190 RESEARCH IN CHINA. 
immediate cause of the damming of the valley, as there is no rock sill 
at the outlet. The ravine at that point is filled with torrential wash and 
loess, constituting a typical deposit of stream-laid Huang-t’u (Fig. B, 
Plate XXV), and at the present time the little brook that flows northeast- 
ward to this point and then turns to the west to empty into the pond is 
building its alluvial cone across the valley. Thus the immediate condition 
is that of a valley dammed by the wash of a tributary stream. Within this 
basin the streams are now carried in elevated canals, and the waters are 
thus ponded at a level such that they can be used to irrigate the plain, 
which is in a high state of cultivation. 
On both sides of the inclosed basin, which has just been described, 
northwest of Wu-t’ai-hién, are ridges which carry rounded hill-tops. The 
Huang-t’u formation lies upon the slopes of the hills, except where they are 
too steep, and extends over the saddles between them. It is not generally 
present on the summits, which are scoured by the driving winds. Yet 
there are hill-tops on which it occurs to a thickness of 10 to 20 feet, 3 to 6 
meters, and of such extent that it is terraced and cultivated. ‘These 
isolated patches resemble a capping of sandstone on a shale hill. The 
topographic form is that of a mesa, and the inference that we draw from it 
is that the loess cap is but a remnant of a once more extensive formation. 
Neither wind nor water could deposit the material in the situation in which 
it occurs, unless the adjacent valleys were likewise filled to a depth of 300 
feet, go meters, above their present floors. We regard these mesa-like 
cappings as remnants, which indicate an ancient level of the upper surface 
of the Huang-t’u formation, and infer that since that condition prevailed 
there has been extensive erosion. 
The valley of the Sing-ho below Wu-t’ai-hién presents very interesting 
occurrences of the Huang-t’u formation. On the northeastern side of the 
river is a terrace 125 to 150 feet, 35 to 45 meters, high, which is com- 
posed entirely of Huang-t’u. The material is nearly all fine loess and 
exhibits indistinct stratification, as though deposited in still water. It 
also shows the usual vertical cleavage. From this terrace the surface 
curves gradually upward to the near-by hills and becomes tangent to their 
slopes. On the left or northeastern bank the Huang-t’u continues for 5 
miles, 8 kilometers, below Wu-t’ai-hién, and forms the northeastern side of 
the canyon for nearly 3 miles, 5 kilometers. The opposing slope through- 
out that distance is a steep surface of slate and limestone about 1,200 feet, 
350 meters, high. While the wide valley about Wu-t’ai-hién belongs to a 
mature physiographic phase, the canyon is a feature of a younger and 
more recent phase—a feature due to warping. The deposit of Huang-t’u 
opposite the wall of slate and limestone can only have been made since 
