GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. "I 



Were gouged out, either because of the structure or softness of the rock or be- 

 cause of the more vigorous ice action for a limited distance. At the heads of 

 the separate valleys, broad semicircular amphitheaters, or cirques, were cut out, 

 an interesting series of which is shown in plate xxxix, figure 2, at the right side 

 of the view. 



From observations made upon the plucking power of glaciers in the various 

 valleys, the writer is quite prej^ared to admit the sufhciency of glaciers as engines 

 of erosion, especially where the ice is very thick, concentrated in its action, and op- 

 erates for long time over stratified, or much jointed formations. In addition 

 to the plucked mountain peaks observed in the Yoho Valley (page 79), there 

 is to be seen in Paradise Valley, lying between Lake Louise and Moraine 

 Lake, a very striking case of plucking, in which very heavily bedded quartzite 

 has been bodily removed. The upper stratum is 10 to 12 feet thick, and about 

 the margin of the stratum, the upper surface of which is very perfectly glaciated, 

 immense blocks, some of them as large as small houses, have been started a short 

 distance and then left. Apparently in the waning stages of the glacier the 

 ice had been iinable to get hold of blocks which it had been able to pry loose 

 from the parent bed. This occurs near the head of the valley and it is difficult 

 to resist the conclusion that hundreds of feet of similar, or less resistant rock 

 may have been removed between this ledge and the mouth of the valley. Glaciers 

 with their basal la5'ers shod with hard rock debris would be able to erode slowly. 

 The amount of erosion accomplished in this way would depend simply upon the 

 time, but it has probably always been small, when compared with that due to 

 plucking. It seems likely that pure ice can have only an insignificant effect 

 upon ordinary rock, when simply pushed across it. The greater the pressure the 

 more the melting, and unless disruption of the rocks occurs, the only effect 

 would be to give the rock a polish. 



d. Pleistocene deposition. During the maximum stages of glaciation there 

 were so few overtowering cliffs above the neve fields and the ice streams them- 

 selves that very little supra- and englacial material was carried. In consequence 

 during the stages of halt, that must have succeeded one another in the retreat 

 from the outermost position attained by the trunk streams, no great, conspicuous 

 moraines "W'ere formed. Not until the glaciers had retreated to near the heads 

 of the valleys do we find prominent terminal moraines. At this stage the level 

 of the ice and snow has dropped below the cliffs so that it is possible for the 

 glaciers to acquire, in many cases, a heavy load of rock debris upon their upper 

 surfaces. Glaciers like the Yoho have still been unable to build prominent 

 moraines from materials carried supraglacially. The detritus resulting from 

 the destruction of rock strata in the valleys and over the rocky slopes was 

 carried near the bases of the glaciers, or pushed and rolled along between the 

 ice and its bed. This resulted in the making of much ground moraine, much 

 of which remained in the valleys in places favorable for its lodgment. During 

 the maximum stages of glacial development much of ~this subglacial material was 

 carried bej'ond the mountains and deposited as till, or it found its way into the 



