RECONNAISSANCE IN CENTRAL TURKESTAN. 
207 
colder and wanner epochs, a series of terraces would be formed closely resembling 
those found to-day, provided only that the epochs constantly diminished in intensity 
and duration. Those streams in which a graded condition had been reached before 
this series of climatic changes began would have their terraces cut entirely in gravel, 
while the others would have terraces composed partly of gravel and partly of rock. 
The streams of this class are as a rule deepening rather than widening their valleys. 
It is noteworthy that this series of climatic changes corresponds essentially 
with the series inferred from the old moraines, and the agreement of the two greatly 
favors the tlieorj' that accounts for both. In this connection two points need 
emphasis. In the first place, it has been shown that the moraines give e\idence of 
interglacial epochs alternating with glacial epochs rather than of a single glacial 
A— limestone. B, C, D— successive fillings of gravel. 
Fig. 144. — Cross-section of the Ispairan Valley, showing successive periods of cutting 
and filling. 
period which came to an end with a series of partial glacial retreats separated by 
intervals of glacial rest. The terraces show even stronger evidence of the same 
conclusion. The deposition of gravel and broadening flood-plains on the one 
hand and the cutting of narrow trench-like valleys on the other do not represent 
successive stages of the same kind of climate change ; they represent contrasted 
climatic conditions — namely, the systematic alternation of colder and wanner 
climates. In the second place, the terraces decrease in width, height, and preser\^ation 
in the same wa)- that the moraines decrease in size and in the degree of weathering 
and erosion to which they have been subjected. In both cases the oldest examples 
are large and well developed, but not ven,- well preserved ; the others are successively 
smaller and better preserved. All these points of agreement increase the proba- 
bility that the theory of a glacial period consisting of interglacial epochs is correct. 
One difficulty presents itself — the number of terraces and of moraines does not 
always agree — the maximum of the one is nine, of the other only five. Part of 
this discrepancy is due, as has been already explained, to the fact that some of the 
terrace series are complicated by one or two minor terraces, which seem to be due 
merely to ordinan- swinging of the streams; but even with due allowance for this 
complication, there seems to be some evidence of a sixth unexplained terrace. It 
may therefore be suggested that the first of our glacial epochs was not actually the 
first, but merely the maximum, and was preceded by others which have left no trace 
save in doubtful terraces. This, however, is at present pure conjecture. 
