SUBAERIAL AGENTS 309 



the open valleys where it intersects softer and more destructible 

 rocks. 



Rivers also produce changes in topography by constructional 

 processes., as in their flood plains and terraces, processes which 

 are most notable in the lower parts of the course, and which gain 

 increased efficiency through a subsidence of the region. 



Degradation is most rapid on the hillsides which border river 

 valleys, because of the removal of waste by the rivers. Away 

 from the streams the denudation of the country is much slower, 

 because the waste is less readily removed. Those points will 

 longest remain standing above the general level which are com- 

 posed of the hardest rocks and are farthest removed from the 

 principal lines of drainage. 



The subaerial agencies act with the greater efficiency the more 

 elevated the region upon which they operate. Consequently, so 

 long as the region be not again elevated, denudation operates at 

 a continually . diminishing rate. The strong relief of hill and 

 valley is carved out with comparative rapidity, but the more 

 nearly the country is reduced to base-level, the more slowly does 

 degradation proceed, and the final stages of base-levelling must be 

 exceedingly slow. Nevertheless, if no renewed upheaval takes 

 place, the loftiest and most rugged land surface must be eventu- 

 ally cut down to that level. The universal and permanent base- 

 level is, of course, the sea; but other local and temporary 

 base-levels may for a time control the development of certain 

 areas. Tributaries cannot cut below the main stream into which 

 they flow ; a lake forms the base-level for the streams which sup- 

 ply it, until the lake is removed by draining away or being filled 

 with sediment. Regions, like the Great Basin, whose drainage 

 finds no outlet, may have base-levels either above or below the 

 level of the sea; e.g. the surface of the Dead Sea of Palestine is 

 1308 feet below the Mediterranean. 



It is perhaps a question whether any large region has ever 

 remained stationary (with reference to the sea-level) for a suffi- 

 ciently long time to be absolutely base-levelled. On the other 

 hand, there is abundant evidence to show that such areas have 



