THE QUESTION OF CAUSE 



133 



clear that portions of the anticline pass be^'ond the vertical and are sup- 

 ported by the enveloping till. 



Interpretations 



With reference to questions of cause, two features of the overturning 

 are sjiecially significant : one, that the overthrow is restricted to the part 

 projecting above the surrounding rock ; the other, that the crest is thrown 

 toward the southwest, or in the direction of the local movement of the 

 Pleistocene ice-sheet. These peculiarities point to the glacier as the cause 

 of the overturning, and the deformation is thus distinguished from the 

 ordinary anticlinal ridges of the old lake bottoms, Avhich are clearly sub- 

 sequent not only to the ice-sheet but to the glacial lakes. 



On the other hand, the disturbance can not have been preglacial, for 

 in that case ice erosion would have destroyed the ])rojecting rock ridge. 



FicuRE 2. — Section at Thirtymile Point. 



Showing antieline in Medina shale and associated ridge or terrace of till. For position, see pre- 

 ceding figure. The cliff at left is 20 feet liigh. 



Indeed, the preservation of this frail ridge of crushed rock, and of the still 

 softer ridge of till, shows that there was little ice scour after the disloca- 

 tion occurred. 



The only assumption as to time which seems to accord with all the 

 facts dates the deformation just before the ice at this locality ceased to 

 move. The edge of the glacier stood here when the uplift took place, 

 there was enough subsequent motion to overturn the projecting anticline, 

 and then the ice stopped or its face was melted back. 



The association of the disturbance with the ice margin leads naturally 

 to the further hypothesis that the glacier was responsible for all the 

 phenomena — that the forward thrust of the glacier was able, under the 

 critical conditions obtaining at the ice margin, to tear from its secular 

 moorings a broad mass of rock and slide it a iey^ feet forward and up- 

 ward. If that is what actually occurred the slidden block was at least 

 300 feet broad and 20 feet thick, and its dimensions may have been very 

 much greater. 



An alternative hypothesis connects the dislocation with the general 

 fault system of the region — if its trivial slips may be dignified by that 



