DESERTS. 263 
remarkably free from surface moraine; and for the same reason, viz, that the 
mountains above are almost wholly mantled with ice. We must believe that the 
old epoch moraines, in spite of their immensity, were accumulated and brought 
forward under the ice and by its margins. 
In my report of 1903 it was shown that considerable mountain movement 
took place apparently between the first and second glacial epochs. The northern 
flank of the Trans-Alai is truncated by what appears to be a fault-scarp, displacing 
the first-epoch moraines and broad-trough valleys and dissected by the narrower 
trough valleys of later glaciation. The Kizil Art valley, principal Trans-Alai 
tributary, has the twice-troughed form due to uplift, from its heading cirque 
nearly to its mouth in the Alai; there the bottom of its lower trough sinks under 
the flood-plain of later alluviation, half-drowning the great second-epoch moraine 
that rises at the mouth of the valley to stretch 10 miles out into the Alai. 
—— 
/ 

Fig. 441.—Alluvial Terraces in the Lower End of the Alai Valley. 
Portions of its second-epoch moraine lie apparently faulted up on the terraces 
of its upper, broad-trough stage. Half-way down the Alai there is an uptilted 
mass of alluvium jutting out from its northern side as though more complicated 
deformation took place. In the eastern half of the valley tributaries from the 
north are deeply drowned in alluvium, as contrasted with those from the south, 
uplifted Trans-Alai, which have been canyoned postglacially. Terraces begin 
to flank the Kizil Su toward the outlet of the valley; and of these the higher are, 
as shown above, the result of cutting down since the first glacial epoch, while the 
lower appear to have been since the second. By early explorers these were taken 
for shores of an ancient lake; but they incline gently downstream, diverging some- 
what above it, ascend into tributary valleys from the north as characteristic 
alluvial terraces, and in no way resemble those of a lake. It is, therefore, obvious 
that the Alai valley has suffered two distinct periods of mountain movement— 
