104 
EXPLORATIONS IN TURKESTAN. 
(B, fig. 72) has not been destroyed by the widening of the valley (Y), this is not 
because the widening of the valley has been prevented by the lateral encounter of 
unusually resistant rocks, but because the river spontaneously ceased its destnictive 
work when the lower valley was significantly narrower than the upper \'alley (V), 
instead of continuing to widen the lower \-alley so as to combine the two terraces, 
A and B, in a single terrace, A'. All the terraces of the two Kugarts and of the 
Alabuga-Narin Valley, and at least some of those of the Chu system, thus appear 
to result from successive reductions in the power or in the period of river action. 
The same holds true in the case of the terraces in the Kopet Dagh, above described. 
Here, as well as there, it is not likely that the terraces now seen record all of the 
terrace-making episodes, but onl)- the decreasing maxima iu a complicated series. 
It should also be pointed out that the terraces of New England and of the Tian 
Shan appear to be of unlike age. Those of New England are eroded in loose 
sands or clays, and are all of later date than the last glacial epoch. Indeed, their 
production may have required less than half of post-glacial time, for the valleys iu 
which they were car\^ed were aggraded, after the 
ice retreated, by the same rivers that are now 
degrading them ; and the existence of the ter- 
races shows that less material has been removed 
than was previously deposited. The terraces of 
the Tian Shan, on the other hand, are usually 
eroded either in rather well knit gravels or con- 
glomerates, as along the two Kugarts, or in partly consolidated sandstones and 
claystones, as in the Narin fonnation, while in the Kopet Dagh they are car\-ed in 
calcareous shales. In all these cases the terrace materials are strong enough to stand 
up in steep bluffs. None of these terraces are in glaciated valleys. The earlier 
terracing appears to be much more ancient than the latest moraines in the high 
mountain valleys. It is therefore quite conceivable that, as Mr. Huntington has 
concluded, the successive glacial epochs and the successive terracing epochs, each 
of decreasing intensit}', may be synchronous, and may be common results of a 
series of climatic changes. Whether it is finally proved that the terraces result 
from climatic changes, or whether the terraces are in part the result of cnistal 
movements, there appears to be good ground for thinking that the time internals 
marked by the terraces may be correlated over ver}- considerable distances, and 
that the time inter\-als thus established ma}- be eventually placed in the same scale 
with those indicated b}- the glacial records ; and that thus a good beginning toward 
the establishment of a Quaternar}' time scale will have been made. 
Fig. 72. — Ideal section of Terraces. 
