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154 GEOLOGY OF NEW YORK CITY
Long
Island is a fusi-form strip of land 120 miles long and 23 miles wide at
its widest point, and split at its eastern end into two lobes, to which
whalers have expressively applied the names of North Fluke and South
Fluke, from the suggestion of a whale's tail. Its higher elevations
are on the north, in an irregular line of hills which attain a height
of 420 feet at High Hill, and which cross Brooklyn and terminate at the
Narrows. This range of hills divides eastward and projects two ribs
into the north and south arms of land inclosing Peconic Bay, while,
approaching the southern limit of Long Island Sound, its declivities
plunge sometimes steeply into that body of water, and its outlines are
deranged by picturesque and deeply emphasized indentations. South of
this northern marginal rim the island presents a sloping plain to the
ocean, which inundates its southern shores, forming many embay-ments,
beyond which a thin reef of sand makes a slender fence against the
outer sea and creates a chain of semi-inland bays: Jamaica, Hempstead,
Great South, Moriches, Shinnecok.
The
basal, hard, crystalline rocks underlying the island have been alluded
to. Immediately above these it is believed that the Cretaceous beds
lie, and while some of them form visible topographic features, they are
for the most part buried, and their position and succession is
determined by wells. The hidden sources of water supply which Long
Island is now seeking are to be found, in part, in the Cretaceous
sands. The beds of the Cretaceous are described as consisting of
strata, of a depth of 300 to 400 feet, of sand and plastic clays, often
brightly colored, resting on the crystallines. These pass upward into
light-colored quartz sands, with occasional clay beds, on the north,
and into fine gray lignite-bearing sands and clays on the south shore.
The greensand marls of New Jersey, with their marine fossils (the
teachers may see these latter abundantly displayed in the fossil-shell
layer at Atlantic Highlands, N. J.), are absent in the Long Island
Cretaceous, but plant remains are present, indicating shoal water or
near-shore conditions. The Cretaceous gravels are
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