CERTAIN PHASES OF GLACIAL EROSION 203 



Fig. 6, from the chain of Mont Blanc, represents the Glacier 

 des Grandes Jorasses on the Italian side of the rugged mountain 

 mass of the same name. Other similar glaciers to the left and 

 right have etched their basins into the upper slopes of this great 

 mountain rampart. These glacier-filled basins are deeply sunken 

 and are as broad or broader near the base of their cirque walls 

 than they are farther down toward the ends of the present ice 

 tongues. At their heads they are terminated by precipitous 

 rock walls. Extremely precipitous cliffs come down to the Glacier 

 de Rochfort from the Aiguille du Geant and the col between it and 

 the Aiguilles Marbrees. From these rock walls behind the ice 

 there is a very decided change in slope to the gentle incline of the 

 glacier floor below. In just the same way there is a very abrupt 

 change of slope from the precipitous rocks of the Grandes Jorasses 

 and Mont Mallet to the very moderately inclined surface of the 

 Glacier des Grandes Jorasses at the foot of these steep cliffs. It 

 is at the point where these cliffs join the less inclined basin floor 

 beneath the glacier that the greatest cutting has occurred. Such 

 a profile of cliff and floor coming together at a sharp angle is quite 

 unlike any gully erosion developed by ordinary running water in 

 mountains of massive crystalline rocks. The greatest cutting has 

 been beneath the glacier in the neighborhood of the bergschrund 

 and directed backward into the mountain. 



II. CERTAIN SIGNIFICANT POSITIONS OF CIRQUES 



In our sketch of the initiation of cirques, we gave preference 

 to cases located on leeward aspects of eminences favorable for 

 snow lodgment but unfavorable for the concentration of running 

 water. We noted that if the lee brow were protected by its snow 

 covering, the crest should slowly shift toward the protected spot 

 and the protecting snow-cap should shift in turn to its lee and thus 

 combine to shape forth an asymmetrical mountain horn. On the 

 other hand, if the snow mass becomes a superior erosive agent 

 when it begins motion, and digs out a broad basin which in turn 

 adds to the catchment of snow, and if at the same time the embry- 

 onic glacier stopes headward, it, in its way, moves toward a summit 

 position. It is clear that rainfall does not concentrate toward 



