STUDIES IN THE SIERRA^ 



By John Muir 



NO. V. POST-GLACIAL DENUDATION 



WHEN Nature lifted the ice-sheet from the mountains 

 she may well be said not to have turned a new leaf, but 

 to have made a new one of the old. Throughout the unnum- 

 bered seasons of the glacial epoch the range lay buried, 

 crushed, and sunless. In the stupendous denudation to which 

 it was then subjected, all its pre-glacial features disappeared. 

 Plants, animals, and landscapes were wiped from its flanks 

 like drawings from a blackboard, and the vast page left 

 smooth and clean, to be repictured with young life and the 

 varied and beautiful inscriptions of water, snow, and the at- 

 mosphere. 



The variability in hardness, structure, and mineralogical 

 composition of the rocks forming the present surface of the 

 range has given rise to irregularities in the amount of post- 

 glacial denudation effected in different portions, and these ir- 

 regularities have been greatly multiplied and augmented by 

 differences in the kind and intensity of the denuding forces, 

 and in the length of time that different portions of the range 

 have been exposed to their action. The summits have re- 

 ceived more snow, the foothills more rain, while the middle 

 region has been variably acted upon by both of these agents. 

 Again, different portions are denuded in a greater or less de- 

 gree according to their relations to level. The bottoms of 

 trunk valleys are swept by powerful rivers, the branches by 

 creeks and rills, while the intervening plateaus and ridges are 

 acted upon only by thin, feeble currents, silent and nearly in- 

 visible. Again some portions of the range are subjected every 

 winter to the scouring action of avalanches, while others are 

 entirely beyond the range of such action. But the most influen- 

 tial of the general causes that have conspired to produce ir- 



* Reprinted, as revised by the author, from the Overland Monthly of November, 1874. 



