RECONNAISSANCE IN CENTRAL TURKESTAN. 197 
enough for the stream to cut a valley 400 or more feet deep across the soft strata 
which stretch along the base of the mountains. A second advance of the ice 
widened this valley and deposited another moraine on its floor. 
After this the ice seems to have again retreated, while a second epoch of 
normal erosion ensued. This was of considerable duration, since it was suflBcient 
not only to account for the considerable differences in amount of weathering 
between the second and third moraines, but also to allow the cutting of a valley 
300 or 400 feet deep through the second moraine. That the second moraine was 
so deeply eroded is made clear b\' the fact that the third moraine lies more than 300 
feet lower than the second. A glacier can not deepen its valley where it is depositing 
a moraine ; therefore the erosion of the valley nmst ha\-e been perfonned by water 
after the ice had retreated. Tlie moraine, the third of the series, which was fonned 
after this epoch of erosion lies in a valley of almost the same fonn as that of to-day. 
The moraine itself consists of a broad, rounded lobe which rises abruptly in a 
smooth, steep slope from the extensive gra\el plain of the Alai basin and has 
' . 2. 3.4'=mO''aines of the respective epochs. IV'=gravel deposited in the valley as the fourth glacier was advancing. 
Fig. 139. — Cross-section o( the Taka Valley, looking nortK. 
the characteristic, though slightly subdued, topograph)- of a moraine. Through 
the middle nins the verj- broad flood-plain of the Taka Su. The moraine is more 
complete than either of its predecessors, because the lower parts of the earlier 
moraines have been cut oflT by the broadening of the main Alai basin in the soft 
Tertiaries. Upstream the third moraine is prolonged for 2 or 3 miles in the form 
of a terrace of the same .sort as that of the second moraine below which it lies. 
The next interglacial epoch, that between the deposition of the third and 
fourth moraines, seems to have been shorter than its predecessor; nevertheless 
there must have been an interval of retreat during which aqueous erosion again 
deepened the Taka Su \'alley. This is shown by the fact that the fourth moraine, like 
both the second and third, is prolonged far upstream in the fonn of a terrace co\-ered 
with glacial debris and ha\ing along its inner edge an old stream channel. The 
terrace is smaller than the one above it, just as that is smaller than its predecessor. 
The main body of the fourth moraine lies directly upon the third, about a mile 
