CH. XVIII THE VALLEY OF KARA-SHAHR 277
of oases which stretches along the foot of the Tien-shan, and which from ancient times to the present day has served for the great northern highroad of the Tarim basin. But Korla is also the point nearest to the great valley of Kara-shahr which debouches within half a day's march from it. This valley descends wide open from the great plateaux of Yulduz at its head, favourite grazing-grounds of nomadic races from Huns down to the Mongols of the present day, and has at all times formed the easiest approach for nomadic raids and invasion.
The Kara-shahr valley at its southern end and near the town from which it takes its name widens out into a subsidiary basin occupied for the most part by the large lake known as Baghrash-köl. It is from this lake, which acts as a big natural reservoir for the waters of the Kaidu-gol or Kara-shahr river, that Korla derives its abundant supply of irrigation, and the Konche-darya that considerable and constant volume of water which enabled it in ancient times to act as a main feeder of the `Dry River' for the greater part of the year.
For reasons probably connected with its present, mainly Mongol, population the naturally fertile area fringing the Baghrash lake holds nowadays but comparatively little cul-
E tivation. But as the Chinese records show, it was different
in ancient times when this territory of Yen-ch`i was of eco-
1 nomic and political importance. There is evidence of this
to be seen in the great extent of the ruined circumvallation known as Baghdad-shahri which marks the position of the old capital close to the northern shore of the lake. There the effect of subsoil moisture impregnated with salts, together with atmospheric conditions less dry than in the Tarim basin proper, has completely destroyed all structural
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