116 GEOLOGY OF NEW YORK CITY
Kemp
says, " It must have been a shallow trough now worn completely away,
but its presence would simplify the problem of the development of our
local drainage lines on the east side of Manhattan Island; they would
then be uniformly due to the relatively easy erosion of limestone."
There are yet, however, some elements of proof lacking.
The
shores of Greater New York embrace those with a rocky nucleus, as those
about Manhattan Island, and those of sand, clay, and earth, as those
along the coasts of Long Island and Staten Island. The latter, from the
softness of their material, have been shaped into generally smooth
contours with exceptional prolongations and outlying shoals and bars.
These undergo mutations with the storms and tides as the sand, whirled
and drifted, moves bodily up or down the coast.
The
former, being more resistant from their mineral compactness, although
far older in time, are margined with islets, prominences, inlets, and
knobs. The islands and reefs in the Upper Bay are such rock nuclei; the
strips of hard gneiss forming islands in the East River and the
immature coast line further east towards New Rochelle fall into the
same category.
There
is discovered in the submarine survey of the sea-bottom near the
entrance to the New York Lower Bay a well-defined submarine valley,
also an area of clay bottom extending about one hundred miles seaward,
and, thirdly, a deep ravine at the edge of the continental slope. There
was formerly found in this region a series of " deep mud holes " in a
straight line off the entrance to the harbor. Later these mud holes
were believed to indicate part of a submerged channel continuous with
the axis of the Hudson River valley. Ten nautical miles east by south
from Sandy Hook there is a depth of 114 feet, in a depression or gulch
which extends southerly about ten miles, then turns to the eastward for
five miles, and maintains thereafter a straight course of fifteen miles
to a deeper ravine crossed by a bar, which seems to lie across the
latter's mouth at its immediate debouchment into
