

HISTORY OF LANDSCAPE 181 



but their gradients are steep, and rapids often occur in their 

 course. 



At any given period the maturity of a river will express the 

 difference between the power of the river itself and the activity 

 of the lateral agencies. In rainless countries lateral agencies, 

 such as rain and frost, are in abeyance on a river's course, though 

 the river may bring water and stones from distant mountains, 

 sufficient to carve out a course which may remain gorge-like for 

 a long period. The rapid torrent of a mountain district will also 

 easily outstrip lateral agents, until its gradient has been lessened 

 and its erosive power diminished In the low grounds the gently 

 flowing river is doing little or no downward cutting, and all the 

 work is that of the lateral agencies. 



As, however, the valley widens and its slopes become more 

 gradual, the travel of soil and subsoil slackens, and finally it stops : 

 friction is too great and erosion has come to a standstill. The 

 river gradient is too gentle to give the water sufficient velocity 

 to carry any denuded material at all ; the slope of its flanks too 

 gentle to provide it with any new material to carry or use as an 

 erosive agent. The system has passed through maturity to old 

 age and death. A country in which rivers have reached this 

 stage is said to be at base-level ; its slopes have become so gentle 

 that they can hardly be distinguished from a plain, or from the 

 plane which is the ultimate result of marine denudation. It will 

 not be reduced absolutely to sea-level, but very nearly, and no 

 further denudation can take place on it unless some important 

 change occurs. It is at the end of the cycle of denudation, 

 which began when its rocks were first lifted above the area where 

 they were formed, when the first rain fell on it and the first 

 water flowed off it. 



If now the region were lifted or tilted, the streams would be 

 rejuvenated and would begin to flow with renewed velocity. In 

 consequence they would probably, as a general rule, start afresh 

 in their old courses and would begin to deepen their old valleys 

 again. As they did so they would first have to remove and carry 

 out to sea the bulk of their gravel and alluvia, before reaching 

 down to the solid rock below. It is extremely probable that 

 patches or terraces of these materials would be left behind, which 



