Diya PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
These questions arise: What is the explanation of this valley, of which a part 
has no trunk-stream? Was it ever a true river-valley? Is it the lower portion 
of a valley of which the head-waters have been captured by a higher branch of 
the Oxus? Was it possibly the valley of the Kizil Su, now captured at Obu-garm 
by the Vaksh River? 
East of the open plains the valley-floor has been uplifted and dissected by 
the Hyak Darya, which plays the part of a true trunk-stream for that portion. 
Following it up to about 18 miles above Faizabad, we find the channel made by 
this stream, at first deep with high terraces, decreases in depth and finally opens 
out onto the ancient valley-floor again. Here the floor is a sweeping concave 
about 3 miles wide between the mountain sides, with spacious tributary fans and 
talus cones, and there are no terraces. It stands at an elevation of 6,100 feet 
and forms a rich summer pasture, resorted to by Usbeg nomads. 
The remarkable fact about this floor is that it continues rising east beyond 
the heading tributaries of its stream, and we thus confront another portion of the 
valley with no trunk-stream. For 3 miles it is a wide grass plain with not even a 
tributary descending to its borders. Then, while it is still rising east, there begin 
Ss 

Fault scarp 

Fig. 450.—Cross-profile of the Hissar Valley. 
gully systems developed back into it from a stream which joins the Kizil Su near 
Obu-garm. These gullies join into a gorge deepening rapidly with terraces inclin- 
ing east, while the old floor above apparently still rises east. The gorge finally 
develops into a canyon 2,000 feet deep in hard limestone and softer rocks; then 
widens again at Obu-garm and debouches into the Kizil Su with a total depth of 
3,000 feet. 
The high-grade plains or first-stage terraces of the Kizil Su appear to have 
a height of about 3,000 feet at this point, and, if so, conform with the old uplifted 
floor of the Hissar valley. Here there is no doubt that the gorge of Obu-garm 
has captured the head of the Hissar valley. The valley at the divide is so broad 
that it must have been formed by a large stream. It is possible that the Kizil Su 
was that stream and that the mountain movements that first broke up the mature 
topography wrought this change in its course. 
In connection with the valley of Hissar it is important to know the valley 
form of its northern tributaries. One of these, the Sardai-miona, was briefly 
studied in descending from Kak Pass over the Hissar Mountains. For 40 miles 
it is a gorge over 2,000 feet deep, of which the last 25 miles narrows to a granite 
