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Fordham Gneiss

Where the Skyscrapers Are



Dense, dark green to black, banded, grainy-textured, it punctuates the unseen underbelly of Manhattan. It was formed hundreds of millions of years ago in a crucible of immense heat and pressure, a tectonic upheaval as volcanoes erupted and the continental plates of Pangaea, the supercontinent, ground against each other. They divided, creating a vast gulf that would separate the Eastern Seaboard from North Africa. It is a rock. It is an island.

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Manhattan is a geologist’s dream. But sophisticated on-site analysis of what lies beneath the surface is a relatively recent phenomenon. Construction of Water Tunnel No. 3, as deep as six hundred feet below street level, the Second Avenue Subway, the Flushing Line Subway Extension, and the Long Island Rail Road’s East Side Access project to Grand Central Terminal under Park Avenue opened a basement window for geologists to confirm their vision of how Manhattan was formed and why skyscrapers sprouted downtown and in midtown but not in between.

Depending on where you live in Manhattan, you can’t honestly say it’s not your fault. What geologists found was a wide variety of metamorphic rock—formed as tectonic plates collided—and distinct geological fault lines along Dyckman Street, 125th Street, Morningside Drive, and Canal Street, suggested by water coursing through the paths of least resistance, fractures and fissures that reached across the spine of Manhattan between the East and Hudson Rivers. While Manhattan schist is the best known of the rock formations that form the city’s subbasement, the island is also defined by amphibolite, by Inwood marble farther uptown, and by Fordham gneiss, which predominates in the Bronx, on Roosevelt Island, and on the Lower East Side (and protruding on “C-rock” opposite the Columbia University athletic complex).

Gneiss (pronounced “nice”) dates back a dazzling 1.2 billion years, when earth-shattering continental collisions caused sedimentary rock to recrystallize into contorted black-and-white-banded metamorphic rock. It is the oldest natural New York object. (The oldest objects in New York are 4.6-billion-year-old meteors and 10-­billion-year-old stardust—actually, presolar grains in primitive chondrites—at the American Museum of Natural History. The oldest handcrafted object in Manhattan is considered to be the obelisk known as Cleopatra’s Needle, dating from 1450 B.C. and installed in Central Park in 1881.)

The interlayered rock formations belowground are analogous to the intermixed neighborhoods on the surface. The granites are folded into tunnel walls exposed by monstrous rock-boring machines. The undulating formations are the bedrock that defines Manhattan’s skyline. In midtown, bedrock is just below the asphalt. To build the World Trade Center, seventy-five feet of fill, glacial till, and muck had to be excavated until bedrock was reached. In between downtown and midtown, the bedrock surface dips into a deeper trough and the ground is relatively squishy, which means that a century or so ago, building a skyscraper there would have been too challenging for contemporary engineering. Today, while it may be prohibitively expensive, such construction is technologically possible. Good rocks, geologists like to say, make good foundations and good tunnels.

Underground Manhattan is laced with unseen, taken-for-granted tunnels, the latest of which is the East Side Access, 170 feet below Park Avenue. It stretches from the East Sixty-Third Street tunnel under the East River, which it shares with the subway from Queens, and terminates at East Thirty-Sixth Street, just below the Union League Club. (A Manhattan portion of the sixty-mile-long third water tunnel, which has been under construction for four decades and is scheduled for completion around 2020, opened in 2013; the Long Island Rail Road’s direct East Side Access is now expected to start around 2020.)

Legally, landlords own the land beneath their property to the center of the earth, so tunnels require easements, which, in the case of government agencies, can be obtained through negotiation or by exercising the right of eminent domain. An advance team of geologists mines the excavations to verify topographical details of the original shoreline and underground water courses still derived from the pre-development 1865 map of Egbert Viele (a civil engineer and congressman), to adjust engineering specifications to the conditions that are discovered, and to leave a geological record for posterity. Finding amphibolite and similar rock formations migrating like baked taffy—one geologist likened the pattern to a Charleston Chew—in both Manhattan and Morocco provides evidence substantiating Alfred Wegener’s once ridiculed theory of continental drift.