Indeed, the evidences, so far as they have been examined, appear to warrant the conclusion that the region of the western Tian-shan, from Lake Issyk-kul southwards, was in great part the scene of probably five successive glacial periods, each being less severe than the period which immediately preceded it.
After great suffering Hsiian Tsang reached Igu, the seat of a Turkish principality, and pursued his way along the southern foot of the T'ian-shan, which he crossed by a glacier pass (vividly described) in the longitude of Lake Issyk-kul.
From Issyk-kul there is a sharp rise of6000-9000ft.
This last is continued without a break past the western end of Issyk-kul, being directly prolonged by the Alexander Mountains, although parted from them by the gorge of Buam or Bom, through which the Issyk-kul probably once drained.
It forms the barrier between the Issyk-kul and Balkash lakes, the elevation of which is about 5000 ft.
Above the Issyk-kul and lifts its summits higher than 13,000 ft.
Dzungaria, Kulja, Issyk-kul, Ferghana) for a considerable distance towards the east, greatly facilitate access to the loftier plateau lands of Central Asia, and have from time immemorial been the highways of human intercourse between East and West.
Three of these are in the government of Semiryechensk in Central Asia, all belonging to the Tianshan system: - (r) the Terskei Ala-tau, south of and parallel to the lake of Issyk-kul; (2) the Kunghei Ala-tau, and (3) the Trans-Ili Ala-tau, both N.
South of Lake Issyk-kul, which appears to be a hollow of tectonic origin, runs the Terskei Ala-tau, separating the lake from the high valley of the Naryn.
Again, while the Ili (Kulja) valle y lies at 1300 ft., the Issyk-kul has an altitude of 5300 ft., the Koshkar basin, in which the river Chu has its source, reaches 6070 ft., the Son-kul valley 9430 ft., the Ak-sai valley, farther east, 10,000 to 11,150 ft., and the Chatyr-kul on the north side of the Terek Mountains 11,200 ft.