EVIDENCE FROM GLACIAL PHENOMENA 29 



abrasion. But alongside the glacier the forces of rock-destruction are 

 greater in number and intensity. It has been shown that the combined 

 resultant of the several erosive factors in stream glaciers — weight or press- 

 ure, slope of channel or velocity of flow, and friction on the sides and 

 bottom of the channel— is most effective along the sides of the glacier, and 

 in addition to this we also recognize along the glacier edge the coopera- 

 tion of weathering, and of frost-work, suggesting that at the bergschrund. 

 The products of erosion are more or less removed from the plane of con- 

 tact along the glacier edges and carried downward toward the bottom (to 

 burden the bottom ice and there prevent erosion), while new tools of 

 marginal erosion are constantly supplied by land-wash and from the 

 lateral moraine. 



Probably the most important and conclusive paper in English relating 

 to the mechanics of glaciers with reference to their erosional work on 

 their channels was published in 1894 by W J McGee * reasoning from 

 studies in the Mono Lake region. His general conclusion is summed 

 up in the last sentence of his paper : 



11 It follows that these features do not necessarily imply extensive glacial exca- 

 vation or indicate that glaciers are superlatively energetic engines of erosion." 



Figure 1.— Truncation of Tributary Valleys. 



This theoretic diagram illustrates possible truncation of tributary valleys by glacial widening 

 without deepening of the main valley. 



Some of the special and positive results of his analysis are stated in 

 more positive terms, as follows : 



11 It follows that the general tendency of glaciers must be to widen rather than 

 to deepen the valleys they occupy, and to transform V to U canyons. . . . 

 It follows again that the characteristic glacial canyons must be only modified 

 stream-canyons" (page 359). 



He shows that one effect of changing a V-shaped valley to U shape is 

 to cut off the lower or terminal part of the convex profile of the smaller 

 tributary and ungraded valleys, and to thus produce what have been 



* W J McGee : " Glacial canyons." Jour. Geol., vol. ii, 1894, pp. 350-3C4. 

 V— Bum,. Geol. Soc. Am., Vol. 16, 1904 



