244 bulletin: museum of comparative zoology. 



group of markings, and affords reason to believe that both groups are 

 effects of the release of shearing stresses set up in the roche moutonnee 

 by a striating boulder. 



Packard's early theory of the furrows has been amended to a form 

 which may be stated in his own words : 



" The curved and crescentic or gouge shape of the mark appears to be clue to 

 the fact (1) that the glacier carried or pushed a more or less angular boulder 

 ov.r a granite nubble or spur, so that the pressure was greater than at other 

 points in the valley [the Aar Valley] ; (2) the more or less rounded boulder, 

 with its lower or under side perhaps somewhat flat, and so situated that the 

 iee rested only on the top, occasioned greater local pressure than where no 

 boulders were present ; (3) the boulder meeting with a slight obstacle suddenly 

 stopped, and the ice pushing it from behind caused it to slightly tip, so that an 

 immense pressure was brought to bear on the small surface, causing the forma- 

 tion of a gouge-like crescentic hollow, with the concavity towards the origin 

 of the motion, i.e., facing up the valley. 



" In making my first explanation I wrongly inferred that there might be an 

 ' advancing and receding motion of the glacier,' so as to cause the stone to 

 turn over. 



" In some way, then, due both to the striking or pushing force of the glacier, 

 and to the local pressure resulting from the presence of a boulder between the 

 ice and the rock-surface, the boulder was not as with the rest of the ground- 

 moraine, pushed gradually and slowly onward, but hitched, thus causing it to 

 break off the lunoid fragment on a surface peculiarly liable, under great local 

 pressure, to exfoliate." 1 



If the writer correctly understand Packard's view, both his hypothesis 

 and the shear hypothesis agree in removing the furrows from the category 

 of "chatter-marks," which imply that the gouging-tool must have been 

 lifted clear of the rock-surface in order to deliver its smiting blow. 

 That true lunoid furrows may be formed at intervals along an otherwise 

 continuous groove is shown in Figs. 32, 33, and 34 of Chamberlin's 

 classic paper. 2 It is clear, in each case, that the boulder must have 

 pressed the ledge with great force as it traversed the inter-furrow space. 

 Packard makes the hollow as well as the tendency to exfoliate date from 

 the time of the actual passage of the boulder over the rock ; but he does 

 not show how localized downward pressure could produce a crescentiform 

 hollow with a steep scarp facing upstream. 



The main conclusions from these considerations are : First, that cres- 

 centic cracks and lunoid furrows may be distinguished in the field ; 

 secondly, that while often very abundant, they form valuable criteria 



i Amor. Geo]., Feb. 181)0, p. 105. 



2 U. S. Geol. Surv.,7tli Ann. Rept.,1885-8G, p. 219-220. 



