50 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



70 rings of growth and of the latter 77 rings. It is over this morainic heap that 

 the drainage brook from the glacier cascades. The swift stream and its load of 

 hard angular sediment have a perceptible rounding effect upon the corners, 

 edges, and faces of even the hardest quartzites. This effect was most strikingly 

 shown in quartzite boulders lying in the bed of a glacial stream coming from 

 the Asulkan ridge. 



e. Dirt bands. Under this term there was described by Forbes, in 1843, 

 (Travels through the Alps of Savoy, p. 162), a superficial feature of certain 

 glaciers which is of much interest and. possibly, of much importance. It is 

 found in those glaciers which change their slope sufficiently to give rise to a 

 distinct system of transverse crevasses, not necessarily to a cascade or ice-fall. 

 The phenomenon was not understood by Forbes himself and, by various writers 

 since, 1 has been confused with the dirt zones, described upon page 39, which are 

 the outcropping edges of variously marked strata. It is to the keen observation 

 and shrewd interpretation of Tyndall that we are indebted for the true explana- 

 tion 2 of the feature. The Victoria and the Lefroy glaciers furnish an excellent 

 opportunity for the study of dirt bands under very simple conditions, as 

 well as the dirt zones for comparison. The two types of structure may be gotten 

 upon the same photographic plate and are well shown in plate iv, figure 2. In 

 very simple form the dirt bands may be seen cutting across the dirt zones upon 

 the lower Lefroy, owing to the abnormal position of the latter. Under ordinary 

 conditions the two would be more or less conformable and possibly difficult 

 to separate. 



The typical dirt bands are of such a nature that they can be seen most strik- 

 ingly at a distance of a half-mile or more from the glacier and at a considerable 

 elevation above it. When once seen, however, it is possible to locate them in a 

 very general way while upon the surface of the glacier itself. In the summer of 

 1904, from the summit of the Devil's Thumb, which overlooks the Victoria Glacier 

 from a height of 8,000 feet, there could be counted 23 soiled streaks passing across 

 the glacier. Beginning near the crest of the ice slope opposite the nose of Mt. 

 Lefroy, the bands were narrow, straight, and extended nearly across the glacier. 

 They showed so dimly that there was uncertainty in regard to the count, until 

 they had been gone over a number of times. Upon the face of the slope they 

 became" more distinct, curved so as to be convex down-stream, and correspondingly 

 shortened. A few of them could be traced around into the transverse crevasses 

 which had not been completely closed. Beyond the foot of the ice slope the 

 bands became still better defined, especially upon the southern, or up-stream 

 margin, narrower, more closely .placed, and changed their shape from arcs of 

 circumferences to hyperbolas. Towards the lower end of the series the bands 

 became much shorter, the arms extending into and blending with the surface 

 debris, and their apices appeared to mark the locus of maximum surface velocity 



' Agassiz, Geological Sketches, vol. i, pp. 244 and 254; Russell, Glaciers of North America, p. 43; Hess, 

 Die Gletscher, p. 169. 



* Glaciers of the Alps, pt. n, chapter ,26. 



