BASINS AND GORGES OF THE CIIU. 97 
Alexander range on the north. Near the soutliern border of the basin are some 
ridges fonned of clay beds, probably Tertiary', tilted to the south and eroded, as in 
fig. 62, and thus suggesting progressive deformation of the bordering mountain 
blocks, as in the Narin basin. The passage of the Juvan-arik through the clay 
ridges is marked by terraces at three levels. Farther on there is a broad plain near 
the grade of the present rivers. The topography of the district must hav^e been 
ver^- different when the clays were deposited, for the rapid river is now and has 
long been washing coarse waste in abundance from its gorge in the Yukok-tau, 
The clays may, therefore, be provisionally referred to an early period of defonnation, 
before the surrounding region had gained a strong relief. Their defonnation and 
progressi\-e degradation may be associated with the stronger dislocation and dissec- 
tion of the inclosing ranges. 
The Urta-takoe soon leaves the Kach-kar basin by a rather narrow valley, and 
enters a second basin in the center of which lies Urta-takoe post-station, just south 
of a superb fan that is washed from the block range on the north. The longitudinal 
valley here has even,- appearance of being aggraded, especially to the east, where 
the waste that is washed in from the higher range on the south has built up a long, 
slightly convex filling against the middle of the smaller range on the north. 
Mention of this has already been made in connection with the block moimtains of 
this district. The ri\er runs northward through a gorge and thus reaches the 
western end of the Issik Kul basin, where a great volume of gravels has been 
deposited and afterward more or less dissected. Some of these gTa\-els will be 
mentioned in the section on the lake basin. 
The river that we have been following is called the Chn after passing the west 
end of Issik Kul. For the next 20 miles it follows a rather open \-alley westward, 
with an extraordinar)' exhibition of terraced alluvial deposits, including cream- 
colored clays and hea\y gravels. Then the deep and wild Buani gorge is followed 
northward. The river here flows at great speed in most tumultuous fashion for 
miles together. Its descent is so rapid that the road alongside of it was often 
undesirably steep. The iutrenchment of the gorge is evidently still in active 
progress ; yet even here, where the walls are steep and ragged in resistant rocks, 
and where there is often not even the beginning of a flood plain, some small 
tributary streams enter the Chu practically at grade. At Kok-muinak station the 
gorge opens upon a wide basin, where the river has made some fine terraces by 
cutting down through its fonner gravels and into the rock beneath. 
The persistent alternation of open longitudinal valle\s with silts and gravels 
and of narrow transverse gorges with bare rock walls, taken with the ungraded 
character of the river in the gorges, gives strong evidence of subrecent displace- 
ment of the ranges in the Chu basin, and thus confirms the inferences based on 
the form of the mountain blocks. 
THE KOGART TERRACES. 
Terraces occur in all the valleys that we followed. The best examples will be 
briefly described, beginning with those of the (western) Kugart, where we first 
entered the mountains. This valley seems to have been eroded to a much greater 
