1882. 183 Trans. N. Y. 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 
too 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 imperfectiy preserved, near the village of West Farms, about 
two miles south of those described, in the same valley. 
