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



duced or even abolished on the opposite side. The hollow under the finger 

 is a direct result of the pressure and the curving ridge is an indirect 

 result, the intermediate factors being a complex system of internal strains 

 and stresses. 



I conceive that an analogous condition obtains in the rock bed as a result 

 of the oblique pressure under the hypothetic boulder ; that there is a cen- 

 tral depression of the surface (figure 7) ; that this is margined on one 

 side by a curved elevation; and that there are internal strains and 

 stresses; but the strains are comparatively small and the slopes of de- 

 formation are very gentle, because in rock the strain limit is quickly 

 reached and rupture ensues. The hypothesis assumes that rupture in this 

 case is initiated in the surface of the rock, along the inner slope of the 

 curving ridge (figure 7), and is propagated obliquely downward, forming 

 the conoid fracture (figure 8) of the crescentic gouge. 



Figure 8. — Theoretic Deformation of Rock beneath a Glacier. 



This ideal section illustrates the theoretic deformation of rock beneath a glacier by 

 differential pressure in connection with an embedded boulder. The arrow indicates the 

 direction of the pressure. The direction of ice motion is from right to left. The conoid 

 fracture of the crescentic gouge is shown at left of the boulder. Compare figure 7. 



In the absence of a rigorous analysis of the stresses associated with the 

 deformation, the correlation of the conoid fracture with the curved ridge 

 is an assumption only; but having made that assumption it seems pos- 

 sible to base on it certain inferences tending to throw light on other ele- 

 ments of the gouge. In the production of the deformation the rock 

 compressed vertically under the boulder experienced horizontal dilatation 

 whereby the ridge was pushed up, and the ridge itself experienced hori- 

 zontal compression. The region of the fracture was thus subjected to 

 horizontal compression just before the rupture, and as soon as the fracture 

 had been formed the wedge of rock above it was relieved of horizontal 

 compression and recovered its original horizontal extent. The wedge 

 had also been bent, its upper surface constituting the crest of the ridge, 

 and when it was detached beneath it tended to recover also its original 

 unbent form by lifting its edge. This change was resisted by the pres- 

 sure of the overlying ice, with the result that the wedge became affected 



