PHILOSOPHICAL CONCLUSION 47 



more conspicuous by such moderate widening of valleys and steepening 

 and smoothing of the valley walls as the ice has possibly done. 



PHIL OSOPHl CA L CONCL USION 



Even a small amount of ice wear may be conspicuous, while the actual 

 amount of erosion will commonly be indeterminate. However, there 

 are decided limitations to the possible degrees of erosion, and ignorance 

 of the limit gives no warrant for excessive claims, even if it does give 

 the opportunity. When along with the quantitative uncertainty of the 

 amount of ice-work there is combined qualitative doubt in tbe diagnosis 

 or interpretation of physiographic features, then the matter becomes 

 chiefly a question of mental attitude and personal judgment. 



The claims for extreme glacial erosion have been almost entirely 

 founded on physiographic characters — deep lake basins, fiords, and hang- 

 ing valleys. This was true of Ramsay and his followers in England, of 

 Helland and his school in Norway, and of the group of eminent physiog- 

 raphers in America. The assumption is made, without any attempt 

 at proof except the physiographic argument, that Pleistocene glaciers 

 could abrade thousands of feet in granitic rocks. The illogical argu- 

 ment may be briefly stated as follows : Hanging valleys are common in 

 glaciated regions ; they are not thought to be a normal product of stream 

 work ; they may be explained by glacial deepening of the trunk valleys ; 

 therefore the trunk valleys have been ice-deepened. But all the geologic 

 evidence is to the effect that it was impossible in the time available for 

 the glaciers to cut deep valleys. The advocates of ice erosion have never 

 presented any proof from living or extinct glaciers that ice has made or 

 is making or could possibly make a deep valley in hard, unweathered 

 rock. The position is taken behind a bulwark of analogy and assump- 

 tion. 



The burden of proof is properly on the advocates of unlimited ice 

 erosion; but it would seem wiser to essay the task of explaining anom- 

 alous topography by the operation of ordinal and competent agencies. 

 It is far more probable that some physiographic element has been over- 

 looked, or that knowledge is deficient, or interpretation in error, than that 

 Norwegian and Alaskan glaciers did a kind and amount of work which 

 glaciers in general have not done, are not doing, and apparently can 

 not do. 



We do not yet know the erosional conditions and effects resulting from 

 rapid uplift of high mountains with cores of crystalline rocks of hetero- 

 geneous structure and under the probably warm and humid climate of 

 the pre-Pleistocene. " Hung up " valleys will probably be found a 



