The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Gignoux, M. “Les formations marines pliocènes et quaternaires de l’Italie du Sud et de la Sicilie.” Ann. l’Univ. Lyon, n.a., Vol. 1, fasc. 36, Paris, 1911, pp. 693.

Depéret, C. “Les anciennes lignes de rivage de la côte française de la Mediterranée.” Bull. Soc. Geol. de France, ser. 4, Vol. 6, pp. 207-30.

Douvillé, R. “Espagne,” Handbuch regional Geol., H. 7, 1911. (Includes Gibraltar and Balearic Is.)

Hume, W. F., and Craig, J. I. “The Glacial period and climatic change in North-east Africa.” Rep. Brit. Assoc., 1911, p. 382.

CHAPTER VII
ASIA DURING THE GLACIAL PERIOD

The great area of Asia is at present but little explored for glacial traces, but a certain amount of evidence has been collected, and the data from the various mountain districts are consistent enough to map out the general trend of the history of the continent during the Ice Age.

The earth-movements which brought about the present configuration of Asia were completed as regards their major details by the close of the Tertiary period. These movements left a number of great basins closed in on all sides by enormous mountain walls; at first all these basins contained lakes, and the subsequent geographical history has consisted largely in the gradual silting up of the lakes and the development of more and more arid conditions. The fluctuations of the Ice Age were superposed on this secular desiccation, but except in northern Siberia the part played by glaciation in the history of the country has been relatively small.

Consider for a moment the relief of Asia. The orographic centre may be taken as the great Pamir plateau, the “Roof of the World,” with an average elevation exceeding the height of Mont Blanc, diversified by ranges of mountains exceeding 25,000 feet in places. East of this is the great plateau of Tibet, 10,000 to 17,000 feet, bounded on the south by the mighty Himalayas, and on the north by the mountains of Kuen Lun. On the north the Pamir plateau is bounded by the Alai range, passing north-east into the Tian-Shan mountains, rising to 24,000 feet in Khan-tengri. Still further north-east comes the Altai range, with an elevation of 9000 feet. East of Lake Baikal lie a series of ranges averaging 8000 feet in height, and passing into the Stanovoi range of eastern Siberia and the mountains of Kamchatka.

The Himalayas, owing to their heavy snowfall derived from the south-west monsoon, bear numerous great glaciers, but with the series of ranges extending from the Pamirs to north-east Siberia the case is different. These ranges all rise above the snow-line in places, but owing to the scanty snowfall they bear at most a few small glaciers on their northern sides, and none at all on the slopes which face towards the deserts of western China, and in all cases the glaciation is very slight in comparison with their elevation.

This distribution was characteristic also of the Ice Age. In the Pamirs there is evidence of two periods when the glaciers had a greater extent; in the first they extended to a level of 5000 feet, in the second to 7000 feet. The present limit of the glaciers lies at about 10,000 feet. The first glaciation was remote, for the moraines are worn and weathered, but the second was much more recent, for the moraines are fresh, and in some cases there are still masses of “dead” ice buried beneath great accumulations of debris and occasionally exposed by slips.

In the Tian-Shan mountains there are remains of two glaciations. The earlier was the greater, and the glaciers descended well below 10,000 feet. This glaciation was followed by a long interval, when the erosion of the rivers converted the U-shaped glacial valleys into V-shaped gorges. A second glaciation descended to a level of 10,000 feet, and again developed U-valleys to this level; the end-moraines of these glaciers are young and fresh-looking. In the Altai range there were also two glacial periods. In the older and greater the snow-line was depressed by 3000 feet. The glaciers attained a length of twelve miles and descended to a level of only 3000 feet above the sea. The second glaciation was less extensive.

So far we have been dealing with small mountain glaciers only. But in north-eastern Siberia we find a different state of affairs. The Stanovoi and Verkhoiansk mountains were heavily glaciated, and during the first glaciation were probably the centre of an actual ice-sheet similar to that of Scandinavia. The ice descended the valleys of the rivers Yana, Indijirka and Kolyma and covered the New Siberian Islands, which were at that time connected with the mainland. The upper valley of the Lena was also heavily glaciated by an ice-sheet moving southward, probably from the Patom highlands. When this glaciation drew to a close the source of supply among the mountains ceased, and the ice on the lowlands and in the lower parts of the river valleys was left stranded as “dead” ice. When the mountains became free of ice, the re-born rivers carried great quantities of moraine-clay and other debris with them, and flooding the ice-surface over wide areas deposited their load above the ice. In course of time the remains of the ice-sheet were deeply covered by a layer of earth and stones, which prevented the ice from melting and preserved it to the present day. This is the probable origin of the well-known “fossil ice” of Siberia. Other theories have been put forward, such as the freezing of ground water during the winter, but none are satisfactory, and that given here was generally adopted by Russian geologists.

