REPORT OF THE CHIEF ASTRONOMER. 585 



SESSIONAL PAPER No. 25a 



Within the mountain group the ice of maximum depth — 2,800 feet — lay 

 over the Wigwam valley. The average depth of the ice-cap from the eastern 

 divide to Tobacco Plains was about 1,200 feet. The discharge of the ice was 

 in a general southward direction and was largely concentrated in the Wigwam 

 valley and in the Rocky Mountain Trench, though some of the ice flowed east- 

 ward into the North Flathead glacier before beginning its southward journey. 



In the Galton-MacDonald mountains where crossed by the Boundary belt, 

 only twelve peaks projected, as nunataks, above the surface of the ice-cap and 

 the summit of -the highest of these was not more than about 500 feet above the 

 ice. The total area of the nunataks must have been under three or four square 

 miles, or about three to four per cent of the area. In that area about twenty 

 glacial cirques have been mapped, along with a half dozen others mapped in the 

 MacDonald range, east of the Ice Divide. 



The western slope of the Galton range and the eastern slope of the McGil- 

 livray range delivered many high-level streams of ice to the Rocky Mountain 

 Trench where the ice-cap was of exceptional thickness. Observations in both 

 ranges showing that the continuous cap enveloped all slopes below the present 

 7,300-foot contour, and the Kootenay river at the Boundary being at approxi- 

 mately the 2,300-foot contour, it follows that, at the time of maximum glacia- 

 tion, the cap was here about 5,000 feet thick. It is altogether likely that, 

 toward the close of the Pleistocene period, the trench was still occupied by south- 

 ward-moving ice, a majestic glacier twelve to fifteen miles in width and scores 

 of miles in length. 



Distinct lateral moraines formed by the trench glacier were observed east 

 of Tobacco Plains at various elevations from 500 to 2,000 feet or more above 

 the valley floor. When these deposits were made the glacier must have had a 

 width of at least twelve miles. As shown by the deep groovings and by the 

 development of numerous roches moutonnees, the trench was the scene of intense 

 abrasion. 



An indication of the notable excavating power of the ice moving down the 

 trench is perhaps given in the fact that the branch valleys in some cases are 

 hanging hundreds of feet above the floor of the trench. As in the case of 

 Cameron Falls at Waterton lake, the probabilities are in favour of the view that 

 these discontinuities of stream gradient are due to more rapid excavation by 

 trunk glaciers as compared with branch glaciers. Such may be the origin of 

 the 500-foot cascade on Phillips creek where it tumbles into the trench. It 

 should, on the other hand, be noted that this cliff may possibly be structurally 

 determined by the retreat of the underlying limestone eastward, down the dip 

 of the beds. (Plate 53, Fig. A.) Since the second explanation cannot be 

 entirely excluded, Phillips creek is not to be surely placed in the class of valleys 

 which ' hang ' because of differential glacial erosion. 



All of the striae observed on the floor and side slopes of the trench are 

 directed southward and faithfully parallel to the axis of the valley. The trench 

 glacier completed the work begun during the existence of the ice-cap. The 

 geological work of the cap and that of the trench glacier conspired to produce 

 much of the existing relief of the vallev floor at Tobacco Plains. This relief 



