PHYSIOGRAPHY OF NORTHWESTERN CHINA. 219 
of the rate of the process, which is of much interest in the study of the 
physiographic history of the region. The conditions of retreat are very 
favorable at this point, since the work of the stream and the activities of 
the natives in widening their fields in the valley have combined to hasten 
the recession. 
The brooks which occupy this old valley are comparatively insig- 
nificant. Their drainage area is circumscribed by the summits of the 
immediately surrounding ridges. They are autogenous streams, occupying 
the valley of a once much larger river, probably of the Hu-t’o-ho. 
Below Huang-t’u-chai the route follows the broad valley of the north- 
ern branch of the Fén-ho and reaches the city of T’ai-ytian-fu, which lies 
partly within reach of the floods of the river, partly on the low slopes of 
Huang-t’u, stretching toward the eastern hills. Off to the northwest the 
valley is bounded by a bold escarpment of Sinian limestones, at which the 
river emerges from a picturesque canyon. The view from the summit of 
the escarpment discloses a deeply cut plateau, which exhibits an upper, 
maturely eroded surface traversed by youthful canyons. 
The basin of T’ai-yiian-fu may be called the sixth and most southern 
of the northern loess basins. It is connected with the Hin-chéu basin 
by the old river valley of Huang-t’u-chai, and the two lie with their major 
axes nearly parallel and en échelon, the southern one being offset to the 
southeast. This is a characteristic of the structural forms of the great arc 
of normal faults to which these downwarps belong. 
The T’ai-yiian-fu basin is much larger than any of those to the north 
of it. It is 70 miles, 110 kilometers, in length from northeast to south- 
west, approximately 25 miles, 40 kilometers, across, and has an area 
approaching 2,000 square miles. Had we detailed maps of its surface and 
of the surrounding mountains, we would no doubt find expressive features 
in which to read the details of its history; but the general phenomena, 
so far as we were able to observe them, do not appear to differ from those 
of the Hin-chéu basin. Along the northwestern margin of the plain the 
mountains rise with a moderately steep, but nowhere precipitous, face 
and are cut by well-developed valleys at right angles to their trend. 
Details of form vary with the character of the limestone, shale, or sand- 
stone, but in general a similar topographic phase extends from the 
plain to the summits of the hills and there is a unity of development in 
the slope which contrasts with the younger transverse ravines. Low 
hills forming the points of the spurs just above the plain are similar 
in character to higher hills which rise from the upper slopes. The whole 
presents the aspect of a surface developed under uniform conditions— 
that is, at a common altitude—and which has since been warped, so 
that some parts of it now lie along the margins of the plain, while other 