During the long warm interglacial which followed, the surface of the thick earth-layer covering the ice bore low-growing herbage in the same way as any other earth-surface. (A parallel to this is found in Alaska, where the glaciers terminate among the forests, which actually grow over the moraines covering their snouts.) The rivers cut their way down through the earth and ice, exposing ice-cliffs, which were quickly buried by talus from above. The mammoth and woolly rhinoceros roamed the land, and their tusks remain in great numbers as the “fossil ivory” of Siberia and the Arctic Ocean. Still more remarkable is the fact that mammoths have been found buried entire, and preserved by the frozen ground to the present day. It is difficult to say how the animals reached such a position, but most probably they sank into swamps formed during the summer and were quickly frozen.

In western Europe the mammoth and woolly rhinoceros are regarded as indications of severe climate, but their presence in north-eastern Siberia in large numbers is evidence of a climate probably somewhat warmer than that of the present day, especially as regards the length of the vegetation period. Probably the winter snowfall also was less than now. It is difficult to see how the fauna could have moved from, say, the New Siberian islands into a warmer climate each winter, for the winter climate becomes markedly more severe as one penetrates south from the Arctic coast into the interior. It is possible that the mammoth and woolly rhinoceros hibernated during the winter.

After this interglacial there came a recrudescence of glacial conditions. In this case, however, the Stanovoi and Verkhoiansk mountains and the Patom highlands were not buried in an ice-sheet, but became the centre of great valley glaciers, which reproduced the well-known glacial phenomena—corries, glacial terraces, U-valleys, etc. The ice extended down the great river valleys, leaving a typical moraine landscape on either side, and again reached the New Siberian islands. In course of time the climate ameliorated, again commencing in the south, and again the ice of the glaciers was buried. In the New Siberian islands the happenings are summarized very expressively by a rock-section described by Vollossovitsch. The bottom of the section is formed by the older layer of “fossil ice.” Above this is a sandy clay with remains of meadow vegetation and shrubs, followed by a fine clay with remains of alder and white birch, and the bones of mammoth and rhinoceros. Above this comes another layer of “fossil ice,” followed by clay with the dwarf birch, Arctic willow, and bones of musk ox, horse and later mammoth. After this the coastal regions sank beneath the sea for a time and marine clays were formed in a climate somewhat warmer than the present. When the land rose again the conditions resembled those now prevailing.

Though not part of Asia, reference may be made here to the glaciation of Spitzbergen, which runs strictly parallel with that of northern Siberia. The first glaciation was of the “ice-sheet” type, originating in the region north of Storfjiord, filling the whole of that fiord and extending south of South Cape. Barentz Land and Stans Foreland were at least partially ice-covered. The ice-floor of Spitzbergen, which resembles that of Siberia, may have originated during this glaciation. This was followed by subsidence to 230 feet below present level, and the ice retreated, giving place to an “interglacial,” during which frost was very active and largely obliterated the traces of the ice-sheet. This “interglacial” was followed by a second extension of the ice, which affected the valleys and fiords only, leaving the plateaux free. This again was followed by subsidence and a warm period.

In southern Kamchatka there was a great development of ice, but in the form of a network of glaciers rather than of an inland ice-sheet. In the east the ice reached the sea, but on the west it left a zone forty to sixty miles broad, and up to a thousand feet high unglaciated, so that there was the same difference then as now between the rainy east side and the drier west side of the peninsula. The present snow-line in the centre of southern Kamchatka is about 5500 feet, and at the maximum of the glaciation it must have been fully 3000 feet lower.

This glaciation was followed after an interval by a second, which was confined to the mountains. The moraines of this glaciation are much fresher than are those of the earlier one.

In Japan the mountains were only just high enough for glaciers to develop in the north. The moraines are old and weathered, and their meaning has been disputed; but recent work by Simotomai and Oseki seems to have established their glacial origin. The depression of the snow-line necessary to produce them—about 3000 feet—fits in very well with that observed in adjoining parts of the continent. The phenomena were confined to small hanging glaciers in the Hida mountains which cut out corries and descended to a level of about 8000 feet, leaving small morainic ridges. This glaciation was probably contemporaneous with the earlier and greater glaciation of Siberia. To the succeeding interglacial may be attributed the marine deposits found near Tokio containing corals, at present living some distance further south. No trace of any subsequent glaciation of Japan has yet been found.

