282 R. S. TARR RECENT ADVANCE OF GLaCIERS IN ALASKA 



transmission of this great wave; in fact, this difficulty has led me to 

 question whether it may not be necessary to seek an explanation in an 

 earthquake of more remote date. Facts bearing on this question are not 

 at hand. 



So far as I know, no similar phenomenon is reported from other re- 

 gions for comparison. It is well known that normal climatic variations 

 cause a slow wave of advance, but exactly what would follow upon a sud- 

 den and abnormal accession of snow supply is not clear. So far as I am 

 able to consider the question, however, I am led to believe that it would 

 not be proper to make use of the results of normal climatic variations as 

 an exact parallel in a consideration of the effects which would follow 

 upon a sudden accession of enormous quantities of snow and ice. It 

 seems probable that so great an increase in supply would start a wave of 

 such power as to crowd upon the rigid and nearly stagnant ice of a wast- 

 ing glacier. Such a crowding, by applying a rapid thrust against the ice, 

 might well push it so hard and fast as to cause it to slide forward, and 

 cause it to move by massive breaking instead of by interstitial movement. 

 It is conceivable even that the effects of such a wave might advance faster 

 than the wave itself. 



To state the latter point more fully, when a great wave of advance, 

 starting in the neve region, rieaches the glacier proper, it might exert 

 sufficient pressure to push forward the ice in front of it, and thus cause 

 it to break, pile up, and advance even down to the very margin. In this 

 way the effects of a great wave of advance might well extend clear to the 

 end of the glacier long before it would be possible for a normal wave to 

 reach the terminus. Some such process as this seems demanded by the 

 fact that miles of ice are suddenly, in a period of not over ten months, 

 changed absolutely from a surface easily traversed to a labyrinth of 

 crevasses. 



If ice near the bottom of the glacier is moving by viscous flow, the sud- 

 den application of pressure, due to great accession of supply, may cause 

 a more rapid flow of the bottom layers, and thus greatly rupture the rigid 

 upper layers. 



Glaciers that are not advancing 



That some glaciers are advancing and others not, suggests the possibil- 

 ity that there is some selective action, as a result of which certain glaciers 

 are immune from the effects which have caused such a remarkable ad- 

 vance in others; but this may be only . apparent. If the cause for the 

 advance is earthquake shaking, it ought in time to affect most, if not all, 



