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.
BIBLIOGRAPHY
Many of the more important references are in Russian, and for these reference is made to summaries in other languages.
Sevastianov, D. P. “On the glaciation of the extreme north-east of Siberia.” J. 12 Congr. Russ. Nat., Moscow, 1910, No. 10, p. 491. (Russian, see Geol. Centralblatt, 15, p. 205.)
Riesnitschanko, W. “Ancient and modern glaciers of the south-western Altai.” Mem. Russ. Geogr. Soc., 48, 1912, p. 357. (Russian, see Geol. Centralblatt, 19, p. 131.)
Komarov, W. “On the Quaternary glaciation of Kamchatka—Travels in Kamchatka in 1908-9,” Vol. 1 (Russian, see N. J. Min., 1915, Pt. 2, P. 117).