92 NOTES ON INDIVIDUAL MAP SHEETS [Chap. IV
height 2,391 for the western edge of that basin, are derived from this levelling operation. The height of Besh-toghrak (2,340), obtained as the mean of several observations, was accepted as datum point. 208 The record of levels, as shown in Appendix C, proves a continuously descending slope from the ancient lacustrine basin to the dried-up Lop sea bed.
A descriptive account of the ground along the caravan track, as seen on my first passage, is given in Desert Cathay, i. pp. 525 sqq.; ii. pp. 1 sqq., and a summary of its characteristic features in Serindia, ii. pp. 549 sqq.; cf. also Geogr. Journal, 1916, xlviii. pp. 129 sq. The topography of the ground south of the Su-to-ho, along the ancient Chinese border line or Limes (C,D.4), and of that on its western flank which the terminal - marsh basin of the Su-lo-ho served to protect (C.4), has been touched upon passim in the account given of my fruitful archaeological explorations on this Limes portion, in Desert Cathay, ii. pp. 92-158. The physical character and historical topography of the ground along the several sections of the Limes has been discussed in the detailed record of those explorations, in Serinclia, ii. Chapters xvll-xix; see in particular pp. 683 sqq., 656 sq., 662 sqq., 693 sqq., 705 sqq. There frequent reférenee has been also made to the evidence furnished by the ancient remains and records regarding the physical conditions, water levels, etc., prevailing here during the first centuries before and after Christ.
Though limited in extent and fairly uniform in character, the surveyed area of this sheet possesses special geographical interest. It comprises the wide trough of the terminal course of the Su-to-ho, separating the southernmost Kuruk-tagh from the gravel glacis of the easternmost Altin-tagh.
Astronomically observed latitudes.
To the north of the marshy depression (B,C. 4) in which the present bed of the river ends, we have other branches, now dry, of an ancient delta descending into an earlier terminal basin ; the lacustrine character of this is clearly marked by its strings of Mesas. 21
Immediately to the west of this basin, the ` Su-lo-ho trough' is continued in the previously mentioned valley leading down from Besh-toghrak to the eastern bay of the dried-up Lop sea bed. There is evidence supporting the, belief that through this northern basin the Su-lo-ho drainage reached the ancient Lop sea until a recent geological period. 22 It is probable also that percolation from it, even within historical times, helped to facilitate the use of the Béshtoghrak valley for the early Chinese route towards Lou-Ian. It certainly accounts for the desert vegetation to be found today in this valley, notwithstanding the utter barrenness of the Kuruk-tagh on one side of it, and of the high ridges of drift-sand on the other.
Along the Su-lo-ho course and the line of spring-fed marshes which accompany it, such vegetation is abundant. But the configuration of the ground precludes irrigation, and the evidence of the plentiful documents recovered from the ruined watch-stations of the Limes makes it certain that already in ancient times no cultivation existed along this desolate border-line.
Corrections. A. 4. R. B. Làl Singh's Camps 50 and 51 of 1913 should be shown eire. 6 miles S.W. of Kosh-kuduk and cire. 7 miles S.W. of Yantak-kuduk, respectively. Against Camp 50 of 1913 should be added the height 2,500.
C. 4. The height of Toghrak-bulak should be corrected into 2,837 (see Appendix B) :
1906-08. Besh-toghrak, Camp 152 (at well ; B. 4)
Toghrak-bulak, Camp 154 (on left bank of river bed; C. 4) .
20$ It must, however, be noted that the mercurial barometer observation taken in 1913 indicated for 136sh-toghrak a height of 2,010. ft. only; cf. below Appendix B.
21 Exactly corresponding strings of Mesas are found at the end of the narrow plateau-tongues jutting ont into the actual terminal basin of the Su-lo-ho (C. 4), as correctly shown in the ' Detailed Map of the ancient Chinese Limes', 3 miles to 1 inch, in Plan
83 of Serindia, vol. iii.
I regret that the representation of this feature, characteristic also of other lacustrine basins along the Su-to-ho, e. q., the Khara-nbr and the lagoons below it (Sheet No. 38. A. 4), has been omitted in the small-scale map. The origin of these Mesas has been ex. plained in,Serindia, ii. pp. 576, 589, 642.
s2 See Serindia, ii. pp. 551 sq.