86 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



of the boulder and it would be glaciated not only upon the stoss and upper sur- 

 face, but upon the lee side as well. A certain relation must exist between the extent 

 of lee-side glaciation and the thickness of the ice, which, if known, would give 

 some data for estimating the maximum thickness of certain Pleistocene sheets. 



4. CREVASSES. 



The crevassed condition of much of the n6v6, especially that in the main 

 direction of flow, has been noted. Faultings and dislocations occur, disclosing 

 the stratified nature of the ne"ve and subjacent ice. This crevassing is due, 

 apparently, to irregularities in the bed, rather than to differential motion, and 

 indicates that the ice here attains no great thickness. About the margin of the 

 neVe" field there occur, here and there, breaks where the ne"ve" has withdrawn 

 from a portion still clinging to the rocky wall. These are the bergschrunds 

 described upon the Victoria and Wenkchemna glaciers (pages 22 and 67). On a 

 line between Perley Rock and the western end of Mt. Lookout, there opens up a 

 series of transverse crevasses as the ice begins its descent into the valley and its 

 velocity is accelerated (plate xxxn, figure 2). The ice is unable to yield to the ten- 

 sional strain and forms long V-shaped gashes at right angles to the stress. These 

 become convex down stream because of the more rapid central movement of the 

 ice. Conditions are here favorable for the formation of Forbes' dirt bands (page 

 50), but the ice soon plunges over the quartzite ledges, is shattered in every 

 direction and all structure lost. Upon the crest of the cascade a network of 

 crevasses opens, dividing the ice into irregular angular blocks. These become 

 melted upon all sides and assume the form of pinnacles and steeples, seracs, 

 displaying beautifully the stratified structure of the ice (plate xxxv, figure 2). 

 Reaching the bottom of the cascade these blocks are jumbled together, many of 

 them completely melted and the remainder frozen together into a great ice con- 

 glomerate (plate xxxv, figure i)'. As pointed out upon page 44, it is quite 

 conceivable that some of the basal layers might be able to descend the slope 

 without having their structure destroyed. 



The rapid central movement of the ice, due to the high average slope of the 

 bed, gives rise to a very complete system of marginal crevasses, extending in- 

 ward and upward, and showing conspicuously when the glacier is seen from a 

 height. In the lower third the ice does not feel the restraint of the valley walls, 

 spreads laterally because of its own weight, and there are opened longtitudinal and 

 radial crevasses, some of them extending to the margin of the ice. As the 

 surface is continually lowered by melting, only the bottoms of some of the 

 shallower crevasses remain, and these appear simply as short gashes in the other- 

 wise smooth surface. 



5. ICE STRUCTURE. 



There is evidence upon the glacier's left, back a short distance from the nose, 

 that the stratification in the basal portion of the glacier is not completely de- 

 stroyed in passing the cascade. Traces of the stratification may be seen dipping 



