174 EXPLORATIONS IN TURKESTAN.
well covered with grass and by contrast seem fertile. The mountains on the southern border of the basin reach a height of from 15,000 to 1 7,000 feet and have been carved into truly Alpine forms by numerous large glaciers. Another basin is that of the Ak Sai and Chadir Kul (Tent Lake), where glaciation has played a relatively small part. The ridge south of this basin forms the Chinese border and has already been described. The valleys descending thence to the Kashgar basin are of the same nature as those on the north slope of the plateau toward Issik Kul. They are cut in somewhat softer strata, however, and hence are wider, and for the saine reason the interstream areas are more dissected into sharp hills. Vegetation is almost absent because of the dryness of the climate, and the older contorted limestones and slates stand naked in black and gray, while the later strata are bright with red, pink, and green.
Chadir ,Kiel.—The lake of Chadir Kul, at the head of the Ak Sai basin, near the southern side of the Tian Shan plateau, is a small sheet of water about 16 miles long by 6 wide. It is in the midst of a barren, mountain-girt plain, and does not overflow, in spite of the snowy heights that surround it and of a drainage area which, according to the Russian maps, is five times as large as the lake itself. This has not always been the case, however. The plain of Chadir Kul is composed largely of fine silt which could hardly have been deposited by aggrading streams so near their mountainous headwater area, but which might easily have been deposited in a lake. In one place a cut some 15 feet deep along the side of a brook discloses fine silt full of sphagnum, on which rests a little gravel. The lake formerly had an outlet at the head of the Ak Sai basin, where the plain contracts to a distinct channel about a third of a mile wide, bounded on either side by a terrace 4o or 5o feet high. Across the mouth of this outlet lies a little ridge of sand and gravel io or 15 feet high, apparently an abandoned and dissected beach. It does not extend quite to the northern terrace, being separated from it by an open gap of 200 or 300 feet. Apparently the outlet was first stable long enough to allow the cutting of the broad valley and the terraces on either side. Then a change of some sort caused the building of a beach and the partial closing of the outlet, through which, however, a stream still ran for a time before another change caused the lake to retire to its present level. Around Chadir Kul itself nothing was seen to show what these changes were and why they occurred. From the evidence of other places, which will be discussed later, the changes seem referable to alternate expansions and contractions of the lake under the influence of glacial epochs and inter-glacial epochs. The outlet of the lake is later than at least one epoch of glacial action, for while the terrace on the south side of the broad channel is composed of ordinary gravel, the other terrace consists partly of moraine stuff full of bowlders of schistose slate ranging tip to 3 or 4 feet in diameter. This must have come from the valleys just to the north, where there are other moraines, and it may have blocked the outlet and caused the lake to expand. The lake as a whole, however, seems to be due to a slight swell or bulge in the basin floor between the Ak Sai basin and its continuation in that of Chadir Kul.