glaciers of the canadian rockies and selkirks. i 29 



8. Theories of Glacial Motion. 



The one question that continually arises in the minds of all glacial students, 

 with tantalizing frequency is — what is the nature of this glacial motion? Are 

 we any nearer an acceptable hypothesis than we were nearly seventy years 

 ago when the serious study of glaciers was begun? Possibly! Without attempt- 

 ing the discussion here of the various theories that have been proposed, the writer 

 desires to record his convictions after his Pleistocene studies in the Lake Erie 

 region and his four consecutive seasons about the Canadian glaciers. All are 

 now agreed that sliding, expansion by freezing or changes in temperature, 

 general melting under pressure and regelation cannot fully and completely 

 account for the known facts of glacial motion. Probably all will admit that, 

 under certain circumstances, every one of these factors may find its application. 

 In the n6v6 region it is possible that a certain amount of rolling and sliding may 

 occur amongst the granules, producing some motion, such as we may see in a pile 

 of beans or peas. Farther down in the glacier, although the granules are inti- 

 mately interlocked, it is quite probable that they would permit of a certain 

 amount, possibly considerable, motion between their adjacent faces. If we 

 introduce the idea of a partial melting of the granules, those portions of them 

 subjected to especial stress, or friction, will yield under the action of gravity 

 working from above, or behind, and will permit other granules to yield. Upon 

 relief of pressure, the water will be frozen to the original granule or distributed 

 to neighboring granules, as discussed in the preceding paragraph, and thus 

 the movement of the glacier may arise entirely from the alteration and growth 

 of its component granules. We must assume that the heat necessary for the 

 partial liquefaction of the granules is developed within the glacier as the result 

 of pressure and friction and not that it is derived from the atmosphere, or the bed.' 

 However, any heat communicated from such a source will make it that much 

 easier for internal changes to take place. As presented by Chamberlin and 

 Salisbury, this theory of granular change accounts more satisfactorily for the 

 glacial phenomena observed than any other, in which no molecular movement of 

 the solid granules is assumed. 



The original idea of plasticity ascribed to glaciers by Rendu and developed 

 by Forbes is based upon the conception that the molecules of firm ice will yield 

 continuously to a stress, without producing visible rupture. The stress may be 

 of the nature of a thrust, or of tension. This theory has been rejected by the most 

 prominent physicists who have turned their attention to the problem of glacial 

 motion, because of their unwillingness to admit that ice could possess this and cer- 

 tain other properties apparently inconsistent with plasticity. The difficulty so far 

 as rigidity alone is concerned is removed by our knowledge that such substances 

 as lead, tin, and iron may be made to flow by pressure, under ordinary tempera- 

 tures. As soon as direct experiments were made to test the plasticity of ice it 



1 In a'recent'pamphlet entitled, " The^iscous'i;5.'the'Granular Theory of Glacial Motion," Mr. O. W. 

 Willcox has endeavored to'show that the heat developed through pressure or impact of the granules would 

 be couducted away as rapidly as it is generated (p. 18). 



