By Marcia Anderson
Skyscrapers seem to dominate Manhattan, but look a little closer and you will see very clearly that they are actually clustered in two distinct areas. Downtown is home to the new Freedom Tower, the Financial District, EPA and Federal Plaza. Midtown includes the Empire State Building, Chrysler Building, Park Avenue and Times Square.
The surface bedrock over much of New York City provides solid anchorage to support the high buildings. Although there are two areas of this strong bedrock at or close to the surface, there is a valley in between where the building heights are lower. In the region south of 30th Street the strong bedrock begins to fall away, and at Washington Square it descends deeply into the earth. The whole region between midtown and Wall Street, including Greenwich Village, SoHo, and Chinatown, would be underwater were it not filled with fathoms of debris left over from the Ice Ages. In this middle area where the bedrock is buried deeply beneath sediments, it is far more difficult to build tall buildings with structural integrity, since such buildings have to be anchored on solid bedrock, not on sediments and glacial till.
The Rocks that Form Manhattan.
According to the American Museum of Natural History, the island of Manhattan is built on three layers of rock known as Manhattan Schist, Inwood Marble and Fordham Gneiss. The layers were once flat, laying like a sandwich. The Inwood Marble was metamorphosed from limestone by heat and intense pressure, and formed the 150 to 500 foot thick marble beds beneath the Harlem River, East River and the Harlem lowland. It is visible above ground forming a ridge from Dyckman Street on the Upper West Side northward to Marble Hill. The billion-year-old Fordham Gneiss lies above the surface in the Bronx, and forms the Riverdale and Grand Concourse ridges.
The Manhattan Schist runs from the Henry Hudson Bridge on the north end to the Battery on its southern tip. However, it dips abruptly several hundred feet below ground at Washington Square, and gradually ascends beginning at Chambers Street (New York City nature). The three rock layers are now interfolded and shape the topography of Manhattan. Where Manhattan schist is found close to the surface, you can build high, and so Downtown and Midtown host Manhattan’s tallest skyscrapers.
What Caused the Great Dip in the Manhattan Schist?
About 500 million years ago, the African continent, was on a collision course with the ancient North American continent. 450 million years ago, New York City was situated where the continental plates collided and was trapped roughly in the center of the newly formed super-continent, Pangaea. The impact of the colliding continental plates pushed up the land located in-between the two plates, like an accordion, forcing mountains upwards, forming the soaring Tectonic Mountain Range. The collision folded the bedrock into dips and folds which account for the deep valley filled with sediment and till (New York City geology). Today, all that remains of these mountains are their stumps which are the bedrock of modern-day NYC. Being buried and compressed under such a vast mountain range has made the Manhattan Schist an exceptionally hard rock. However, the dips that were folded down formed valleys that were later filled with accumulated loose sediments and rocks during the Ice Ages.
But the story of New York City does not end there. The area that NYC sat on was weak, unstable, and prone to earthquakes, and later volcanoes. The volcanoes triggered a mass extinction of plants and animals, by depositing huge amounts of hot magma and poisoning surrounding waters and air with sulfur and other toxic compounds. About 200 million years ago, a dark, volcanic rock, called basalt, was formed by these volcanic eruptions, and created the Palisades. At this same time, the super-continent broke apart, and NYC wound up on the coast again. Much of the later success of NYC is owed to its location on the coast and its deep water harbor.
The minerals in the Manhattan Schist can also prove this story of the bedrock of NYC’s long history. The shiny flakes of mica and crystals of garnet only form at very deep depths, under extensive pressure for millions of years. Look for them, in the outcrops scattered throughout Central Park.
About the Author: About the Author: Marcia is with EPA’s Center of Expertise for School IPM in Dallas, Texas. She holds a PhD in Environmental Management from Montclair State University along with degrees in Biology, Environmental Design, Landscape Architecture, and Instruction and Curriculum. Marcia was formerly with the EPA Region 2 Pesticides Program and has been a professor of Earth and Environmental Studies, Geology, and Oceanography at several universities.