Geoscience Reference
In-Depth Information
5.2.1 Fossil river systems of the Sahara
Early French observers, like Chudeau (
1921
), Kilian and Petit-Lagrange (
1933
),
Joleaud (
1934
), Urvoy (
1935
;
1937
;
1942
)andLambert(
1936
), commented on the
role of previously wetter climates in forming the alluvial terraces so common along
the now more or less defunct drainage systems of the central Sahara, such as the
Saoura in Algeria or the Azaouak and the Tafassasset in Niger, but were unable to
specify when the climate was wetter or how much wetter it was.
The advent of radiocarbon dating in the 1950s (see
Chapter 6
) revolutionised the
study of prehistoric Saharan environments and prehistoric cultures (Alimen et al.,
1966
). Jean Chavaillon, supervised by Henriette Alimen (Alimen,
1955
; Alimen and
Chavaillon,
1963
), distinguished geologist, prehistorian and founding director of the
former CNRS Laboratoire de Geologie duQuaternaire at Meudon-Bellevue near Paris,
completed an exploratory study of the depositional history of the alluvial terraces
along the Saoura, in which Chavaillon equated fluvial downcutting with increasing
discharge and aggradation with a trend towards aridity (Chavaillon,
1964
).
Georges Conrad assailed this overly simplified model of river behaviour a few
years later (Conrad,
1969
). Detailed stratigraphic and paleoecological investigation
of the late Pleistocene 'Saourian terrace' revealed lignites interbedded with coarse
sands fining upwards into shell-and pollen-bearing marls and clays. Clearly, one late
Pleistocene depositional cycle encompassed a variety of local and regional envir-
onmental fluctuations. Interestingly enough, George Williams's study of piedmont
deposits near Biskra, undertaken independently of Conrad's (who told me that he
was equally unaware of Williams's work), reached somewhat similar conclusions
(Williams,
1970
).
The Saharan uplands were logical places in which to study geomorphic changes in
the headwaters of once mighty rivers. Pierre Rognon's detailed and eclectic study of
the Hoggar valleys (Rognon,
1967
) was shortly followed by comprehensive French,
Swiss and German studies of alluvial deposits around Tibesti in which attempts were
sometimes made to correlate separate and often undated river terraces with the soon to
be outmoded Alpine glacial sequence of Europe (Maley et al.,
1970
; Messerli,
1972
;
Winiger,
1972
;Jakel,
1977
). Despite later efforts to deduce the likely hydrological
conditions under which certain Saharan fluvial deposits had formed (Rognon,
1976a
; Rognon,
1976b
), it soon became apparent that climatic interpretation of river
terraces was fraught with difficulty and all too often riddled with circular reaso-
ning (Williams,
1976a
).
5.2.2 Evidence of once widespread lakes in the Sahara and Afar deserts
An awareness of the difficulties involved in correlating and correctly interpreting
river terraces, especially when there were no fossils or dateable organic remains,
