182 EXPLORATIONS IN TURKESTAN.
THE QUATERNARY PERIOD.
In our survey of Central Turkestan we have found that its geological history was long characterized by a remarkable unity. The geological series is uniform in the main, though not in detail. Much of the country in late Tertiary time was reduced to the stage of mature or old topography ; and now, after broad deformation, the basins continue to be aggraded plains, and even the mountains retain much of their Tertiary maturity, although exhibiting marked results of revived erosion. When the country was divided into strongly marked provinces by the Quaternary deformation, a considerable diversity was introduced between the mountains or plateaus on the one hand and the basins on the other. Both the pre-Quaternary unity and the Quaternary diversity were due largely to internal causes—to tectonic movements or to lack of movement. In the remainder of this report I shall consider a series of changes of a different character, which seem to have nothing to do with movements of depression or elevation, but appear to depend upon external controls. The changes now considered were climatic and seem to have affected all parts of the country at the same time, although in different ways. As the changes continued to take place through a large part of Quaternary time, they furnish the basis for a definite time-scale of wide application. They involve a series of oscillations between glacial and interglacial epochs. The plan of study outlined by Professor Davis at the beginning of our work in Turkestan directed attention to the evidence of possible climatic changes shown (1) in ancient moraines ; (2) in terraces, especially along streams flowing from moraines; (3) in lakes and lake deposits ; and (4) in deltas and flood plains of streams which do not • reach the sea. In examining evidence of the first three classes it was found not only that climatic changes have occurred, but that there has been a series of changes of decreasing severity ; it has, however, not yet been possible to correlate exactly the changes shown by one class of evidence with those shown by another. In the fourth class there should also be indication of climatic changes if the facts elsewhere observed have been rightly interpreted, but as yet this class of evidence has not been detected.
GLACIATION.
DISTRIBUTION OP GLACIERS AND AMOUNT OP EROSION.
During the two months' journey from Issik Kul to Marghilan a considerable number of glaciers, possibly fifty, were seen among mountains ranging from 14,000 to 18,000 feet in height. Most of the glaciers were small and ended close to the base of their cirques. The largest was that of Khoja Ishken* in the Alai Mountains, close to the Bokharan boundary, at the head of one of the innumerable Kok Sus or Blue rivers. It is a small example of the valley type of glaciers commonly associated with the Alps. Its length, so far as can be judged from very incomplete maps, is 5 or 6 miles. None of the glaciers descend to a low elevation,
*On the Russian map, scale io versts to the inch, this is called the Adramova glacier, but as the Kirghiz in the neighborhood use the name Khoja Ishken, I have adopted the latter.
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