EROSION HISTORY OF THE YANGTZE RIVER 
131 
through similar rock walls, and because they have been unable 
to keep pace with the master stream they are speeded up in 
their lower courses, forming rapids or falls, many of which are 
impassible at any stage of the water. 
The interpretation offered for the region just described is that 
the tops of the ridges represent a former erosion surface, over 
which the streams were flowing. Their present courses were ac- 
quired at that time and an elevation of the land surface took 
place. The streams continued in the channels already formed, 
and so were forced to cut through the ridges. Owing to the in- 
creased gradient due to elevation the inter-ridge areas were rap- 
idly eroded, especially along the axes of the anticlines, some of 
which have been completely broken down to depths of several 
hundred feet, leaving the limbs projected a mile or more apart, 
with open valleys between. The elevation of this old plain is 
about 1,700 feet above the sea, and it is 1,500 miles inland, about 
400 to 500 miles farther in than the gorge intermediate plain 
above mentioned, which has an elevation of about 2,200 feet. 
It is apparent frem these figures that if the river is following 
its former course, the two plains are not of the same age, unless 
there has been differential warping to account for the discrep- 
ancy of 500 feet. Only detailed work in the field will determine 
whether the two plains are really two or one. 
At the city of Suifu, a little less than 100 feet above the pres- 
ent river there is a terrace which gives undoubted evidence of 
having been the bed of the river at a higher stage. The plain 
stretches back from the river to a width varying from half a 
mile to mere than a mile. It is so thickly strewn with water 
worn cobbles and bowlders that over wide areas it has not been 
cleared for use. In places small patches have been cleared and 
the rocks heaped into high walls. Such plots are rich, and yield 
good crops. 
