the importance of the territory already in ancient times is amply attested by archaeological remains and historical records alike. If we compare this fact with the great extent of that portion of the cultivated area which now, as the map shows, depends solely on Karèz irrigation, the progress made by desiccation in this region receives striking illustration.
SECTION V.—THE SU-LO-HO BASIN
East of the Lop depression our surveys extended to a series of drainageless regions which, if not comparable in size to the Tarim basin, yet resemble it
Extent of Sn-lo-ho closely in various physical features of importance and are linked up
with it also in historical interest. Proceeding from the extreme eastern end of the Lop depression across a dry lake-bed once apparently draining into it, I we enter directly the lowest portion of the basin of the Su-to-ho, containing this river's delta and terminal marshes. 2 This basin, as shown on Sheets Nos. 35, 38, 40, extends for some 220 miles from east to west with an average width of 30-40 miles between the foot hills of the ranges which bound it. These are the western Nan-shan in the south and the utterly barren Pei-shan northward.
The Su-to-ho, a considerable river, fed by glaciers and eternal snows of the Central Nan-shan, descends into the basin at its eastern end. After
Course of Su-lo-ho. breaking through a succession of ranges in gorges, of which those to
the south are as yet unexplored, 3 it skirts a low divide separating its basin from one much smaller adjoining eastwards. Below the oasis of Yii-mên-hsien the Su-lo-ho turns sharply westwards. Maintaining this course it passes through the Khara-nôr lake, supposed to be its terminal basin until our surveys proved this to he situated some 80 miles further west in the salt marshes already referred to. 4 On its whole course through the basin the Su-lo-ho receives only one affluent, the Tang-ho or Tun-huang river, rising on high plateaus towards Tsaidam. This provides ample irrigation for the large oasis of Tun-huang, or Sha-chou, which occupies its alluvial fan.
Tun-huang is the only settlement of considerable size in the whole region, and its local resources were of special value in ancient times when the great natural
Importance of Tun passage of the Su-lo-ho basin served as the earliest high road of Chinese
expansion into Central Asia. 6 It was for the protection of this important trade-route leading through Lou-lan to the oases of the Tarim basin that the far-flung westernmost portion of the ancient Chinese border line was constructed towards the second century B. C. 6 The extreme aridity of the desert ground, over which it led past the Su-lo-ho and its riverine marshes, explains the abundance and remarkable preservation of the ancient records, etc., brought to light by my explorations along this ruined Limes.
Excepting the narrow belt of desert vegetation which accompanies the river's lower
course and the limited ground capable of irrigation beside the oasis of
Tun-huang, the Su-lo-ho basin consists mainly of slopes of absolutely
bare gravel, corresponding to the ` Sai' of the K`un-lun. Immediately
I See above p. 31.
1 See Sheet No. 35. B, C. 4. See Sheet No. 41. B. 1, C. 1-3.
6 Cf. Serindda, ii. pp. 578 sqq. on the `Geographical features of the lower Sn-lo-ho basin'.
6 The main facts concerning this extension of
the ancient Chinese Limes along the Su-lo-ho basin, as elucidated by my explorations of 1907, are summarized in Chapter XX of Serindia, ii. pp. 722 sqq.
7 See Sheets No. 36. C, D. 1; 38. B. 4; 39: A, B. 1; see also No. 86. B. 4 for the big sand ridges skirting the terminal bed of the Sn-lo.ho.