RECONNAISSANCE IN CENTRAL TURKESTAN. 167 
THE TERTIARY PENEPLAIN. 
The unity of Central Turkestan is shown not only in the wide extent of the 
various members of the rock series, but also in the extensive peneplain which 
truncates them. In all parts of the region there are numerous places where the 
surface of interstream areas presents a smooth, gentle slope quite out of hannony 
with the tilted strata which it truncates evenly without regard to whether they are 
hard or soft, and with the steep-sided \'alle)-s that are being cut in it. These areas 
are therefore regarded as uplifted and more or less dissected parts of a fonnerly 
low-lying j^eneplain of erosion. In the southern part of the Tian Shan plateau, 
for example, the large plateau basin containing Chadir Kul and the Ak Sai River 
and lying at a height of from 10,000 to 1 1,000 feet above the sea is bounded on the 
south by a broad ridge or swell rising to a height of from 13,000 to 14,000 feet. On 
the southern slope of the ridge there is a descent of 9,000 feet to the plain of the 
Kashgar basin in a distance of from 70 to 15 miles — that is, a descent of from 120 
to 600 feet per mile. This descent is sufficient to cause active erosion, which in 
due time will produce the most irregular topography with a ma.xinuun of relief; 
but the valleys of the south slope are not }et profound and the interstream areas, 
though very rugged, rise everywhere to the height of a nearly smooth imaginaiv' 
surface ascending from the Kashgar basin to the broad ridge which incloses it on 
the north. This surface is evident in the hard Paleozoic formations and can be 
detected even in the soft Tertiaries. A broad ridge with such a slope descending 
from it must soon become very rugged by reason of the headward erosion of the 
streams ; but here, especially in the eastern portion, the southern ridge of Tian 
Shan is still quite smooth and level, and its surface is indifferent to rock structure ; 
hence its elevation from the condition of a low-lying peneplain must be com- 
paratively recent. A smooth plain (plateau) of large extent stretches northward 
from the ridge, sloping at an average rate of about 100 feet per mile toward the 
Ak Sai basin and tnmcating the almost vertical slates and limestones of Paleozoic 
age (see fig. 124, p. 172, southern end). Not far to the west, in the district southeast 
of Chadir Kul, there are a number of eas)- passes across the same ridge, which 
there forms the Chinese boundar)-. Two of these passes, Kara Kennak and Kuzzil 
Kur, are in the soft upper members of the Tertiary series, although at an ele\ation 
of over 12,000 feet. 
It is evident that the smooth imaginary' surface to which the tops of the hills 
rise on the southern slope of the Tian Shan, above described, and its more actual 
continuation in the plateau, which tnmcates the Paleozoic .strata farther north, 
could not have been fonned by an)^ known process under the present conditions of 
altitude and drainage ; nor could the weak Tertian,- strata of the passes farther west 
have been long preserved in their present fonn at the elevation at which they now lie. 
In order to reduce the deformed strata to .so smooth a surface the Tian Shan region 
must have stood many thousand feet lower than now, until it reached a late mature 
or oldish stage of erosion, deser\'ing to be called a peneplain, over large areas. The 
present altitude of the region nuist be due to uplift and warping of the peneplain 
