134 G. K. GILBERT DISLOCATION AT THIRTYMILE POINT, NEW YORK 



title. In general the shale shows no trace of disturbance, its small south- 

 ward dip being barely perceptible, but within 20 miles of this locality I 

 have noted 10 small faults or groups of faults. With a single exception 

 they belong to the thrust class. The hade ranges from to 60 degrees, 

 and the vertical displacement from 2 to 20 inches. In 5 instances the 

 upthrust block moved southward, and other directions were southwest, 

 west, and east. With some of the faults are associated local dips not ex- 

 ceeding 10 degrees. The faults are believed to be preglacial. The crucial 

 evidence on this point — their relation to the ice-planed top of the rock 

 mass — was not obtained, but the}^ have the smooth walls characteristic 

 of deep-seated shear-planes instead of the ragged faces ordinaril}^ asso- 

 ciated with superficial fractures. 



It is conceivable that compressive strains such as caused these small 

 faults accumulated under the ice-sheet and found relief at its retreating 

 margin in the production of the Thirtymile Point dislocation. The ice- 

 margin would be a critical line for such strains, as they would be there 

 reinforced by strains arising from the horizontal thrust of the glacier. 



The second hypothesis encounters a quantitative difficult3^ If a dis- 

 location of 20 inches is the greatest associated with the diastrophism of 

 all the periods from Silurian to Neocene, a dislocation of 6 feet seems an 

 excessive product of Pleistocene diastrophism ; but this difficulty is miti- 

 gated b}^ the fact, shown by shorelines of glacial lakes, that the region 

 was subject during the ice retreat to differential uplift involving warping 

 and straining. 



The first hypothesis is of interest to glacialists because of its bearing 

 on the question of the efficiency of the glacier snout as a plow. If the 

 ice could loosen and push along a hill of firm shale, it would seem compe- 

 tent to crowd weak strata of clay and marl into such a chaotic heap as 

 is exhibited by the Marthas Vineyard Cretaceous. 



Should this description rouse enough interest to induce others to ex- 

 amine the locality, their visits should not be long delayed. This part of 

 the coast is specially exposed to the attack of storm waves and is rapidly 

 beaten back. The fractured anticline is a zone of weakness, and the 

 attack is there peculiarly effective, so that after a ver}'' few }' ears nothing 

 will remain of the present features of dislocation except the subaqueous 

 traces of the anticline. 



