GLACIAL STUDIES IN GREENLAND 59 1 



cna naturally raise the question whether the projection of the 

 upper layers is due to actual overthrust of the upper layers or 

 merely to the more rapid melting of the lower layers, or to a 

 combination of the two. There is no question but that the dirty 

 layers absorb the solar heat with more facility than the cleaner 

 ice and are melting backward more rapidly, and that some of 

 the irregularities which the vertical faces of this and other gla- 

 ciers present are due to this differential melting. There is, on 

 the other hand, little room for doubt that the upper layers of the 

 glacier move faster than the lower ones. This is in accord with 

 the generally accepted doctrine that the upper portion of a gla- 

 cier moves faster than the lower, a doctrine based upon observa- 

 tion as well as theory. It is, however, an open question whether 

 the differential motion is localized along the planes which sepa- 

 rate the layers, or whether it is distributed through the mass. It 

 is not advisable to enter at length upon the discussion of the 

 question here, but these very striking phenomena merit special 

 study with reference to it. It will be observed that the under- 

 sides of the projecting layers are distinctly fluted. This might 

 easily be interpreted as a demonstration in itself of the move- 

 ment of the upper layers over bowlder-shod under layers, result- 

 ing in the grooving of the over-running masses. It appears 

 clear, however, that to some extent at least the fashioning of 

 these flutings in the form in which they are now seen is due to 

 the action of water descending the face of the glacier and flow- 

 ing backward on the under side of the projecting layers. This 

 does not, however, dismiss the hypothesis that the initiation of 

 the fluting was due to differential motion between the layers. 

 The fact that the debris has been carried in between the layers 

 is in itself very significant respecting the hypothesis of one layer 

 sliding over another. But this differential motion might have 

 been confined to the point where the debris was carried in and 

 may not be taking place in this terminal part of the ice. So 

 this class of evidence, though it is pertinent to the general ques- 

 tion of a shearing motion between the layers, is not altogether 

 unequivocal in its application here. 



