2o6 
EXPLORATIONS IN TURKESTAN. 
XBRRACES AS A RESULT OF CLIMATIC CHANGES. 
Let us see if the other theory, that of climate changes, is more satisfactory. 
Starting with a climate and topography similar to those of to-day, what would be 
the effect of successive epochs of glacial and non-glacial climate or of colder and 
wanner climate? Judging by what has been found true in other parts of the 
world, the effect would be just what we find in the terraces of Central Turkestan. 
During the epochs of colder climate aggradation would take place in all the valleys 
where the streams had already attained a graded condition ; and that condition 
would Isoon be established in those valleys where the slope was relatively gentle, 
Fig. 143.- Terraces of the Kok Kiya, on the Tian Shan Plateau, al an elevation o( I 1,500 (ect. In 
the foreground and in the middle distance the stream has been caught upon limestone and has cut 
merely a narrow gorge, while in the gravel between it has excavated a broad flood-plain with 
terraced sides. 
though the streams were not previously graded. The graded streams would then 
swing sidewi.se, and very broad flood-plains wotdd be fonned. When warmer 
conditions again prevailed the streams would once more begin to cut downward ; 
the few streams that had already reached grade previous to the period of aggradation 
would cut into the gravel till a new grade was reached, and then, if time allowed, 
they would broaden their flood-plains once more. Of the other streams, the 
majority would soon cut through the gravel coating of their flood-plains and 
intrench themselves in the solid rock beneath. If another cold epoch ensued, the 
previonsl}- graded streams would begin to aggrade and would fill their valleys in the 
same way as before ; the others would become graded and would open their valleys 
and fonn flood-plains once more. Thus, In- a succession of alternations between 
