RECONNAISSANCE IN CENTRAL TURKESTAN. 173
the Jukuchak pass, is full of old moraines spread in a broad, uneven sheet and enclosing numerous lifeless ponds and lakes. In other basins the moraines have not advanced so far and the streams have cut slight terraces in the gravel silt or the Tertiary strata which lie on the floor of the depressions and form the plains. Such in general are the basins and ridges of the most typical portion of the Tian Shan plateau.
In the very center of the plateau is a valley of erosion of quite a different character. South of Chakur Korum pass the Kara Kul River, one of the main branches of the Narin, flows in a young valley i,000 or z,000 feet deep, with a narrow bottom and steep walls like those which characterize the valleys on the north slope -oh the plateau. The road descends from the pass to the river by a narrow side valley with walls a thousand or more feet high, and in some places perpendicular
Fig. 125.— Scene in the Yak,Tash;,Basin, in the northern part of the Tian Shan plateau, looking northwest. In the foreground the basin-floor is covered with a flat moraine holding numerous ponds ; beyond are smooth-topped mountains cut by glacial valleys.
for several hundred feet. Near its mouth, where the flood-plain widens a little, the valley is suddenly blocked by a barrier nearly 200 feet high, lying directly across the path of the stream. This barrier is the moraine of a little glacier terminating far up on the precipitous side of the valley. The moraine is so porous that the stream flows directly through it with no apparent check, although the gravel flood-plain above the barrier is broader than below.
Among the elevated basins of the Tian Shan plateau one of the largest is that of the Mudirum Su. Its upper portion is a desert of old morainic waste unrelieved by vegetation ; the lower portion is also full of moraines, but they are