A PECULIAR BELT OF OBLIQUE FAULTING 605 



are the Big Snowy Mountains on the north, the Little Belt Moun- 

 tains on the northwest, the Snowy Range on the southwest, and 

 the Big Horn Mountains on the southeast."^ In three of these 

 ranges the uphft has been so great that the pre- Cambrian complex 

 is now exposed in the central core. Igneous outbursts have occurred 

 in the general region, though not recognized within the Lake 

 Basin field itself. The general setting of this remarkable faulting 

 is admirably portrayed in this Bulletin, and many suggestive 

 details are brought to the front, but as no definite explanation 



of the faulting is offered, I here- 



with venture to suggest some of /// ^/// //////////// ///7 



the possible factors which may 



have given rise to the phenome- * " 



non. 



POSSIBLE EXPLANATIONS 



L^:^^^^:^^^^^^^ 



There are two famiHar pro- ^^^; 2.- Diagram to illustrate the 



, . , , , formation of oblique crevasses along the 



cesses which are known to pro- ^^^^^^ ^^ ^ ^^^^.^^ ^^^.^^ ^ ^^^ ^^^^_ 



duce parallel fracturing in a zone tion indicated by the arrow. The 



such as described. The first is crevasses point upstream as they extend 



illustrated by the development ^^ard the middle of the glacier where 



■^ ^ the motion is more rapid. 



of oblique crevasses along the 



margins of valley glaciers. As the middle portion of a glacier moves 

 more rapidly than the ice near the sides, a line on the glacier connect- 

 ing points A and B (Fig. 2) will, after a time, become A'B'.^ Since 

 A'B' is longer than AB, there has been stretching along that line. 

 In fact, it can readily be shown that A'B' represents the direction 

 of elongation, or major axis of the strain ellipsoid, as developed 

 by Leith.^ Tension therefore develops along this line, and is 

 relieved by fractures at right angles to it. Hence it is that the 

 lateral margins of valley glaciers are commonly riven by a great 



^ E. T. Hancock, op. cit., p. 132. 



' John Tyndall, Glaciers of the Alps (i860), pp. 318-19. 



^ C. K. Leith, Structural Geology (1913), pp. 16-21. 



It is of course to be recognized that, since the rate of motion changes more 

 rapidly near the margin of the glacier than toward the middle, the line A'B' will be 

 curved instead of straight, and ideally the relation should be represented by many 

 small strain ellipsoids instead of one large one. 



