44 F. H. Lahee — Crescentic Fractures of Glacial Origin. 



duction of the fractures, since many sets are not accompanied 

 by striae or grooves. The scratch associated with a group of 

 fractures denotes merely a greater intensity of the force which 

 first occasioned these fractures ; or, in other words, a set of 

 fractures, in itself, is evidence of a less intense force than the 

 set accompanied by a groove. 



The distribution of this force within the bed-rock seems to 

 have been tensional. u If the friction on some spot is greater 

 than on the surrounding area, the rock just beneath that spot 

 is moved forward in relation to the surrounding rock through 

 a minute but finite space. This relative movement involves 

 compression about the downstream side of the affected rock 

 and tension about its upstream side rupture occur- 

 ring when the tensile stress exceeds the strength of the rock,"* 

 and this degree of tensile stress appears to have been arrived 

 at rhythmically, as in the case of chatter-marks, since the cres- 

 centic fractures are rather evenly spaced along the axis of any 

 given series. Thus the crescentic fracture resembles the 

 chatter-mark not only in form and orientation, but also in 

 the method of operation of the forces involved. 



In addition to the conditions already mentioned, another is 

 indicated by the position of the fractures, especially (and 

 almost entirely) upon gently northward-sloping ledges on the 

 north side of the hill near its crest. This is where the grav- 

 ity component of resistance offered by the hill to the onward 

 motion of the ice would be practically nil and would be pass- 

 ing into a gravity component of assistance to such motion. 

 To the north of this place the ice was shoved up ; to the south 

 of it, the ice was pushed down. Topographic form seems to 

 have controlled, to some extent, the production of the frac- 

 tures.f 



Summarizing, we infer that the conditions important in the 

 formation of crescentic cracks are : (1) a hard, brittle bed-rock ; 

 (2) a relatively heavy body of ice moving over this rock ; (3) 

 the presence in the ice-base of abundant rock fragments either 

 in contact, or nearly in contact, with the bed-rock : (4) the 

 origin of local tensile stresses within this bed-rock, near its sur- 

 face, by virtue of frictional resistance between it and the frag- 

 ments ; (5) a gentle slope of the overridden surface toward the 

 direction from which the ice is coming; and (6) the position 

 of this surface near the crest of a hill, that is, between slopes 

 opposed to, and dipping with, the ice motion, and conse- 

 quently where there should be in process material changes in 

 the relations of the interacting forces. 



Harvard University, Cambridge, Mass., Oct. 24, 1911. 



* Gilbert, G. K., op. cit., p. 304. ' 



f The writer finds no mention, in the literature, of the exact relation 

 between the topography and other occurrences of the crescentic fractures. 

 The conclusion just presented is drawn from the Northey Hill locality only. 



