|History of New York Geology|
|New York City’s Ancient Bedrock
New York City is the site of ancient earth-shaking and earth-shaping events.*(see diagrams below.) The bedrock that anchors the city’s skyscrapers tells a story of a place going back more than a billion years. The island of Manhattan is built on three strata known as Manhattan Schist, Inwood Marble, and Fordham Gneiss. Schist forms the island’s spine from the Henry Hudson Bridge on its north end to the Battery on its southern tip; it dips abruptly several hundred feet below ground at Washington Square, and makes a gradual ascent beginning at Chambers Street. These dips and rises account for the gap between “midtown” and “downtown” in the Manhattan skyline, since tall buildings had to be anchored on solid bedrock, and not on the glacial till that fills the valleys. The contemporaneous Inwood Marble, metamorphosed from limestone, forms beds 150 to 500 feet thick beneath the Harlem River and adjacent regions known to geologists as Inwood Lowland; it underlies the East River and the Harlem Lowland and above ground forms a ridge from Dyckman Street on the upper west side northward to Marble Hill. The billion-year-old Fordham Gneiss erupts to the surface in the Bronx, forming the Riverdale and Grand Concourse ridges. The three strata are complexly interfolded. Each layer tells its own geological story from which we can reconstruct the geological map of New York City.
The granite gneiss was formed a billion years ago, during what is known as the Grenville orogeny, or mountain-building episode, that marked the creation of an ancient super-continent. It is the crystalline basement rock of the Canadian Shield, the most ancient part of the North American continent. It did not become a part of our region until another cataclysmic mountain-building event, the Taconic orogeny, overthrust and intensely folded it with layers of schist and marble, forming the Manhattan Prong. Together these three intensely folded strata shape the topography of Manhattan.
The Taconic orogeny occurred around 450 million years ago, when the continents began to move toward each other. It was the first of a series of tectonic events that culminated in the creation of Pangaea. The Laurasian land mass, comprised of North America, Europe and Asia, began its collision course toward Gondwana, comprised of South America, Africa, Australia, India and Antarctica. This tectonic plate movement generated tremendous pressure and heat beneath the earth’s crust, forcing lava up through volcanic vents. Volcanoes erupted off the northeastern coast of the young North American continent, spewing lava that cooled and gradually formed a volcanic island arc. Winds blew volcanic ash into a shallow marine basin, where sediments accumulated in mineral-rich layers that were gradually compressed into shale. As the sediment-laden oceanic crust slid beneath the lighter continental crust, during the process of subduction, the volcanic island arc accreted to the continent. The mica-rich shale, subjected to the intense heat of the earth’s mantle, was recrystallized and transformed into schist. In a similar process, limestone reefs that had formed over millions of years off the northeastern coast were metamorphosed into marble during subduction.
Cameron’s Line is an extensive thrust fault zone that marks the boundary where North America’s eastern edge accreted to the volcanic island arc. It trends southwest to northeast from Staten Island into western Connecticut. East of Cameron’s Line lies what geologists call the Hartland Terrane, metamorphosed rocks (gneisses and schists) of deep water origin; west of the Line lies the Manhattan Prong, composed of metamorphosed rocks of shallow water origin – Fordham Gneiss, Inwood Marble (metamorphosed limestone), Manhattan Schist, and Lowerre Quartzite (metamorphosed sandstone). These are by no means arranged in simple layers like the leaves of a book; over hundreds of millions of years, they were intensely folded and overthrusted, pushed up into mountains, eroded and weathered, buried under thousands of feet of sediment, and exposed by glacial scouring.
Where to see the rocks
Central Park and Inwood Hill Park are two of the best places to see outcrops of Manhattan Schist. From the high vantage point of Inwood Hill Park, on the northern tip of Manhattan, one can observe the three bedrock strata in the landforms: schist underlies the ridge of Inwood Hill; marble underlies the Harlem River below; and gneiss surfaces as Riverdale Ridge across the river in the Bronx. Across the street from Inwood Hill Park, in the pocket park on Isham Street at the corner of Seaman Avenue, you can see a lovely outcrop of Inwood Marble.
Best rock exposures outside Manhattan
The New Jersey Palisades along the Hudson offer a dramatic window into the rifting of Pangaea, as continents broke apart at the seams beginning 200 million years ago. Lava erupting through rift valleys intruded into sandstone layers. When the lava cooled, it formed blocks of dolerite enclosed in layers of sandstone. As the rift valleys subsided, the sandstone and dolerite layers were tilted and exposed; over time, the red sandstone envelope eroded away, leaving the dramatic black vertical blocks of the Palisades. In other places the lava flowed over the sandstone beds (known as the redbeds), hardening into basalt. Cliffs along the Connecticut River Valley expose the layers of red and brown sandstone capped by black basalt.
Geology Tours. Sidney Horenstein of the Museum of Natural History leads tours.
New York Geology reading list
Graff, M.M. and Thomas Hanley. Rock Trails in Central Park. New York: Greensward Foundation, 1976.
McCully, Betsy. City at the Water's Edge: A Natural History of New York (RIvergate/Rutgers. 2007).
Merguerian, Charles and John Sanders. Geology of Manhattan and the Bronx. New York: The New York Academy of Sciences, 1990-1991.
Roberts, David C. Geology of Eastern North America. Peterson Field Guides. Boston, New York: Houghton Mifflin, 1996.
Van Diver, Bradford B. Roadside Geology of New York. Missoula, Montana: Mountain Press Publishing, 2003.
New York geology links
|'The Falconer,' Central Park
|Isham Park marble outcrop|
|Central Park schist outcrop|
|Pelham Bay gneiss outcrop|
|New Jersey Palisades|
|Rifting of Pangaea in New York region Credit: USGS|
|Plate tectonics. Credit: USGS|
|Geological time-scale. Credit: USGS|
|Updated August 2011|
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