Geology 



world where glaciers exist, for example, the Merjelen 

 See on the flank of the Great Aletsch Glacier, in which 

 case, however, the col at the head of the valley "0" 

 in Fig. 32 being lower than the surface of the glacier 

 takes the overflow, and the channel is cut in that 

 position instead of being through the spur. 



The position and level of many such lakes, which 

 formerly existed in the British Isles, can be determined 

 by those of their overflow channels which still persist as 

 streamless valleys in a position where the existence of 

 a stream would be impossible under existing conditions. 



On ascending the glacier still farther we found that 

 the tributary valleys were occupied by glaciers which 

 had shrunk away from the main stream and were no 

 longer confluent with it, though one, the uppermost of 

 its class, still contributed ice to the main flow through 

 the medium of avalanches. This glacier terminated at 

 the top of a precipice on the side of a mountain, and as 

 it was slowly pushed over the edge by the weight of the 

 snow behind, mass after mass broke away and came 

 thundering down upon the surface of the main glacier. 

 Naturally we had to give this place a wide berth, but 

 we were able to see by means of field-glasses that much 

 rock ddbris came down with the ice and contributed in 

 no small degree to the rock-train material. 



When we came to examine the first tributary that 

 was actually confluent with the main stream, we found 

 that it possessed two rock-trains, one on either side, and 

 that these consisted of material which fell upon its surface 

 partly as avalanches and partly as stone-showers from 

 overhanging cliffs of rock. 



These stone showers, we found, took place chiefly in 



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