of surface water, has been scooped out and sculptured by the same powerful east wind which is constantly at work in the Su-lo Ho basin into small ridges and trenches invariably running east to west.4 These reproduce the Yârdang formations of the Lou-lan area in the Lop Desert with striking fidelity, though on a smaller scale. The lesser depth of the scourings, usually between two and five feet, is accounted for by the fact that, as we shall presently see, the time that has elapsed since the disappearance of surface water and protective vegetation is here fully a thousand years less. On the other hand, the thickness of the surface layer of loess is here limited, and it is obviously due to this that I found it in places completely abraded and carried off right down to the underlying fine gravel. Much of the sand brought down originally from the detritus-covered hill range is coarse, and its corrasive action here must considerably aid deflation, i. e. wind-erosion pure and simple. The broad belt of eroded loess stretched away eastwards as far as the eye could reach, bare of all sand. But within the area of the old oasis surviving scrub has helped to retain small dunes in places, as already stated, and within the ruined town the protection still afforded by the massive walls has caused the drift-sand to accumulate in big dunes.
In connexion with the physical features just described it may be as well to state at once what Traces of
my survey of the ground, rapid as it had to be from want of time, allowed me to ascertain as to the ancient
water-supply once available for the ruined town and the cultivated area near it. That it could not have been derived from the spring-fed marshes, the drainage of which irrigates the present oasis of Chiao-tzti, is certain. A look at the ground was enough to show me that all these springs lie con siderably below the level of the area of deserted occupation. No clear trace of any stream could be seen from So-yang-ch`êng on the bare gravel glacis that slopes up for miles to the foot of the hill range. But when I was riding, towards the end of my visits, to the site across the wind-scoured ground to the east of the ruined town,6 my eye was caught by a low gravel-covered ridge coming from the south-east. It proved, as expected, to carry the line of an old canal with its banks still clearly marked on the top. The coarse sand and pebbles which the water used to bring down from the foot of the hills had necessarily in the course of time helped to raise the canal bed, as noticed in every oasis from Khotan to An-hsi. Subsequently, when the canal ceased to carry water, the same heavy deposits protected it from the force of the winds which have, since irrigation ceased, been continually cutting up and lowering the ground on either side. Thus the top of the canal banks has come to lie I0—I2 feet on the average above the level of the eroded ground on either side.
' The photograph reproduced in Desert Cathay, ii. Fig. zro, shows these very clearly.
° Fig. 238 shows a little of this in the background.
° I much regret that want of time and of transport suited to
the season prevented the extension of survey work in this direction and the clearing up of the topographical question involved. The delineation of the hills to the south-east of Ch'iao-tzû, as shown in Map No. 83. s, c. 4 from Rai Ram