covery
of a large pot-hole, doubtless glacial in origin, on the northeast
corner of the excavation for that structure, at the corner of Fulton
and Church Streets. It was uncovered beneath the hard pan, and
had been formed in the underlying schist. It had considerable
dimensions, being seven feet in diameter, about five feet deep ( ?),
and held at the bottom an accumulation of pebbles and mud.
Other
evidences, more naturally referable to the consequences of the
presence of the ice sheet, are found in rivers which, by reason of the
glacial accumulations in the preglacial beds, have been diverted, or,
as it were, ejected, and have been forced to find new channels. Such a
remarkable fact seems proved by Professor J. K. Kemp within the limits
of Greater New York and, in this case, illustrated by the Bronx and at
a locality where a public reservation (Bronx Park) places it
permanently within the observation of all.*
The
gorge of the Bronx River, so strikingly shown a little southwest of the
Lorillard House, has, it is assumed, been cut through the gneiss ridge
at a time when it was compelled to desert a previous channel which
passed southwest from Wil-liamsbridge, and which approximately followed
the line of the railroad track for some distance in the limestone beds
in which its course north of Williamsbridge now runs. Pushed aside a
little below Williamsbridge, it began its attack on the gneiss ridge to
the southeast. Pot-holes west of the gorge, which have been excavated
or bored in the rock by water running with some velocity and carrying
sand or gravel, can yet be seen. Without dogmatizing, Professor Kemp
suggests that the Bronx, as a preglacial stream was, upon the
resumption of its duties with the passing away of the Ice Age,
confronted by a drift bank filling up its old channel, and that it
filed through the gneiss hill, cutting its way down as the land
gradually rose. The photograph (Fig. 6$) shows the west wall
♦The
teachers will find a comprehensive discussion and description of
glacial grooves, etc., in the Seventh Annual Report of the United
States Geological Survey, by T. C. Chamberlin.
