PHYSIOGRAPHIC OBSERVATIONS. I47 
down of tlieir floors since the close of the Alai glacial epoch, and that, although most 
of this cutting down took place before the close of the Kunnndi glacial epoch, the 
upper portion of at least the Kizil-Art darya is still cutting down in rock bottom. 
The valley fonns of those side streams coming down from the north, from the 
Alai range, might well be called a reverse of those from the south or opposite 
side of the Alai \'alle\-. We had a good general view of manj- northern tributaries 
from a distance, and made a detailed study of the Sari Tash \'alley, which heads at 
Katin-Art pass. This belongs to that class of valleys which had in the past the steep 
V-shape, containing torrent streams, but which have since been largely buried by 
waste, forming a broad, flat surface over which their now sluggish streams meander. 
In the Sari Tash Valle}' only peaks of the rock core rise from the nearly talus-buried 
mountain sides (figs, no, in). There is practically no transportation; the talus is 
stationarj' and coated nearly to the top with loess and grass. The stream meanders 
(^=talus; i5=liraestone ; a^=anuvium.) 
Fig. 1 10. — Section looking north up the Sari Tash Valley. 
between grass banks to within half a mile of the coast, where it winds between 
interlocking fans of talus. A few versts east of the mouth of this valley there rises 
an island spur separated from the rest of the range by Alai Valley alluvium. 
EXPLANATION OF CONDITIONS. 
It is evident that the whole side-valley system of the northern border has been 
deeply submerged in alluvium, whereas the system opposite, on the southern border, 
has been cut down. These reverse conditions might be explained by a simj)le tilt of 
the region decreasing the grade of southerl)- flowing streams and increasing that of 
northerly flowing streams ; but, as will be shown later, that would not fit in witli 
what has happened to the Alai Mountains. The simplest explanation seems to be 
a sinking of the Alai Valley with the region including its northern tributaries. If 
this took place with a fault on the southeni border, it ought to have left a fault cliff" 
which might explain tlie escarpment at the base of the Trans-Alai range, which 
escarpment has been described under observations on glacial geolog}-. 
Suess states on authority of Ivanof that the Alai \'alley is partiall\- surrounded 
by lake terraces. He considers this valley as orogenicallj- the eastern extension of 
the Zerafshan and drains the supposed lake through a gorge cut back bj- a capturing 
