130 G. F. WRIGHT LOESS IN NORTHERN CHINA AND CENTRAL ASIA 



we have been describing, and is about 30 miles from the base of the 

 bordering mountains, which limit the plain on the northwest, and about 

 80 miles inland from the sea. From near the border of the mountains 

 to the sea through Peking the slope of the surface is pretty uniform, 

 averaging about 6 feet to the mile, so that the surface of the loess at the 

 entrance to the pass at Nankau is about 600 feet above tide. The slope 

 from Peking to Nankau, however, is considerably greater than it is on 

 the other side toward the sea. Issuing from the pass at Nankau, a very 

 distinct delta extends out on the plain for a distance of between 5 and 

 6 miles. This delta consists of a rather confused intermingling of loess 

 with sand and gravel and occasional fragments of rock a foot or two in 

 diameter. This coarser material occurs near the surface as much as 

 4 miles outside the mouth of the gorge, the surface sloping to that dis- 

 tance in a direct line at the rate of 50 feet a mile, making 200 feet in the 

 first 4 miles ; but on the southwest side the descent is abrupt, leaving a 

 long low plain several miles wide between the delta and the mountains 

 in that direction. 



On the contrary, on the northeastern side the deposits of loess, at 

 nearly the same level with the head of the delta, stretch for many miles 

 along the base of the mountains toward the Ming tombs. In many 

 places here we passed between perpendicular sections of loess 15 to 20 

 feet in height. They were especially prominent in the vicinity of a 

 small stream coming down from the mountains about half way between 

 Nankau pass and the Ming tombs, a distance of about 10 miles ; but the 

 larger stream coming down from the mountains into the amphitheater 

 around which the Ming tombs are built has worn a broad deep channel 

 in the sedimentary deposits, and occupies a bed 50 or more feet below 

 the general level. This bed is thickly strewn with boulders several miles 

 away from ,the base of the mountain. The portion of one of these 

 boulders projecting out of the ground measured 9 by 6 by 3 feet. 



From the situation of these deposits, it would seem pretty clear that 

 they sustain a definite relation to the comparatively small streams com- 

 ing down into the plain from the mountains to the northwest. Of these 

 the Bishaho, which comes through Nankau pass, is the largest. But it 

 seems difficult to resist the conclusion that at the time of the deposit of 

 the deltas waterlevel was met at the base of the mountains at the eleva- 

 tion of 600 feet, which is that of the head of the delta spreading out from 

 the Nankau pass. It seems also clear that at the time of the main depo- 

 sition the material to which the stream had access was much more abun- 

 dant than it is now, for at the present time all these streams are rapidly 

 eroding material at these higher levels and transporting it to lower levels. 

 The Bishaho, having completely abandoned the line of its old delta, now 



