20 6 
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. 
