308 G. K. GILBERT — CRESCENTIC GOUGES ON GLACIATED SURFACES 



ceases. It begins somewhere on the upstream slope of the projection and 

 ceases at its crest. Thus the conditions for localization of pressure by 

 this method have the same distribution in relation to prominences of the 

 rock bed as that observed for the crescentic gouges. 



If the ice beneath the boulder is clear of debris, it is probable that a 

 large differential pressure can not be developed without causing the ice 

 to iiow away and bringing the boulder into contact with the rock bed. 

 The result to the rock bed of such contact would be a deep scratch or 

 groove, and grooves are not the normal associates of crescentic gouges. 

 It seems necessary, therefore, to suppose that when crescentic gouges were 

 made the direct contact of the boulder was in some way prevented — and 

 the means of prevention is not far to seek. If only the sand and other 

 fine detritus normally abundant in the base of a glacier be assumed to 

 saturate the ice beneath the boulder, a cushion is provided quite compe- 





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Figure 5. — Longitudinal Section of lower 

 Part of a Glacier. 



The section is supposed to be at a point 

 where it passes a projection of the rock 

 bed, and illustrates the deflection of lines- 

 of flow and the temporary compression of 

 the lowest layers of ice. 



Figure 6. — Longitudinal Section of loiter 

 Pari of a Glacier. 



The section is on the upstream side of a 

 projection of the rock bed and illustrates 

 the changing relations of an embedded 

 boulder to the system of flow lines. Trie 

 ice motion is from right to left. 



tent to prevent actual contact of boulder and rock bed and at the same 

 time transmit the pressure of the boulder to a small area of the bed. 



A complete discussion of this hypothesis would include a mathematical 

 analysis of the mechanics of the conoid fracture. Only the elastician is 

 competent to make such an analysis, and I have not attempted it. Never- 

 theless, as I have not been able to ignore altogether that aspect of the 

 subject, I shall venture a few lay suggestions. 



As the conoid of percussion is symmetric about an axis normal to the 

 surface receiving the blow, and as the conoid of the crescentic gouge is 

 asymmetric, it may be inferred that the direction of the force producing 

 the latter is oblique to the rock surface. In a general way all pressures 

 of the ice upon glaciated bed-rock must be oblique ; otherwise there would 

 be no forward motion; but the particular pressure to which appeal has 

 been made in connection with the crescentic gouge is the result of a com- 

 pression of the ice in the direction normal to the rock face, and should oe 



