282 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
with stratified and cross-bedded alluvial grit (fig. 462). Upstream the proportion 
of alluvium and size of its fragments steadily increase, and about 30 miles above 
Samarkand the river is flanked by a con- 
glomerate cliff 50 feet high, with an over- 
lying coat of loess as well as some thinner 
beds of it lower down. Another fact of 
interest over that same distance is the grad- 
ual transition of color from yellow loess 
at Samarkand to light-gray loess there. 
Fig. 460.—Section near the End of the Yarkich Glacier. Since the flood-plain is necessarily much 
wider at Samarkand, dust has to drift about more there before coming to rest 
as loess and thus has more chance to oxidize. 

THE TARIM BASIN. 
EVIDENCES OF PERIPHERAL UPLIFTS. 
Passing to the Tarim, western basin of the Gobi, we find its border ranges 
gashed by gorges with high terraces. The Kizil Su gorge of Tarim and what was 
seen of its tributary topography is so remarkably similar to that of the Markan 
Su that one can not help drawing the conclusion that this type is persistent through 
that region. Uppermost in both we find the old graded-down topography and 
uplifted piedmont gravel-plains, while between this and the present flood-plain 
there are various terraces, badly preserved because of the gorge’s narrowness. 
There are also massive remnants of an alluvial conglomerate several hundred feet 
thick, recording a refilling that took place, a backing-up of waste into the graded 
widths established after the first uplift and before the second. The region of 
Aikart Pass, on the Markan Su route to Kashgar, is a massive piedmont formation 
of half-consolidated gravels from which project some remnant peaks of the under- 
lying highly-tilted red gypsiferous series. It appears to be a piedmont series, 
laid down during the long preglacial erosion that resulted in the old topography 
referred to. The whole is now dissected by well-developed valley systems to a 
depth of over 4,000 feet, having been refilled with 200 feet or more of waste, after- 
wards reexcavated. Some idea of the amount of silt in these streams was obtained 
in seeing two basins over half a mile wide, that were formed by landslides only 
thirty years ago, but now filled with red silt. 
At about 25 miles east of Aikart Pass the Aikart valley is confronted by a 
high transverse fault-scarp of uptilted piedmont. After making a short bend 
it cuts through this with a narrower and flat-bottomed valley with vertical sides, 
exposing a piedmont conglomerate inclining gently east downstream. In the 
mountain valley above there has been a broad refilling terrace cut on one side by a 
narrow gorge 220 feet deep. But in the uptilted piedmont those traditions do 
not hold and we have a flat-bottomed channel with narrow terraces rising, perhaps, 
30 feet above stream and converging with it downwards. 
