GLACIAL SCULPTURE 207 



The importance of plucking may also be inferred, as in 

 fact it has been by Dana, from the abundance of boulders 

 in the moraines of an ice-sheet In the waste deposited 

 by an alpine glacier it is not easy to discriminate 

 plucked boulders from the boulders which have fallen to 

 the ice from adjacent rock slopes and been carried for- 

 ward as back-load, but the waste carried to the edge of a 

 great ice-sheet like the Laurentide has all been picked 

 up as well as transported by the ice. A portion of such 

 a body of waste must be referred to the mantle of resid- 

 uary and alluvial debris found initially on the land by the 

 expanding ice mass, and this portion of course includes a 

 contingent of boulders ; but there is no reason to regard 

 this factor as of great importance. It is probably much 

 more than offset by the destruction of boulders in the 

 glacial mill, for in the making of the rock flour which 

 constitutes the body of glacial till, the abrasion of the 

 coarser waste carried by the bottom ice may approach, 

 or even exceed, the abrasion of the rock floor. While 

 these various factors do not admit of definite valuation, I 

 think it fair to say that the ratio of boulders, on the one 

 hand, to clay and sand, on the other, in the waste deposits 

 as a whole, is something less than the ratio of plucking 

 to abrasion in the erosive work of such an ice-sheet as 

 the Laurentide. 



The study of this subject made such slow progress in 

 the field that opportunities for good photographic illustra- 

 tion were not improved. The views reproduced in plate 

 xvin were taken to show abrasive work, and illustrate 

 plucking only incidentally. The upper view looks up the 

 wall of Muir Inlet. It shows stratified rock, with a boss 

 of more massive, possibly plutonic, rock beyond. A 

 century ago all the stratified rock was covered by Muir 

 Glacier. At the left near the sky-line the strata are seen 

 to be obliquely truncated, and the plane or nappe of trun- 



