3. Inwood Hill Park and Isham Park
Inwood Hill and Isham Parks are on the northern end of Manhattan Island, and
are host to many easily accessible outcrops along park trails away from traffic
(Leave your hammers at home! It is illegal to collect rock samples in all city
parks). The best place to park a car is near Isham Park along Seaman Street
and Isham Avenue. Isham Park borders the east side of Inwood Hill Park Exceptional
exposures of Inwood Marble crop out around the intersection of Isham and Seaman
Avenues (Figure 19). The marble consists of a variety of lithologies ranging
from thin-to-thick bedded, brownish to gray, sugary-textured dolomitic marble
to fine-grained marble, quartzite, and schist. Some layers display
crenulated folding. The sand around the base of the outcrop consists almost
entirely of gray dolomite crystals.
Inwood Marble is known to occur in the subsurface throughout much of Manhattan.
It has been encountered in deep excavations beneath the Midtown area.
|Figure 19. Outcrop of Ordovician Inwood Marble in Isham Park.
Inwood Hill Park consists of a number of playing fields on its eastern side,
with a forested, rocky upland on its western margin. The park is bordered by
the Hudson River to the west, the Harlem River to the north, by Seaman Avenue
and Payson Street to the east, and by Dyckman Street to the south. The steep
hillsides along the western edge of Inwood Park are host to large outcrops of
the Manhattan Formation (Figure 20). The formation consists of gray, garnetiferous
schist and dark-colored amphibolite gneiss. Outcrops of schist and gneiss bearing
garnet crystals up to an inch in diameter occur along the trail directly west
of the intersection of Isham Avenue with Seaman Avenue around the baseball fields.
A trail system leads upward through a beautiful climax forest to high points
overlooking the Harlem River. This area was the last portion of Manhattan to
be developed. Until as recently as the 1940s the Inwood Park was still farmland
and the home of a small village of American Indians.
|Figure 20. Garnetiferous gneiss and schist of the Manhattan Formation
exposed at Inwood Hill Park. Note garnets next to hammer handle (hammer
handle is 1 foot for scale - not for collecting!).