2O6 ALASKA GLACIERS 



clearest examples are on salient masses, but this may be 

 merely a question of exposure, for the finest illustrations 

 of abrasive sculpture are also on salients. Where a heter- 

 ogeneous rock bed acquires an uneven surface by abrasion, 

 the prominences of obdurate rock would be specially ex- 

 posed to plucking, and it is easy to understand that pluck- 

 ing may be combined with abrasion in the reduction of 

 such a tract. 



The unevenness produced by plucking is a minor feature 

 of the sculpture topography. When greater features are 

 considered it is evident that plucking as well as abrasion 

 is more active on salient than on reentrant profiles, for how- 

 ever hackly an ice-worn hill may be in detail, its general 

 profile and contours have the same sweeping curves which 

 characterize the products of abrasion. 



The preceding discussion, which for brevity has been 

 given somewhat deductive form, is largely based on field 

 observation, being the result of an endeavor to understand 

 the varied phenomena of sculpture observed in Alaska. 

 In a region where the evidence of great glacial erosion is 

 overwhelming, where multitudinous hanging valleys, the 

 general obliteration of spurs from the sides of U-valleys, 

 and the dominant and thorough rounding of crests and 

 corners of hills and small mountains, testify to an enor- 

 mous amount of glacial degradation, it was a matter of 

 surprise to find the reduction of the surface to smooth- 

 sweeping curves a somewhat rare phenomenon. By far 

 the greater number of well-exposed glaciated areas, even 

 where the degradation has been profound, abound in low 

 embossments and in more or less angular groins or re- 

 entrant spaces showing little trace of abrasive action. 

 These surface characters presented themselves as facts 

 requiring explanation; and I have come to regard them 

 as indications of the great importance of plucking in the 

 work of glacial erosion. 



