It is most probable that, when allowance has been made for the obliteration of glacial markings, and the region has been better explored, it will appear that the glaciation of Turkestan was on a scale at least as vast as that of the Himalayas.
Glacial deposits certainly do exist in the Permo-carboniferous formations, which are described under that head, but in the true Carboniferous system glaciation may be taken as not proven.
They almost invariably lie on strongly ice-worn platforms of rock, and are obviously hollows produced by the gouging action of the sheets of land-ice by which the general glaciation of the country was affected.
Glaciation.-In the central and western parts of the Tian-shan there exist numerous indications of former glaciation on an extensive scale, e.g.
It has been suggested, however, that a prolonged period of volcanic activity may reduce the air temperature to a marked degree by throwing large quantities of dust into the atmosphere: this will act by preventing the penetration of solar radiation.) During a period of prolonged glaciation water becomes withdrawn from the ocean, for rainfall goes to form solid ice-caps that accumulate upon polar and continental land areas.
Local glaciation has modified the higher levels of the Bighorn Mountains, giving glacial cirques, alpine peaks and many mountain lakes and waterfalls.
The effect of glaciation on the soil is noted in a later paragraph.
In the United States no direct evidence has been found of the low tensperaturewhich brought about glaciation in many other parts of the earth during this period.
The unusual glaciation of the east coast is evidently owing to the north polar current carrying the ice masses from the north polar basin 4 south-westward along the land, and giving it an entirely arctic climate down to Cape Farewell.
No satisfactory solution of this problem has been reached; but the association of the Great Lakes and other large lakes farther north in Canada with the great North American area of strong and repeated glaciation is highly suggestive.