Geoscience Reference
In-Depth Information
Cooke (
1958
) and the distinguished American glacial geologist R.F. Flint (
1959a
;
1959b
) provided thorough and dispassionate critiques of the evidence used to infer
wetter or drier climates in Africa during the Quaternary, noting that lake fluctuations
often reflected volcanic or tectonic influences rather than climate. Flint was careful
to draw a distinction between lake fluctuations that
demanded
a climatic control, as
opposed to merely being
consistent
with a wetter (or drier) climate. Other possible
non-climatic factors pointed out by both Flint and Cooke were changes in run-off into
lake basins caused by changes in river discharge resulting from river capture or the
breaching of a natural lake dam by headward erosion. Publication of
Background to
Evolution in Africa
(Bishop and Clark,
1967
) marked a return to correct stratigraphic
procedures. Bishop (
1971
) provided a history and critique of the East African pluvial
concept and was careful to distinguish between interpretation and field observations.
He also noted that the Gamblian pluvial was Holocene and not late Pleistocene in
age, and categorically rejected the evidence put forward in support of all the earlier
pluvials.
Despite these caveats, the notion that glacial climates were wetter than today in
East Africa and elsewhere was still well-entrenched (Budel,
1977
). In their compre-
hensive monograph on
Desert and River in Nubia
, Butzer and Hansen (
1968
) mapped
widespread late Pleistocene gravels in river terraces along the Nile Valley in southern
Egypt. They argued that in order to carry such a coarse bed load, the Nile must have
had a greater competence and flood discharge than it does today. They concluded
from this that climatic conditions were wetter in the Ethiopian headwaters of the Nile
during the late Pleistocene, thus supporting the notion of a glacial pluvial climate. It is
interesting to note that Fairbridge (
1962
;
1963
) used essentially the same evidence of
widespread late Pleistocene alluviation in the Nubian Nile Valley of northern Sudan
to argue instead for glacial aridity, on the grounds that the river lacked the competence
and flood discharge to carry this material to the sea.
Oddly enough, as we observed in
Chapter 10
, both groups of workers were partly
right. The LGM was indeed more arid in the Ethiopian headwaters of the Blue Nile,
but the more seasonal discharge regime led to widespread deposition of sand and
gravel in central and northern Sudan and southern Egypt (Adamson et al.,
1980
).
Because the White Nile was cut off from its sources in Uganda, it was no longer
able to provide low season flow to the Nile, which became a seasonal rather than a
perennial river, so that a great deal of sediment never reached the Mediterranean.
The East African glacial-pluvial model so vigorously espoused by Nilsson (
1949
)
was finally laid to rest at the Pan-African Prehistory and Quaternary Studies Congress
held in Addis Ababa in December 1971, at which Karl Butzer and his colleagues
presented the first detailed radiocarbon chronology for the Kenyan and Ugandan high
lake levels. Their work, published the following year in
Science
(Butzer et al.,
1972
),
demonstrated that the lakes were high during the early to mid-Holocene and were
low or dry during the late Pleistocene. Using a simple hydrologic model, they were
