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Sierra Club Bulletin 



wherever superior strength of structure or advantageousness of posi- 

 tion admits of such development, just as they are elsewhere in a ver- 

 tical direction. Some of these projections are of a magnitude that 

 well deserves the name of horizontal mountain. That the variability 

 of resistance of the rocks themselves accounts for the variety of these 

 horizontal features is shown by the prevalence of this law. Where 

 the uniformity of glacial pressure has not been disturbed by the en- 

 trance of tributaries, we find that where valleys are narrowest their 

 walls are strongest; where widest, weakest. 



In the case of valleys with sloping walls, their salient features 

 will be mostly developed in an oblique direction; but neither hori- 

 zontal nor oblique mountainets or mountains can ever reach as great 

 dimensions as the vertical, because the retreating curves formed in 

 weaker portions of valley walls are less eroded the deeper they be- 

 come, on account of receiving less and less pressure, while the alter- 

 nating salient curves are more heavily pressed and eroded the far- 

 ther they project into the past-squeezing glacier; thus tending to 

 check irregularity of surface beyond a certain limit, which limit is 

 measured by the resistance offered by the rocks to the glacial energy 

 brought to bear upon them. So intense is this energy in the case of 

 large steeply inclined glaciers, that many salient bosses are broken 

 off on the lower or down-stream side with a fracture like that pro- 

 duced by blasting. These fractures occur in all deep Yosemitic ca- 

 nons, forming the highest expressions of the intensity of glacial force 

 I have observed. 



The same tendency toward maintaining evenness of surface ob- 

 tains to some extent in vertical erosion also; as when hard masses 

 rise abruptly from a comparatively level area exposed to the full 

 sweep of the overpassing current. If vertical cleavage be developed 

 in such rocks, moutonneed forms will be produced with a split face 

 turned away from the direction of the flow, as shown in Figure 8, 

 Study No. I. These forms, measuring from a few inches to a thou- 

 sand feet or more in height, abound in hard granitic regions. If no 

 cleavage be developed, then long ovals will be formed, with their 

 greater diameters extended in the direction of the current. The gen- 

 eral tendency, however, in vertical erosion is to make the valleys 

 deeper and ridges relatively higher, the ice-currents being constantly 

 attracted to the valleys, causing erosion to go on at an accelerated 

 rate, and drawn away from the resisting ridges until they emerge 



