52 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



walls affected more evenly, unless surrounding mountain cliffs interfered. In 

 the healing of such crevasses there would be left a depression, representing the 

 sun's action upon the lips of the crevasse, not simply for one season but through 

 a series, and in this depression the wind-blown dust would collect and the fine 

 debris would be washed by rain and melting ice from the adjacent portions of the 

 glacier, rendering it lighter by contrast. Owing to the more rapid central 

 movement of the ice the bands, at first nearly straight, will begin to curve down- 

 stream and become more and more sharply bent, their apices marking the locus 

 of maximum surface motion. Between them will lie SAvellings, or ridges, having 

 the same general form of the depressions, from which niuch of the finer dirt has 

 been removed. These ridges and intervening dei^ressions may be very inconspicu- 

 ous, as upon the Victoria, or they may become very prominent, as shown upon the 

 Deville Glacier in the Selkirks, forming what Forbes termed ' ' wrinkles ' ' (plate x vi , 

 figure i). They mark that portion of the ice which passed the crest of the 

 slope in the late fall and winter and appear as ridges, partly because of the severe 

 compression to which the ice is subjected and mainly because the adjacent ice 

 has been lowered by melting. Owing to the more rapid movement of the ice 

 down the slope the bands will be farther apart and less well defined, than after the 

 more gentle slope below has been reached and the ice is subjected to longitudinal 

 compression. Upon this more gentle slope they have a better chance to catch 

 and retain the fine debris. Since the sun's action was more powerful at the 

 center of the crevasse, the depression is greater at the apex of the band and 

 persists after that of the extremities has been finally lost by surface melting. 

 In consequence the bands become shorter and shorter and lastly disappear, 

 when ablation has reduced the surface to a general slope and the fine debris is 

 redistributed. Very often it must happen that instead of a single crevasse 

 being formed during the season of melting there would be formed a series of 

 them. Upon a steep slope of the Asulkan they seem to be formed in pairs as 

 shown in plate xvii, figure i, in which it is seen that the ridge of ice separating 

 two adjacent crevasses is acted upon from either side and lowered, assisting in 

 the formation of the depression. The crevasses that are forming the depres- 

 sions, preparatory to the reception of the dirt, may be traced around to the 

 almost healed crevasses at the left, while between them are seen traces of crevasses 

 that have healed with practically no marginal melting. These are presumably 

 those which opened and closed soon enough to escape the rounding action. 

 If the surface slope is too great the depression produced in the ice may not be 

 sufficient to retain enough dust to bring out the series distinctly, as is the case 

 with the Asulkan just noted. Study figure i, plate xxix, from the Yoho Glacier. 

 That the method of formation of these dirt bands is essentially as outlined 

 above admits of no doubt. The question as to whether they are produced 

 annually, or at irregular intervals, needs to be investigated. The average inter- 

 val of those bands originally described by Forbes upon the Mer-de-Glace was 

 711 feet. Opposite his station D the interval was 667 feet {Travels through the 

 Alps of Savoy, p. 165). In a postscript to his volume, p. 420, he gives the move- 



