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Sierra Club Bulletin 



some of the chips. The reason for the greater steepness of the 

 front from A to B than from B to C will be perceived at a 

 glance; and, since the cleavage planes and other controlling 

 elements in its structure are evidently the same throughout the 

 greater portion of its mass as those which determined its pres- 

 ent condition, if the glacial winter had continued longer its 

 more characteristic features would probably have remained 

 essentially the same until the rock was nearly destroyed. 



The section given in Fig. 6 is also taken from the north side 

 of the same valley. It is inclined at an angle of about twenty- 

 two degrees, and therefore has been more flowed over than 

 flowed past. The whole surface, excepting the vertical por- 

 tion at A, which is forty feet high, is polished and striated. 

 The arrows indicate the direction of the striae. At A a few in- 

 cipient cleavage planes are beginning to appear, which show 

 the sizes of some of the chips which the glacier would have 

 broken or split off had it continued longer at work. The whole 

 of the missing layer which covered the rock at B, was evident- 

 ly detached and carried off in this way. The abrupt transition 

 from the polished surface to the split angular front at A, shows 

 in a most unequivocal manner that glaciers erode rocks in at 

 least two very different modes — first, by grinding them into 

 mud; second, by breaking and splitting them into blocks, whose 

 sizes are measured by the divisional planes they possess and 

 the intensity and direction of application of the force brought 

 to bear upon them. That these methods prevail in the denuda- 

 tion of oversowed as well as past-fiowed rocks, is shown by 

 the condition of every canon of the region. For if mud parti- 

 cles only were detached, then all the bottoms would be smooth 

 grooves, interrupted only by flowing undulations; but, instead 

 of this condition, we find that every canon bottom abounds in 

 steps sheer-fronted and angular, and some of them hundreds 

 of feet in height, though ordinarily from one to ten or twelve 

 feef. These step-fronts in most cases measure the size of the 

 chips of erosion as to depth. Many of these interesting ice- 

 chips may be seen in their tracks removed to great distances or 

 only a few feet, when the melting of the glaciers at the close 

 of the period put a stop to their farther progress, leaving them 

 as lessons of the simplest kind. 



