PHYSIOGRAPHY OF NORTHWESTERN CHINA. 205 
erosion, whose bases are buried under deposits foreign to their own develop- 
ment. Where gullies debouch from the hills upon the plains, one finds 
a gulch cut in the loess and sees the gravel from the hills and the loess 
interstratified in its sides. We have here the meeting place of detritus 
brought from above during the season of torrential rain, and dust blown 
from the plains during the season of drought and naked surface. 
Along the foothills north of T’ang-hién and in the valleys northeast 
and southwest of Si-ta-yang, there is a plain which lies at an altitude of 
600 feet, 180 meters, between T’ang-hién and Nan-t’ang-mei, and appears 
to slope southwestward so that between the T’ang-ho and Sha-ho it is 
but 400 feet, 120 meters, above sea. This plain is thinly covered with 
Huang-t’u, which only partly conceals the flat surface of gneiss and lime- 
stone underlying. At the present time the activities of streams are cutting 
deep headwater gulches, and in some instances they have reached through 
an outer range of hills and attacked the plain, as may be seen northwest of 
T’ang-hién. This feature is a valley plain of erosion developed by some 
river. The stream apparently flowed from the northeast, passing between 
T’ang-hién and Nan-t’ang-mei, across the course of the T’ang-ho at Si-ta- 
yang, and thence southward. The abandoned valley lies along the contact 
of soft gneiss with the harder Ta-yang limestone, and thus appears to 
have been adjusted to the rocks, whereas the course of the existing 
T’ang-ho is squarely across the strata, in a canyon which bears no relation 
to the structure. 
The Ning-shan district.*—The name Ning-shan of the principal town, 8 
miles, 13 kilometers, northwest of Si-ta-yang, is applied to a district char- 
acterized by two mountain ranges, between which is a synclinal valley 
floored by Shan-si coal-measures. The mountains, extending from more 
continuous ranges toward the northeast, are cut by the T’ang-ho, by 
branches of the Hang-ho, and by the Sha-ho, into separate groups, the 
rivers flowing across the trend of the ranges. The peaks, which rise 1,500 
to 2,000 feet, 450 to 600 meters, above sea, are bold and sharp in form, 
and near the rivers have steep canyon-like slopes. The town of Ning-shan 
lies in a relatively broad valley, which trends from northeast to southwest 
toward the Sha-ho. A transverse section of the valley from northwest to 
southeast shows that it is developed on the syncline, but exhibits, adjacent 
to the river and for a space of a mile or more on either side of it, a terrace 
at an altitude of 500 to 600 feet, 150 to 180 meters, above sea, or 100 feet, 
30 meters, above the stream. This terrace is planed on the upturned 
edges of the coal-measures, and is thinly covered with Huang-t’u. In 
character and position it exactly parallels the similar plain observed 
between T’ang-hién and Nan-t’ang-mei, and presumably represents the 
* Atlas sheet F I. 
