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MANHATTAN ISLAND 45
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hattan
Island rocks are very rare, or unknown. The following interesting note
of J. Howard Wilson, connected with excavations in Rector Street, is
suggestive of a former boreal climate:
"
In excavating for the new building for the United States Express
Company, on Rector Street, between Sixth and Ninth Avenue elevated, Mr.
Daniel E. Moran, C. E., found resting on the bedrock forty feet below
the surface a small deposit of Venus shells, fragments of wood
and some peaty matter. This deposit was covered by ten feet of glacial
drift which, in turn, was buried under thirty feet of sand, probably of
post-glacial age. The fossiliferous deposit was apparently protected
from the ice action in this spot by a local ledge or shelf of the
bedrock.
The Venus shells resemble very closely those of the recent V. mercenaria Linn., but differ from them somewhat and along a line which seemed to identify them with the variety antiqua of
Verrill from the Pleistocene deposits of Sankaty Head, Nantucket. The
Manhattan specimens were compared with a number of these in the
collections at Columbia University, and the identification was found to
be complete. The variety antiqua is an unusually massive and strongly sculptured variety."
This
shell—a hard-shell clam or quahog—is regarded as symptomatic of cold
currents, and has been found in shell beds on Nantucket Island, in
which were unmistakably associated species of shells decidedly northern
in habit.
Professor
William H. Hobbs * has taken pains to investigate the underlying rock
flooring of Greater New York, and has gathered together a great many
records from the engineering work carried on in the construction of
the East River bridges, the tunnels of the Pennsylvania R. R., the
various municipal bridges connecting Long Island with New York, the
Croton aqueduct, the Subway in Manhattan, the Harlem Ship Canal,
together with the reports of borings, excavations,
♦The Configuration of the Rock Floor of Greater New York, William Herbert Hobbs, Bulletin 270, United States Geological Survey.
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