GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 37 



northern crevasse wall. It is into this depression, which becomes convex down- 

 stream owing to the more rapid central movement, that fine de~bris may collect 

 and give rise to the "dirt bands" of Forbes, to be presently described. As 

 the glacier rounds Lefroy and enters a broader portion of its valley, it has a 

 chance to spread laterally, and longtitudinal and somewhat radiating crevasses 

 are opened which may intersect those having the transverse position. If the 

 glacier is again contracted these crevasses will also be closed, and if any depression 

 is left, it will slope down-stream and not have a tendency to collect debris. 



The more rapid movement of the middle portion of the glacier, when compared 

 with the sides, which are retarded by the friction of the valley walls, induces 

 tensional strains between the central and marginal masses. In consequence, 

 along the sides, there is opened up a characteristic system of marginal crevasses 

 at right angles to the resultant strain. These extend inward and upward, 

 making, theoretically, with the sides angles of about 45. The difference between 

 the central and marginal flow must reach a certain value, and be sufficiently 

 abrupt, otherwise the ice seems capable of yielding without rupture. In this 

 way we may account for the absence of marginal crevasses over the lower west 

 side of the Victoria. The very sudden change in movement, shown in table v, 

 between the margin and the ice of the near-by medial moraine, plates 18 and 17, 

 is evidently responsible for the series of marginal crevasses that are seen between 

 the line of plates and the tributary (see plate in). From their absence upon this 

 side, farther down, we infer that the ice beneath the medial moraine becomes 

 more sluggish as the main flow is deflected westward. Opposite Mt. Lefroy 

 conditions are favorable for their formation and they are well represented upon 

 either side. Opposite the tributary they do not occur, as the marginal ice is 

 sufficiently yielding. Upon the tributary itself these crevasses are well repre- 

 sented, except over the collecting area for the Lefroy. After their formation 

 their inner ends may be swung around until they assume a transverse, or even 

 reversed, position, as seen upon the Aberdeen side of the Lefroy. Here we find 

 one series, averaging N. 51 E. and making angles of about 66 with the margin 

 but ranging from 5 2 to 86; and a second series, many of them nearly closed, and 

 apparently older than the preceding, having an average direction of N. 95 E. and 

 making with the side angles of about 111. 



The size of many crevasses in the spring and their contents of fresh snow show 

 that they may persist through a series of seasons. Sometimes they become 

 partially filled with water which may melt out cavities in their walls and give 

 rise to the most exquisite ice grottoes, a peep into which is worth miles of travel. 

 The closing of crevasses sometimes confines pools of water, often under hydro- 

 static, or ice pressure, and as the surface of the ice is lowered by melting, the water 

 suddenly bursts forth with geyser-like action. The compression of air enclosed 

 in cavities, or brought in by surface streams, often gives rise to a bubbling at the 

 surface and a faint hissing, or chirping sound the "sighing " of the glacier. 



