AFRICA
The Quaternary history of Africa can unfortunately be dismissed in
a very few words. The glaciation of the Atlas Mountains has already
been referred to in connexion with the Mediterranean region. Further
south we have no great mountain chain such as the Andes extending
above the snow-line over the whole extent of the country, but merely
a few isolated peaks. Three of these, all close to the equator, are
known to show traces of a greatly extended glaciation in the past:
Ruwenzori, just north of the equator, on the borders of Uganda and
the Congo, reaching an elevation of 16,794 feet, with the present
snow-line at 15,000 feet, and glaciers extending to 10,000 feet,
formerly bore glaciers extending down as far as 5200 feet; Kenya, on
the equator in Kenya Colony, height 17,040 feet, present snow-line
about 15,000 feet, past snow-line 12,000 feet, and old moraines
at 10,000 feet; finally, Kilimanjaro, 3° S., on the borders of
Tanganyika territory, height 19,320 feet, present limit of glaciers
13,650 feet, past limit 4870 feet. Further south, the Drakensberg
Mountains, between Basutoland and Natal, were glaciated on their
higher summits. In none of these cases have the remains of more than
one glaciation been described, but the mountains are still very
little known and this negative evidence is not conclusive. In the
neighbourhood of Ruwenzori there are several peaks, which approach
12,000 feet, but these were not glaciated, pointing to a snow-line
above this level. Unfortunately the latter piece of evidence is of
doubtful validity since these mountains are volcanic and possibly of
post-glacial age; we may consider, however, that the glaciation of
the central African mountains was characterized by a great increase
in the length of the glaciers with only a slight depression of the
snow-line, conditions showing that the glaciation was due chiefly
to an increase of snowfall, and only in a minor degree to a fall of
temperature. This conclusion is borne out by the low-level beds,
which nowhere show an appreciably lower temperature, but abound in
indications of a former greatly increased rainfall. The first of
these is the former greater size of the African great lakes.
Abyssinia, as we have seen, was probably drier than at present, but
further south the rainfall must have been considerably greater. Lake
Kioga stood 600 feet above its present level and was connected with
Lake Victoria. Lake Victoria and the smaller lakes were twice their
present size, and most of the broad valleys were filled with water.
Lake Magadi is the attenuated relic of a vast sheet of water, and
other great lakes have disappeared entirely. One of these, in the
Rift valley, south of Lake Naivasha, has been mapped by Professor
Gregory and named after Professor Suess. Part of this decrease of the
lakes has undoubtedly taken place within historic times, and part may
be attributed to changes in the drainage, but there remains enough
evidence to show that some time in the Ice Age the great lakes were
very much larger than the present.
Mr. E. J. Wayland, the Government Geologist of Uganda, informs me
that in the old basin of Victoria Nyanza there are masses of gravel
which may be two or three miles in breadth, the surface of which
forms two terraces at different levels. Above the level of these is
an old peneplain with ancient beach gravels. Mr. Wayland considers
that this peneplain was formed probably during the Pliocene by the
first Victoria Nyanza occupying a basin between folds. The initial
high level was due to the want of an outlet, but may have been
amplified by other causes. The level of the lake then sank gradually
to a considerably lower level, after which it rose again nearly to
its old level and remained there for a considerable time. During this
period the great gravel deposits were formed; they contain flood
deposits, especially near their base. The level of the lake then
sank again and this part of the basin was converted into a valley
occupied by a river. Subsequently the level rose again sufficiently
to carve out the lower terrace in the gravels. Mr. Wayland considers
that the upper terrace may also represent a stage distinct from that
in which the gravels were actually deposited, but the upper terrace
may be the original surface of the gravels. Thus there is evidence of
two Pluvial periods in central Africa, of which the first, probably
corresponding with the great extension of the mountain glaciers, was
the greater. From the archæological evidence it appears to correspond
with the Mindelian glaciation of Europe.
A second line of evidence has been pointed out by C. W. Hobley. At
the entrance to Kilindi Harbour, Mombasa, there is a gap in the coral
barrier through which the fresh water from the river finds its way.
