Thank you for your question.
The answer is extensive veins of quartz.
Journal of the Torrey Botanical Society, Oct-Dec 2004 by DeCandido,
Robert, Lamont, Eric E
"Pelham Bay Park (PBPK), Bronx County, New York is located along the
southwestern shore of the Long Island Sound (40° 52' 30'' N, 73° 47'
30'' W), north of Manhattan and south of New Rochelle, Westchester
"Landform in PBPK was largely determined by several glacial flows
during the Pleistocene Period. Evidence for these glaciers in the park
is in the form of roche moutonees, rock erratics, striae and groove
markings. The underlying geology of PBPK is primarily metamorphic in
origin and includes felsic gneisses, sillimanite schists and
amphibolites, with extensive veins of quartz (Schubert 1968, Leveson
and Seyfert 1969)."
Artifacts found on Davids Island
"NEW ROCHELLE ? Archaeologists digging on Davids Island have unearthed
several small chips of quartz that were broken off by ancient American
Indians fashioning tools or weapons on the island."
"For now, Tetra Tech plans to dig 700 to 900 holes. Brighton said
Friday that it had redirected some of its efforts after the first
pieces of quartz were found Wednesday.
Most artifacts can be found close to the surface, said Chris Borstel,
the Tetra Tech field director on the project, because the ground has
not been changed much by natural forces since the last glacier
glaciers formed New Rochelle NY
glaciers formed quartz