ADVANCE AND RETREAT OF CANADIAN GLACIERS 741 



practically the same duration at Banff as at Glacier; the absolute 

 maximum temperature at Banff (July 4, 1903) is recorded as 81. 8.° 

 The length of hot and cold seasons is approximately the same, and 

 the range above freezing is practically the same. 



Such differences as there are would favor a more rapid waste among 

 the first type of glaciers. Occasionally midwinter temperatures above 

 freezing would favor melting ; the warm, dry Chinook winds would 

 favor evaporation. Since, therefore, these glaciers are wasting with 

 less rapidity than those of the second type, there must be some other 

 cause than the temperature of the two regions. 



Altitude of the snow-line. — In the Selkirks the snow-line is at an 

 altitude of about 7,000 feet. From this level the glaciers descend 

 with a fall of from 1,000 to 2,500 feet. 



In the Rockies the snow-line is higher. It is variable, but is in 

 general at an altitude of about 8,500 feet. The debris-covered 

 glaciers end at a level of about 6,000 feet. 



Topography.— -The general effect of the occupation of a valley 

 by a glacier is to round its outline, changing a V to a U in cross- 

 section. The extent to which this can take place depends (i) upon 

 the structure and character of the rock, and (2) upon the erosive 

 .capability of the glacier. The schists and quartzites of the Selkirks 

 are readily rounded, and broad open U's produced in a relatively 

 early stage of erosion. The result is the production of wide areas in 

 which the accumulation of snow is possible. These gentle-sloped 

 depressions aid the great precipitation and low snow-line in the 

 production of the great neve regions. 



The Rockies, in the region of the first type of glaciers, are composed 

 for the most part of hard unmetamorphosed limestone, with vertical 

 cleavage. Its tendency is, on weathering, to produce cliffs. The 

 resulting valleys are steep-sided canyons even after their occupation 

 by ice. Any widening out that takes place is below the snow-line, 

 and impossible for neve formation. The small size and sluggish 

 character of the glaciers may be effective of the same result. 



The only points of lodgment for snow and ice are in cracks in the 

 faces of cHffs, and here are found the only neves of the region, as 

 feeders of the cliff glaciers. Thus precipitation, altitude of the 

 snow-line, and topography combine to form great neves in the one 



