GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 79 



and arrangement. They are like drumlins in that they consist of ground-morainic 

 material, with but a thin dressing of gravel and sand. They differ essentially 

 from drumlins in that their longer axes are parallel with the former ice margin, 

 instead of at right angles to it. If composed of stratified sand and gravel they 

 would be kames. In short they have the structure of drumlins and the form 

 and position of kames. They so much resemble in miniature the knolls and 

 ridges formed by the last great ice sheet in its retreat from the United States 

 and Germany that their origin becomes of especial interest. They have evidently 

 been formed by the glacier, with its nose consisting of a series of lobes, plowing 

 into the ground moraine previously deposited upon the valley floor. A retreat 

 occurred and then an advance, falling a little short of the first position, by which 

 a series of mounds and curved ridges was pushed up. This was repeated, 

 at least a half dozen times, each advance being somewhat weaker and falling a 

 little short of the preceding. Finally the glacier advanced over the entire series, 

 but overrode them so lightly that instead of being destroyed they were simply 

 smoothed and rounded. In melting back the last time, either from the ice itself, 

 or from the subglacial drainage, or from both sources combined, a thin layer of 

 sand and gravel was deposited over the structures. Had the ice lobes been larger 

 the ridges would have appeared more nearly straight, as we find them in the case 

 of the Pleistocene deposits. 



b. Plucking action. About the nose of the glacier, as has been already 

 noted, and upon the eastern side, toward the rock embossment, there are numer- 

 ous illustrations of the bodily disruption of the rock strata, to which the term 

 plucking is applied. Conditions are most favorable for this action when the 

 strata are thin bedded and jointed, when the strike of the strata is transverse 

 to the glacier and the dip is down stream. Under these conditions the rock layers 

 are ripped off and the bed lowered with relative rapidity, the rock fragments 

 being pressed into the ice and moved forward to assist in further work of a like 

 nature. In this way portions of the bed are much roughened by ice action, 

 instead of being smoothed. The edges and corners of the strata which were able, 

 at the last, to resist the action of the ice will be found to be rounded more or less. 

 A mountain spur, lying between the nose of the glacier and Mt. Balfour was 

 overridden by the ice and experienced this action upon an extensive scale. The 

 mountain is made up of curved, concentric strata, the upper layers being of a 

 dark limestone. Upon the southern side these layers dip to the southwest at an 

 angle of about 30. In passing over the peak from north to south many feet of 

 strata have been removed, those able to resist the action forming a succession of 

 steps upon the steeply inclined slope. One only of these steps is shown in plate 

 xxvii, figure 2, behind the hard crest of which the loose fragments have collected, 

 while upon either side they have been swept clean by the ice. This furnishes an 

 illustration of what is known as a "knob and tail" phenomenon. If the com- 

 bined height of the successive steps were ascertained we should have a figure 

 representing the minimum amount of this plucking action upon the southern 



