1 882. 183 Trans. N. V. Ac. Scz. 



tion, where the wear began, is now represented only by a portion of the 

 rock surface about 5' high and 2' long, which is concave and smooth. 



Some fifty feet southwest of this hole, and fifteen feet above it, there 

 are two other vertical pieces of rock with smoothed and somewhat con- 

 cave surfaces, which, perhaps, may be remains of others, but they are 

 loo imperfect to warrant any description. 



The rock, in which these pot-holes are situated, is a compact fine- 

 grained gneiss, with black mica. It is very much broken up by hori- 

 zontal and vertical cleavage planes. There are not very many loose 

 blocks, however, these having been carried away to the south by the 

 glacier of the ice period. The natural strike of the strata in the vi- 

 cinity is S. about 18° W. The dip is nearly vertical, being in the neigh- 

 borhood of 85° N. W. 



The history of the formation of these holes may be briefly outlined 

 as follows : that, in pre-glacial times, the valley through which the 

 Bronx now flows was occupied by a large stream, which was expanded 

 into a broad and deep comparatively slow-moving body of water, above 

 the point where the holes are now situated ; but, at and below this point,^ 

 it became a rapid, turbulent and shallow river, whose level, as shown by 

 the position of the holes, must have been at least thirty feet above that of 

 the present one ; that the holes were produced, as they have been 

 shown to be in other localities, by the grinding action of some harder 

 stone on the gneiss rock, this stone being kept in a whirling motion 

 by an eddy in the current. During the ice period the valley was 

 deepened, and the narrow gorge, through which the waters of the 

 Bronx now pass from below Williamsbridge to Bronxdale, was ex- 

 cavated, the ice . sheet having carried the debris southwardly and 

 deposited it on the terminal moraine running through Long Island. I 

 am informed by Prof. J. D. Hyatt that there are two other pot-holes, 

 rather imperfectly preserved, near the village of West Farms, about 

 two miles south of those described, in the same valley. 



