DENUDATION 105 



burrows and the sites of rotting roots collapse, the main movement 

 of their bounding walls will be downhill. And indeed, settle- 

 ment of all kinds lubricated by rain and aided by the thrust of 

 frost will be in that direction. So the whole soil cap, except on 

 dead levels, will be gravitating downhill, the only check, and that 

 a temporary one, being provided by the tangle of root fibres. 



At the bottom of the slope flows the stream, with banks of 

 soil and subsoil, whether it reaches the rock below or no. The 

 banks are continually collapsing into the stream, and the material 

 is carried away to sea by it. Yet the stream gets no wider, because 

 the soil of its banks is continually travelling down towards it, 

 making good the loss. Thus disintegration through soil making is 

 steadily going on all over even the best cultivated parts of the land, 

 and throughout such areas this disintegration is providing the 

 streams with material to transport no less certainly than where 

 screes carry down the stones obviously broken from bare rock scars. 

 Only, in the latter case the whole process is easily seen, while in the 

 former the vegetation masks all movement and change, and the 

 landscape a thousand years hence may be undistinguishable from 

 that of to-day. Minute measurement, however, would show that 

 the entire area of the valley sides had been somewhat lowered in 

 the interval ; and material taken not from the river's course only, 

 but from its whole drainage area, would have been carried off by it 

 to the sea. 



This process can, of course, only be carried on so long as the 

 slope of the country down to the stream is sufficient to allow of 

 downward movement against friction and obstacles. Gradually 

 the slope will become more and more gentle ; movement will 

 become more and more slack, until everything comes practically 

 to a standstill. When this condition is reached the country will 

 be one of almost imperceptible slopes, and it will be said to have 

 been reduced to base-level. When this stage has been reached 

 denudation will cease, and it can only be renewed if gradients 

 are increased by an elevation of the land to a higher level. Such 

 base-levels may often be seen near the coast and less commonly 

 in flat plains inland. 



Part of the function of a stream is to deepen its valley by 

 means of the load which it rolls along. But while a small part 



