BASINS AND GORGES OF THE CHU. 97
Alexander range on the north. Near the southern border of the basin are some ridges formed 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 have been very 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 deformation, before the surrounding region had gained a strong relief. Their deformation and progressive degradation may be associated with the stronger dislocation and dissection of the inclosing ranges.
The Urta-takoë soon leaves the Kach-kar basin by a rather narrow valley, and enters a second basin in the center of which lies Urta-takoë post-station, just south of a superb fan that is washed from the block range on the north. The longitudinal valley here has every 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 mountains of this district. The river 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 gravels will be mentioned in the section on the lake basin.
The river that we have been following is called the Chu after passing the west end of Issik Kul. For the next 20 miles it follows a rather open valley westward, with an extraordinary exhibition of terraced alluvial deposits, including cream-colored clays and heavy gravels. Then the deep and wild Buain 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 intrenchment 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 former gravels and into the rock beneath.
The persistent alternation of open longitudinal valleys 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 displacement of the ranges in the Chu basin, and thus confirms the inferences based on the form of the mountain blocks.
THE KUGART 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