R.P. 1962 Soils and landuse in
centralwesternNgena. Ibadan: The Government of Western
Nigeria.
Sowunmi, M.A. 1986 Change of vegetation with time. In
Lawson, G. (ed.) Plant ecologyin WestAfnca. Chichester:
Wiley: 273-307.
Stokes, S. and Horrocks,J. 1998 A reconnaissancesurvey of
the linear dunes and loess plains of northwesternNigeria:
granulometry and geochronology. In Whittle, G.L. and
Kendall, G. (eds) Qyaternatydesertsand climatic change.
Rotterdam:Balkema:165-73.
deSwardt, A.MJ. 1964 Laterisation and landscape develop8: 313-33.
ment in parts of equatorialAfrica. t. Geomorph.
Talbot, M.R. and Delibrias, D. 1977 Holocene variations in
the level of Lake Bosumtwi,Ghana. Aature268: 722-4.
Thomas, M.F. 1962 On the approach to landform studies in
Assoc.Nger7a5(2): 87-101.
Nigeria. i. Geogr.
89
Thomas, M.F. and Thorp, M. 1992 Landscapedynamics and
surface deposits arising from Late Quaternary fluctuations
in the forest-savannaboundary. In Furley, P.A., Proctor,J.
and Ratter, J.A. (eds) Aatureand dynsmicsof forest-saranna
London:Chapman and Hall: 215-53.
boundar7es.
Thorp M.B. 1970 Landscape evolution. In Mortimore, MJ.
Bello Uniuersiy,Occasional
(ed.) Zaria and its region. Ahnnadu
Paper4:33-40.
Tricart,J. 1974 E;xistencede periodes seches au Quaternaire
Dyn.
en Amazonie et dans les regionsvoisines. Reu.Geomorph.
23: 145-58.
and formation. Prog.
Tsoar, H. 1989 Linear dunes-forms
Phys.Geogr.13: 507-27.
Vance HaynesJr., C., Maxwell, T.A., E1 Hawary, A., Nicoll,
K.A. and Stokes, S. 1997 An Acheulian site near bir Kiseiba
an
in the Darb el Arba'in desert, Egypt. Geoarchaeology:
12(8):819-32.
jkournal
International
White, F. 1962 Geographic variation and speciation in Africa
with particular reference to Diospyros.TaxonomyGeogr.4:
71-103.