These gaps are always found opposite the mouths of rivers, and are
due to the inability of the coral polyp to live in fresh or brackish
water. In Pleistocene times the land stood some seventy feet lower
relatively to the sea, and the old channel through the reef at this
height is almost double the width of the present channel, showing
that the river then had a greater volume, i.e. the rainfall in its
basin was greater.
But Africa is noteworthy chiefly for its deserts, and the most
important evidence of climatic change is found in the deserts of
Sahara and Kalahari. From the time of the ancient Greeks it had been
believed that the Sahara was formerly the site of a great inland sea,
and the presence of this sea had even been suggested as the cause
of the Ice Age in Europe, but recent investigations have shown that
this is not so; the Sahara has been land at least throughout the
Tertiary period. There is, however, abundant evidence that during the
Quaternary the rainfall was considerably greater than the present.
The presence of numerous animals closely associated with water, such
as the hippopotamus and even the crocodile, in oases now entirely
isolated, shows that these oases were formerly connected with the big
rivers. The most definite evidence, however, comes from Lake Tchad.
This was formerly of much greater area, but Chudeau and Freydenberg
have made out a whole series of changes from desert conditions in the
Tertiary through pluvial conditions in the Quaternary back to desert
conditions of the present. The sequence is as follows:
1. A regime of dunes.
2. A slow transgression causing a long marshy period, during which
numerous plants whose remains are found lived in the period.
3. A slow regression.
4. A rapid transgression (grey loam).
5. A slow regression (clayey white loam with traces of roots).
6. A transgression (white loam).
7. Establishment of a new dune regime.
In the Chari basin east of Lake Tchad are the remains of fish and
shells, and also small pebbles of sandstone and chalcedony, which are
not local, but must have been brought from the mountains of Tibesti
by the rivers Egnei and Toro when their current was much stronger
than at present. In Senegal, south of the 15th parallel, the present
dune sands are underlain by an alluvial soil, indicating moister
conditions preceding the present climate. There is no means of dating
the moist periods indicated by these phenomena, but it is reasonable
to correlate them with the former extension of the central African
lakes.
Passing south to the Kalahari, we find evidence of a number of
moist stages separated by drier intervals, but they can apparently
be grouped into two main Pluvial periods, separated by a long
interpluvial with steppe-like conditions. One at least of these
Pluvial periods must be correlated with the former immense extension
of Lake Ngami and the Etosha Pan.
From Cape Colony, there is some evidence of moister conditions in the
past, but the Quaternary variations cannot be separated from those of
historic times.
Before leaving Africa some reference must be made to an interesting
suggestion by C. W. Hobley, as to the mechanism of climatic change
in tropical countries. He notes that the north-east and south-west
monsoons extend to a height of only a few thousand feet. Above
them are the very steady “trade winds” connected with the general
circulation of the atmosphere. In Kenya Colony these blow from east
or a little south of east. “Their effect is very marked on the high
mountains of the interior, such as Kenya, Kilimanjaro and Elgon; in
the early morning they are generally quite clear, but about 10 a.m.
the clouds sweep up from the S.S.E. and collect on the mountains and
blot them out from view for the rest of the day. These are believed
to be clouds borne inland by the trade winds, and the moisture they
carry is precipitated mainly on the south and south-east sides of
the mountains.” Hobley suggests that there was formerly a nearly
continuous ridge of high land extending north and south, and this
caught the moisture from the trade winds, so causing the Pluvial
period, the evidence for this ridge being the distribution of alpine
plants on the now isolated high mountains. An alternative explanation
is that the greater strength of the earth’s circulation during
glacial times caused the trade winds to be much stronger and also
to extend to a lower level at the expense of the monsoons, just as
the west winds extended to a lower level in northern Egypt. This
would bring a great deal more moisture to be precipitated on the
mountains, increasing the length of the glaciers and also the volume
of the rivers.
BIBLIOGRAPHY
Scott Elliott, G. F. “The geology of Mount Ruwenzori and some
adjoining regions of tropical Africa.” _Q.J.G.S._, 51, 1895, p. 669.
Hobley, C. W. “The alleged desiccation of East Africa.” _Geogr.
Journ._, 44, 1914, p. 467.
Freydenberg, H. “Le Tchad et le Bassin du Chari.” Diss. Paris, 1908.
Passarge, H. “Die Kalahari.”