DENUDATION 339 



have been deposited after the folding movement was accom- 

 plished. If the newest folded strata and the oldest unmoved 

 strata be of successive geological periods, the date of the upheaval 

 is placed between those two periods and said to close the older 

 one for the particular region involved. The subsequent history 

 of a mountain range after its final upheaval above the sea must be 

 read in its denudation and in the evolution of its topography and 

 drainage. 



Denudation of Mountains 



Mountains as we see them are never in the shape which they 

 would present, were the forces of compression and upheaval alone 

 concerned in their formation. Every mountain range has been 

 profoundly affected by the agencies of denudation, and their 

 ridges and peaks, their cliffs and valleys, have been carved out of 

 swelling folds and domes, or angular, tilted fault blocks. As 

 upheaval is a slow process, denudation must have begun its work 

 as soon as the crests of the folds made their appearance above 

 the sea, or above the level of the ground, so that probably no 

 range ever had the full height which the strata, if free from de- 

 nudation, would have given to it. Upheaval, though sometimes 

 slow enough to allow rivers to keep their channels open, is yet too 

 rapid to be kept in check by the processes of general atmospheric 

 weathering, and so the ranges grew into great uplifts. But as soon 

 as the movement of elevation ceased, denudation began to get the 

 upper hand, for as we have learned, mountains are the scene of 

 exceptionally rapid erosion. The steepness of their sides gives 

 great power to the streams which course down them, they cause 

 the discharge of the atmospheric moisture in rain or snow, they 

 are terribly riven by the frost, and they are frequently cut and 

 gashed by glaciers. For a long period the effect of denudation 

 is to greatly increase the ruggedness of the mountains, carving 

 folds into ridges and cliffs, and ridges into bold and inaccessible 

 peaks, but sooner or later the mountains are worn down lower 

 and lower, and are eventually levelled with the plains from which 

 they spring. In the process of degradation, the synclines often 



