Geology of the New York City Region

6. Pelham Bay Park

The Hartland Formation forms the bedrock beneath the Bronx east of Cameron's Line. The shoreline in Pelham Bay Park is an interesting location to study the crystalline basement rock comprising the Hartland Formation. The park is also a good location to examine a variety of glacial materials and features, and modern coastal processes and habitats.

Pelham Bay Park is on the eastern side of the Bronx bordering the western end of Long Island Sound. The park is accessible from park exits on the Bruckner Expressway and the Hutchinson River Parkway, and is also accessible from the last stop on the IRT no. 6 train (combined with either a long walk or a ride on the BX12 bus to Orchard Beach). Orchard Beach is a developed swimming beach on Long Island Sound on the eastern side of the park.

The park encompasses 2,764 acres of coastal lowland, of which roughly a quarter is set aside as two wildlife sanctuaries. The Thomas Pell Wildlife Sanctuary along the Hutchinson River/Eastchester Bay side of the park and is host to tidal marshes bordered by oak-hickory forests. The Hunter Island Marine Zoology and Geology Sanctuary is on the north side of the Orchard Beach bathing area, and includes the northeastern shoreline of Hunter Island, and all of Twin Islands, Two Trees Island, and Cat Briar Island. Storm waves from Long Island Sound have eroded the coastline along Hunter Island, exposing the bedrock of Hartland Formation. The rock consists of granitic and garnetiferous amphibolite gneiss with numerous quartz veins and migmatite dikes. Migmatite is an type igneous rock that forms when metamorphic rocks begin to melt under high temperature. Felsic minerals melt and are injected into the surrounding rock along joints, faults, and other zones of weakness in the rock. As the igneous material gradually cools, bands of feldspar and quartz crystals form along the edges of the intrusion. The center of the migmatite veins typically consist of larger crystals of feldspar and quartz. The migmatite stands out in outcrops as light-colored bands in contrast to the darker amphibolite gneiss host rock. In some cases, the dikes cut across older dikes and quartz-filled veins; many are folded or display offset by faulting. Overlying the bedrock is a blanket of glacial till from which numerous erratics are weathering. The beach is littered with large erratics derived from bedrock sources nearby.

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Last modified: 3/11/2019