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!).