Studies in the Sierra 



309 



these are so considerably obscured by post-glacial erosion, and 

 by a growth of forests, underbrush, and weeds, that only the 

 patient and educated eye will be able to recognize them be- 

 neath so many veils. 



The ice-sheet of the glacial period, like an immense sponge, 

 wiped the Sierra bare of all pre-glacial surface inscriptions, 

 and wrote its own history upon the ample page. We may read 

 the letter-pages of friends when written over and over, if we 

 are intimately acquainted with their handwriting, and under 

 the same conditions we may read Nature's writings on the 

 stone pages of the mountains. Glacial history upon the sum- 

 mit of the Sierra page is clear, and the farther we descend, the 

 more we find its inscriptions crossed and recrossed with the 

 records of other agents. Dews have dimmed it, torrents have 

 scrawled it here and there, and the earthquake and avalanche 

 have covered and erased many a delicate line. Groves and 

 meadows, forests and fields, darken and confuse its more en- 

 during characters along the bottom, until only the laborious 

 student can decipher even the most emphasized passages of the 

 original manuscript. 



METHODS OF GLACIAL DENUDATION 



All geologists recognize the fact that glaciers wear away the 

 rocks over which they move, but great vagueness prevails as to 

 the size of the fragments, their abundance, and the way in 

 which the glacial energy expends itself in detaching and carry- 

 ing them away. And, if possible, still greater vagueness pre- 

 vails as to the forms of the rocks and valleys resulting from 

 erosion. This is not to be wondered at when we consider how 

 recently glacial history has been studied, and how profound 

 the silence and darkness under which glaciers prosecute their 

 works. 



In this article I can do little more for my readers than indi- 

 cate methods of study, and results which may be obtained by 

 those who desire to study the phenomena for themselves. In 

 the first place, we may go to the glaciers themselves and learn 

 what we can of their weight, motions, and general activities* — 



* Here I would refer my readers to the excellent elementary works of Agassiz, 

 Tyndall and Forbes. 



