THE GREAT ICE AGE 357 



plunge the traveller into the depths below. Still the treacher- 

 ous surface is daily crossed by parties of travellers, apparently 

 without any accident. The whole of the ice is moving steadily 

 along the slope on which it rests, at the rate of eight to ten 

 inches daily — the rate of motion is less in winter and greater in 

 summer ; and farther down, where the glacier goes by the name 

 of the Glacier du Bois, and descends a steeper slope, its rapid- 

 ity is greater ; and at the same time by the opening of immense 

 crevasses its surface projects in fantastic ridges and pinnacles. 

 The movements and changes in the ice of these glaciers are in 

 truth very remarkable, and show a mobility and plasticity 

 which at first sight we should not have been prepared to 

 expect in a solid like ice.^ The crevasses become open or 

 closed, curved upwards or downwards, perpendicular or in- 

 clined, according to the surface upon which the glacier is mov- 

 ing, and the whole mass is crushed upward or flattens out, its 

 particles evidently moving on each other with much the same 

 result as would take place in a mass of thick mud similarly 

 moving. On the surface of the ice there are a few boulders 

 and many stones, and in places these accumulate in long 

 irregular bands indicating the hnes of junction of the minor ice 

 streams coming in from above to join the main glacier. At the 

 sides are two great mounds of rubbish, much higher than the 

 present surface of the glacier. They are called the lateral 

 moraines, and consist of boulders, stones, gravel and sand, 

 confusedly intermingled, and for the most part retaining their 

 sharp angles. This mass of rubbish is moved downward by 

 the glacier, and with the stones constituting the central moraine, 



1 I need scarcely say that I adopt the explanation of glacier motion 

 given by Forbes. "The fuller consideration of the physical properties of 

 glacier ice leads essentially to the same conclusions as those to which 

 Forbes was led forty-one years ago by the study of the larger phenomena 

 of glacier motion, that is, that the motion is that of a slightly viscous mass, 

 partly sliding upon its bed, partly shearingupon itself under the influence 

 of gravity." — Trotter, Proc. Royal Society of London, xxxviii. 107. 