J. S. Lee has recently called attention to the existence of a glaciated area in northern China, the evidence for which consists of moraines and striated slabs found in southern Chi-li, and a glaciated valley with travelled boulders in the north of Shan-si. The glacial deposits in Chi-li are closely associated with a layer of quartzite pebbles which continues southward beneath the loess on the eastern side of the Tai-hang range, and is attributed to either torrential rain or the melting of glaciers. J. Geikie had long ago stated that there once existed ice-masses all over northern China, and considered that the ice came from the Himalayas. This origin is impossible, the probable source of the ice being the Yablonoi mountains in southern Mongolia.

In the Himalayas the glaciers formerly had a much greater extension. The glaciers at present extend downwards to 11-13,000 feet, but old moraines are found at 7000 feet, and near Dalhousie on the southern slopes of the Dholadar range to 4740 feet.

On the northern side of the Himalayas there was a great development of ice over Tibet, but there was not a real ice-sheet such as occurred further north. Oldham records three separate periods of glaciation in Kashmir, but it is not yet possible to discuss the glacial history of the Himalayas in detail. The latter is likely to prove complicated, since the range is still rising, and has probably been doing so either continually or intermittently throughout the Quaternary.

The great development of ice in Tibet, which is now semi-arid, owing to interception of the rain-bearing winds by the Himalayan range, suggests a considerable alteration in the present meteorological conditions. The Tibetan snowfall was probably due to the Mediterranean storms, which now give a small winter rainfall in north-west India, and which during the Glacial period greatly increased in strength and frequency and occurred throughout the year ([Chapters IV] [and VI]), giving the Pluvial period of North Africa. These storms would pass across Persia and continue to the north of the Himalayas, probably breaking up over the Tibetan plateau.

It is evident that, taking northern Asia as a whole, there have been two general glaciations, of which the first was the more severe, separated by a long interglacial, during which, in Japan at least, the climate became appreciably warmer than the present. The first glaciation is related to elevation in the Arctic basin, which closed Bering Strait and united the New Siberian islands to the mainland. It was almost certainly contemporaneous with the first glaciation (Gunz-Mindel) of Europe. The ice began as glaciers on the mountains as in Scandinavia, but, owing to the scanty supply of snow, developed more slowly and only reached the dignity of ice-sheets in north-east Siberia. Then followed subsidence below the present level, wider opening of the Bering Strait, warm ocean currents and a long interglacial. After this there was again elevation and a re-development of ice-sheets, but apparently once only, and not twice as in Europe. This glaciation probably corresponded in point of time more or less with the Rissian, for the post-glacial dry of central Asia appears to have been of enormous period length.

There is one other phenomenon which must be considered in connexion with the glacial history of Asia, and that is the loess. Loess has already been referred to in connexion with the glaciation of Europe, but in China its development is much greater. Richthofen, who first studied this deposit attentively, and to whom we owe the æolian theory of its origin, found that it was formerly deposited in China over a much greater area than that over which it is accumulating at present, and attributes this cessation of growth to the heavier rainfall brought by the Glacial period, which enabled the rivers to cut back their valleys and drain some of the mountain basins, formerly enclosed. He considered that loess can accumulate more rapidly in a closed basin, where occasional floods leave behind them layers of bare sand and mud, easily dried to dust, than in a well-drained river valley where floods are rare.

In western Asia outside the limits of glaciation we have further evidence of at least one Pluvial period in the former far greater extent of all the enclosed lakes, due partly to greater precipitation and partly to decreased evaporation. The Caspian Sea and Aral Sea were extended to several times their present size and united into a single sheet of inland water. Lake Lop-Nor was greatly increased in size, and many of the desert basins, at present dry, were the sites of salt lakes. This is especially the case in central Persia, where there were large salt or brackish lakes.

These Pluvial conditions have not yet been correlated with the glaciations of Asia, but, by analogy with the conditions in America discussed in the next chapter, there is little doubt that they were contemporaneous with one at least of the glaciations, and probably there were two main Pluvial periods coinciding with the two Glacial periods. At Baku, on the shores of the Caspian river, Pumpelly has found old shore lines at heights of 600, 500, and 300 feet above the present level of the water. Still more interesting are the conditions found by Sven Hedin in the Kavir basin of Persia. Here there are lacustrine clays and silts referable to a Pluvial period covered by beds of almost pure salt, suggesting a rapid and complete drying up of the lake. Above this again are further silts indicating a return of Pluvial conditions. In addition to this the succession of silts and clays show that there were several minor fluctuations superposed on the main wet periods, giving ten moist phases altogether.