GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 25 



of the rate at which surface ablation was taking place over the lower portion 

 of the Victoria, accurate elevations were taken upon the series of steel plates 

 used in determining the forward movement. The results are shown in table 

 V, page 31, giving the total melting from July 13 to August 4, 1904, for a 

 period of 22 days of midsummer. The maximum lowering of the ice surface 

 occurred at plate No. 13, nearly two-thirds of the way across when measured 

 from the west side, and amounted to 3 . 8 feet, or a daily average-of 2.076 inches. 

 This plate was located on a portion of the glacier least protected by rock debris. 

 Although this lowering of the surface is due, in the main, to the sun's action and 

 general effect of the atmosphere, usually above the freezing point in summer, 

 there are other minor agencies which would tend to give the same result. One of 

 these is the rain, 1.506 inches of which fell during the period of the observations. 

 Other agencies are subglacial melting and subglacial erosion, longitudinal 

 stretching, or a lateral spreading of the ice, all of which would tend to lower 

 the upper surface. It should be noted, further, that in the 22 days this plate 

 moved forward 60 inches and with the average surface slope of 7° to 8°, the 

 plate would be lowered by this agency alone about one-third of an inch daily. 

 Making this correction the actual ablation, from sun, atmosphere, and rain, 

 would amount to about 1.74 inches daily and for the two main months of July 

 and August would give a total of about 9 feet. Observations upon the lower 

 Lefroy showed that the ice surrounding certain morainic heaps had been 

 lowered during the season by about this amount. No glacial tables of this 

 height can be found, however, becavise of the undercutting effect of the sun's 

 rays and the consequent destruction of their pedestals. The broad medial 

 depression lying just west of the medial moraine (see plate iv, figure 2, and cross 

 section page 30) has been produced by the greater surface melting and this 

 has been permitted by the thinner covering of rock debris, the ice of this portion 

 of the glacier coming from the Lefroy side of the upper Victoria. This depres- 

 sion continues down the glacier for 2,200 feet, where it thins out, apparently 

 from surface melting. With a forward motion here of about 64.5 feet annually, 

 it would require 34 years for the ice to move from the line of plates to the oblique 

 ice face, during which time some 306 feet of ice might be melted away. If 

 the rate of melting and rate of forward movement remained constant, this 

 number would represent the approximate thickness of the ice beneath plate 13. 

 It is very probable that the rate of melting becomes less, owing to the concen- 

 tration of rock debris at the surface, but it is also very probable that the rate 

 of forward movement becomes also less as the ice diminishes in thickness. 

 The work with the spirit level indicated that this plate was originally 393 feet 

 above the lower margin of the ice in this depression, the difference of 87 feet 

 representing the rise of the valley floor in this distance. If this latter figure is 

 approximately correct the inclination of the bed is much less than that of the 

 surface of the ice itself 



b. Surface drainage. Over the entire nev^ area the water from the melting 

 snow, as well as that from the rainfall, is absorbed into the body of the glacier 



