276 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
of Central Asia’s valley terraces have resulted from widespread cycles of uplift, 
in some parts locally interrupted, and that just such a variable glaciology would 
arise from a differential uplift. 
THE ZERAFSHAN AS A LONGITUDINAL VALLEY. 
The Zerafshan valley is perhaps the most valuable of longitudinal valleys 
for our purpose. Rising in the ice cave of its wonderful glacier, amid Alpine peaks 
up to 18,000 feet in height, at the forking of two westernmost members of the 
Tian Shan system, its river flows west for 150 miles as a thundering torrent, between 
the rock walls of its canyon carved in the bottom of a gorge several thousand 
feet in depth. Out of this it abruptly emerges onto the broad steppes, to nourish 
the great oasis of Samarkand and 
those bordering it for 200 miles, 
till the last of its waters filter away 
in the gardens and rice-fields of old 
Bokhara. Once it probably joined 
the Oxusand only about a thousand 
years ago filled the canals of Pai- 
kent, then the most powerful city 
of Central Asia, but now aban- 
doned to the desert dunes, from 
which project its ruined walls. As 
a longitudinal and structural valley 
that of the upper Zerafshan has 
responded to uplift differently from 
those carved transversely in up- 
lifted ranges. To begin with, it 
could not much feel any transverse 
tilting suchas so affected the Taldic 
valley to the east and, since it de- 
bouches from between the ends of 
two ranges where they die out and 
seem to have risen but little, it 
responded slowly up the 150 miles 
to its source. Moreover, there is more chance for a warp in a long valley than 
in ashort one. Lastly, the grade of such a long longitudinal valley is necessarily 
much less than that of transverse valleys heading at the same height on the 
same range. It therefore had more tendency to fill with the waste of glacial 
alluviation, especially during long interruptions of crustal movement when aggra- 
dation of the plains could raise the base-level back upstream, either case giving 
rise to massive terraces of alluvium after the cutting-down of a succeeding uplift. 
All the above distinctions are characteristic of the Zerafshan as well as the 
Kizil Su gorge of Karategin. In general, there seem to have been three cycles 
of erosion before the present, which makes a fourth, as the stream is now rapidly 
corrading. 

Fig. 454.—A Peak South of the Zerafshan Glacier. 
