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 moutonnée 
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 due to 
the fact (1) that the glacier carried or pushed a more or less angular boulder 
over 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. 
“Tn 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.* 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 
1 Amer. Geol., Feb. 1890, p. 105. 
2 U.S. Geol. Surv., 7th Ann, Rept., 1885-86, p. 219-220. 
