PHYSIOGRAPHY OF NORTHWESTERN CHINA. 259 
task, and yet its deep canyon-like valley and the ragged ridges which 
surround it showed that it had most effectively attacked the slope in 
which it had grown. 
When we stood on the summit of the divide between Chi-li and Shan-si 
and saw the features sketched in Plate XXX, it was clear that, within 
the record of the older phases of topography, the characters of the most 
recent uplift and canyon development were inscribed. The typical can- 
yons of the Fén-ho epoch in the Wu-t’ai-shan and in the limestone plateau 
south of Wu-t’ai-hién are clearly shown in Plates IV and XX. Their 
relation to the features of the T’ang-hién and Hin-chéu epochs is well 
expressed in the contours of atlas sheets EF I and D I, and may be seen 
in the photograph of the mountains south of Wu-t’ai-shan, atlas sheet C II. 
It may be noted that the canyons of the rivers flowing from the Loess 
Basins do not extend into the basins themselves; that is to say, features 
characteristic of the Fén-ho epoch are limited to the zone of mountain 
uplift, and this zone is thus differentiated in regard to relative movements 
from that of the Loess Basins. Where we find the mountains and canyons 
there has been relative upward movement; where we find only the features 
of the preceding epochs there has been relative downward movement, and 
the rivers which flow from the latter areas into the canyons across the 
former areas are considered to be antecedent streams which have held 
their courses. 
In describing the relation of the Hin-chéu basin to the basin of Huang- 
t’u-chai it was pointed out that the Shi-ling pass, which lies between the 
two, presents the characteristics of a wind gap, presumably from the 
diversion of the headwaters of the Fén-ho. These headwaters now con- 
stitute the Hu-t’o-ho, which is believed to have formerly flowed through 
the Shi-ling. The diversion is attributed to the differential movement 
of the Hin-chéu basin, which was downward, with reference to that of 
the Ki-chéu-shan and the Shi-ling, which was upward. ‘The face of the 
Ki-ché6u-shan toward the Hin-chéu basin is characterized by the features 
of a great fault-scarp, a typical development of Fén-ho time. The youth- 
fulness of the scarp is shown in the moderate development of consequent 
ravines and in the fact that ridges, formed during the earlier episodes 
of faulting, are cut off by the plane of later faulting. Associated with 
the growth of the fault-scarp, both on the northeast and southwest where 
it runs out, are warped surfaces on which one may still trace the features 
of the preceding epoch, now tilted, however, to an attitude in which they 
are being vigorously dissected. The canyons by which the antecedent 
streams leave the Loess Basins and flow across the mountains enter this 
warped surface and deepen gradually as they pass into it. At the Shi-ling 
the upwarp is covered with loess, which may be in part modern wind 
