178 EXPLORATIONS IN TURKESTAN. 
LOESS. 
The plavas of the Kashgar plain are connected with the interestinj^ geolojjical 
problem of the origin of loess. The deposits of the playas greatly resemble 
certain older dci^osits, having all the typical characteristics of loess; and a com- 
parison of the two at once raises the question whether loess may not be in certain 
cases an aqneons deposit, formed on the flat floor of basins or aggraded valleys 
where streams laden with the very finest silt spread out into thin ephemeral sheets. 
In a previons section mention was made of an anticline of very recent date lying 
south of the village of Artush, near Kashgar. It was stated that this anticline is 
composed of gravel interstratfied with a light yellow material, which is there termed 
silt becau.se of its relation to the gravel, but which has all the characteristics of 
loess. If it occurred without the gravel it would at once be pronounced loess. 
South of the anticline, near the city of Kashgar, the whole country- is composed 
of what looks like typical loess. It stands in i)eq:)endicular walls wherever it is 
dissected, and deep trenches are worn in it by the roads; everywhere a close 
examination of the loess walls shows a faint banding ; slightly sandy lasers and, 
occasionally, little lenses of fine gravel are found interbcdded with the silt. Further 
west in similar deposits heavy gravel overlies and is interstratified with layers pos- 
sessing the essential characteristics of loess, although the}' can hardly be of jeolian 
origin. In two other basins, those of Fergana and Issik Kul, deposits of loess were 
seen, which included gravel-filled channels. These facts suggest that loess may be 
a playa formation and that the Kashgar basin may be a place where loess is still in 
process of deposition. 
THE ALAI MOUNTAINS. 
Of the two remaining physiogra]ihic pro\-inccs little need be said, for in essen- 
tial features they are repetitions of the Tian Shan plateau and the Kashgar basin. 
The Alai province includes not only the Alai Mountains proper, which run east 
and west between Fergana and the Pamir, but also the cross-ridge which runs 
northea.st from the Pamir to the Tian Shan plateau, with some peaks rising to a 
height of 18,000 feet. The Alai range is a portion of the old peneplain tiplifted 
thousands of feet into an arch. It is round on top instead of being somewhat 
corrugated like the Tian Shan plateau. Its width is nuich less than that of the 
latter, and it lacks the broad upland basins of warped peneplain, which are so 
characteristic of the Tian Shan plateau. The Alai has, to be sure, a series of small 
valley basins on the north and the great Alai basin on the south, but these are 
all chiefly due to modeni erosion on weak strata that were infolded before the 
completion of the Tertiary peneplain, (xood examples of the small basins are 
seen on the Terek Su at Guristan, on the Ak Bura at Bopan, and on the Ispairan 
at Pum. All of the basins appear to be places where soft strata had been faulted 
down previous to the completion of the Tertiar}- peneplain ; hence, before the uplift 
of the peneplain, the down-faulted weak strata were inaccessible to the processes of 
erosion. Since the uplift, deep valleys with broad flood-plains have been eroded in 
the weak strata, and the surrounding country has been reduced to the stage of 
mature relief with thoroughly graded slopes. In the more resistant limestone 
