The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Steinmann, J. “Diluvium in Südamerika.” Zs. d. D. Geol. Gesellsch., 58, 1906, p. 215.

Meyer, H. “In den Hochanden von Ekuador.” Berlin, 1907.

Sievers, W. “Reise in Peru und Ecuador, ausgeführt 1909.” München und Leipzig, 1914.

Keidel, H. “Ueber den Anteil der Quartaren Klimaschwankungen an der Gestaltung der Gebirgsoberfläche in dem Trockengebiete der mittleren und nördlichen Argentinischen Anden.” Congr. Geol. Internat., 12, Canada, 1913, p. 757.

Willis, Bailey. “Physiography of the Cordillera de los Andes between latitudes 39° and 44° S.” Congr. Geol. Internat., 12, Canada, 1913, P. 733.

Halle, T. G. “On Quaternary deposits and changes of level in Patagonia and Tierra del Fuego.” Bull. Geol. Inst., Upsala, 9, 1908-9, p. 93.

CHAPTER X
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.