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Sierra Club Bulletin 



how they detach, transport, and accumulate rocks from vari- 

 ous sources. Secondly, we may follow in the tracks of the 

 ancient glaciers, and study their denuding power from the 

 forms of their channels, and from the fragments composing 

 the moraines, and the condition of the surfaces from which 

 they were derived, and whether these fragments were rubbed 

 off, split off, or broken off. 



The waters which rush out from beneath all glaciers are tur- 

 bid, and if we follow them to their resting-places in pools we 

 shall find them depositing fine mud, which, when rubbed be- 

 tween the thumb and finger, is smooth as flour. This mud is 

 ground off from the bed of the glacier by a smooth, slipping 

 motion accompanied with immense pressure, giving rise to the 

 polished surfaces we have already noticed. These mud parti- 

 cles are the smallest chips which glaciers make in the degrada- 

 tion of mountains. 



Toward the end of the summer, when the winter snows are 

 melted, particles of dust and sand are seen scattered over the 

 surfaces of the Sierra glaciers in considerable quantities, to- 

 gether with angular masses of rock derived from the shattered 

 storm-beaten cliffs that tower above their heads. The separa- 

 tion of these masses, which vary greatly in size, is due only in 

 part to the action of the glacier, although they all are borne 

 down like drift on the surface of a river and deposited to- 

 gether in moraines. The winds scatter down most of the sand 

 and dust. Some of the larger fragments are set free by the ac- 

 tion of frost, rains, and general weathering agencies; while 

 considerable quantities are borne down in avalanches of snow, 

 and hurled down by the shocks of earthquakes. Yet the glacier 

 performs an important part in the production of these super- 

 ficial effects, by undermining the cliffs whence the fragments 

 fall. During my Sierra explorations in the summers of 1872 

 and 1873, almost every glacier I visited offered illustrations of 

 the special action of earthquakes in this connection, the earth- 

 quake of March, 1872, having just finished shaking the region 

 with considerable violence, leaving the rocks which it hurled 

 upon the ice fresh and nearly unchanged in position. 



But in all moraines we find stones, which, from their shape 

 and composition, and the finish of their surfaces, we know 



