20 H. L. FAIRCHILD — ICE EROSION THEORY A FALLACY 



use these phenomena as argument for great erosion, as their rarity is 

 proof that the ice did not cut deeply or rapidly as a habit. 



Chatter-marks, the crescentric fractures crossing a line of gouging and 

 concave downstream, are also evidence of the ineffectiveness of the gla- 

 cial plane, not only by their rarity, but by proving that the ice did not 

 hold its tools up stiffly to their work. Roche moutonnee forms prove that 

 the glacier acts as a flexible rasp and not as a plane. The two factors, 

 pressure and rapid flow, which should cooperate in order to produce 

 rapid erosion, do not generally increase together. The reason for this 

 lies in the peculiar mechanics of the glacier, which will be considered 

 later (see page 25), along with a general discussion of the element of 

 abrasion in its wider relation. 



(2) " Plucking" is a term which has done much service in the cause 

 of the erosion argument. Doubtless the ice can grasp and pull or push 

 away projecting blocks that have become loosened by weathering or 

 which are much exposed in saliences. The amphitheaters or cirques 

 at the heads of glaciers are thought to be made by the pull of the ice on 

 the frost-loosened blocks, specially in crystalline rocks with vertical 

 jointing. Conditions somewhat similar may exist alongside the stream 

 glacier, but the effect here is to widen and not to deepen the valley. 

 The mechanical conditions at the bottom of the channel or beneath the 

 glacier are entirely different. There the effect of the ice is to abrade 

 away the saliences, and plucking could be only a part of the initial and 

 temporary process of leveling. The idea that plucking has any part in 

 the deepening of large valleys has no warrant in either observation or 

 reason. 



An operation analogous to plucking, and which may be included under 

 that term, occurs in the case of bedded rocks which have been weathered 

 so as to produce open or weak horizontal joints. Such lifting of weath- 

 ered strata is illustrated in plate 23. It can effectively occur only to 

 the depth of weathering, and the discovery of it may be regarded as 

 evidence that the ice has not eroded below the zone of weathered mate- 

 rial. This kind of work is more common in case of continental glaciers, 

 and specially along the crests of escarpments or elevations. It is con- 

 ceivable that a stream glacier might bear such peculiar relation to the 

 attitude of bedded rocks in -its channel that, along with exceedingly 

 slow action of water or weathering beneath the ice, it might slowly 

 pluck at its channel bottom ; but this is not important, and, as a mat- 

 ter of fact, most of the deep valleys and fiords which have been attrib- 

 uted to ice erosion are in crystalline rocks. 



(3) Capricious behavior of the ice within limits may be admitted ; but 

 the admission that the ice acts differently under im perceptible or unknown 



