weathering
to a fine sand. It extends from the village of Marble Hill southward to
within 300 feet of the little church at the entrance of the Inwood
ravine or Dyckman Street. On the west it abuts against the gneiss. It
extends east, north, and south under the low valley from Spuyten Duyvil
Creek to the bluff surmounted by Fort George, where its conÂtact with
the gneiss is hidden by a great depth of drift.
The
limestone exhibits its various characters along the wall south of the
Seaman Mansion, where the coarse crystallized surfaces are contrasted
with very fine-grained and schistose rock. The view from the heights of
Spuyten Duyvil is inÂstructive. The dome of limestone is plainly seen,
sinking on the west into a valley or crevice, penetrated, a little way,
by the Spuyten Duyvil Creek, whence rise the steeper and higher walls
of the Inwood spur of gneiss. The limestone formation is
characteristic, being undulating and softened into low swells by
erosion and solution. The dip is east.
At
the extreme western end of the Spuyten Duyvil Creek the opening to the
Hudson River suggests a crack, fault, or fissure, but east of this, at
the cut of the railroad (Fig. 25), the line of the creek seems to mark
the delimitation of the limestone on the south from the gneiss on the
north, and the creek has its bed in limestone, as the streams generally
do in Westchester County (Dana). A dominant point of interest from
which topography of this section is well descried is at the end of the
Bolton Road leading up from Dyckman Street, and beyond the House of
Mercy, before the descent is reached which plunges in Spuyten Duyvil
Creek. In the aqueduct shaft, at 180th Street, between 10th Avenue and
the Harlem River, at 165 feet below the bed of the river, at its
center, or 465 feet below the level of 10th Avenue, coarse and compact
limestone was taken out.
This
limestone underlies the Harlem River, and is produced in long
prolongations underneath 4th and 5th Avenues (at I32d Street) and also
under 8th Avenue, interrupted by gneiss, which appears to hold it in
synclinal troughs, while in
