216 THOMAS C. CHAMBERLIN AND ROLLIN T. CHAMBERLIN 



lower valley from either end of it. Behind are the abrupt cliffs of 

 the Matterhorn and the Furggen Grat (Fig. 12). The very sharp 

 angle between the steep wall of the Furggen Grat and the Furgg 

 Glacier which is pulling away from it affords strong evidence of 

 the effectiveness of sapping at this critical location. 



VI. BASAL SIDE EROSION 



The sapping and corrasion along the side-base of a glacial 

 valley, by which the normal V-shape is converted into the glacial 

 U -shape, is perhaps due mainly to the better supply of carving 

 tools furnished the sides of the glacier by infall and inwash from 

 the slopes and cliffs on the valley sides, and by seepage from the 

 side walls. This supposes a general similarity between the con- 

 ditions at the side of the valley and those at the cirque base, 

 except that the direction of glacial motion is different. 



All these distinctive phenomena of glaciers seem to us to be 

 expressions at once of the peculiarities of glacial erosion and of 

 its superiority where conditions favor glacial erosion. 



This view does not, however, make, the superiority universal 

 and unqualified. Obviously it does not exclude the view that 

 snow fields while they remain in the passive state serve as pro- 

 tective agencies. Nor does it exclude the view that the center 

 of a continental ice field, from which the motion is mainly radiant 

 and limited in amount, may be protective rather than erosive 

 when compared with normal weathering. Nor does it exclude the 

 view that valley glaciers in some of their parts may be less erosive 

 than normal wear and weathering would be. But these seem to 

 us rather qualifications of the general proposition that glaciers are 

 effective agents of erosion than contraventions of it. 



