54 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



the Vaux boulder had moved downward in 1905 from its original position some 

 676 feet, or at an average rate of about 113 feet per annum. 



/. Dirt stripes. Somewhat closely related to the dirt bands just described, 

 so far as their method of formation is concerned, are the fine streaks of dirt seen 

 along the margins of most glaciers, sufficiently free from surface debris. They 

 may be found, however, anywhere upon the glacier that the blue bands are 

 well developed, reach the surface at a fairly steep angle and are being subjected 

 to surface melting. The blue bands, being composed of relatively firm, compact 

 ice, are more resistant of the sun's action, than the vesicular ice in which 

 they are embedded and project as delicate ridges, separated by narrow 

 furrows. Into these furrows the wind-blown dust settles and is washed 

 from the adjoining ridges, forming narrow, parallel dirt streaks, or stripes. 

 When well developed, as upon the Lefroy, the glacier has the appearance of 

 having been swept with a coarse wire broom; the strokes having all been long, 

 regular and parallel. The dirt stripes mark the position of the vesicular bands 

 in the ice and the lighter streaks between the position of the blue bands them- 

 selves. In this way the banding is clearly shown at the surface, whereas, other- 

 wise, it might be obscure. Views of these stripes have already been shown in plate 

 xn, figure 4 and plate xm, figures 1,2, Sometimes they run down the face of a 

 crevasse wall (plate xn, figure 4), as though they might be something more 

 than a superficial feature, but a little chipping of the ice shows plainly that they 

 are not. After they have once been formed the dirt stripes will absorb the 

 sun's heat and still further emphasize the small furrows. Running, in general, 

 lengthwise of the glacier these furrows become the sites of minute rills which 

 have a tendency to clear away the fine dirt, as fast is it collects. For this 

 reason, as well as because of the nature of the banding -itself, the individual 

 stripes are not continuous for any considerable distance. They are sometimes 

 so closely placed that 10 stripes may be counted within the distance of an inch, 

 but are usually considerably coarser. 



g. Dust and pebble wells. Where small pebbles, or patches of fine dirt, 

 often black from the presence of organic matter, 1 are thinly distributed over 

 the surface of the ice, heat is absorbed and the ice immediately beneath is melted 

 more rapidly than the surrounding ice. Cavities are thus formed with vertical 

 walls, which for a time retain the water. They sink into the ice for a few inches, 

 until protected from the direct rays of the sun by their own walls, when further 

 melting would be delayed until the general surface was lowered sufficiently 

 to allow the sun to again reach the foreign matter at the bottom. Such wells 

 are shown in plate xvn, figure 2. Although their depth at any one time is seldom 

 greater than a finger's length, still in the course of the season their total length 

 would be 9 to 10 feet upon the Victoria and Lefroy. A thin film of water often 

 freezes at night over the surface and then thaws out promptly when again 

 exposed to the sun. After thus freezing the water is at times drawn into the 



'A sample collected from the Illecillewaet in 1903 contained 14 per cent, of organic matter, enough so 

 that when set away moist in a warm room it soon became offensive. 



