30,000 Rock: A Peek into New York City’s Geological Underpinnings

In a City Where Everyone’s Rushing, Few Ponder the Relentless, Plodding Progression of the Ground Beneath Their Feet

When I moved to Manhattan in August 2010, I already knew the city as a maze of crumbling, gum-stained sidewalks connected to what I considered the real word by ribbons of concrete and steel. Crossing one of the largest of those ribbons — the George Washington Bridge — one day, I found myself immediately lost. Highway entrance and exit ramps curled to and fro in an indeterminate tangle. Before too long, though, I’d regained my composure, and the rather serpentine course I’d taken through Harlem’s grid of one-way streets led me more or less in the direction I wanted to go.

Photo courtesy Stig Nygaard/Wikimedia CommonsBelvedere Castle in Central Park stands atop an exposed part of the 380 million-year-old Manhattan schist.

Just as Harlem’s greasy pavement spat my battered car into Morningside Heights, I found myself face-to-face with a most curious sight  — a building perched atop a huge mass of boulders which looked to be embedded in the side of a small hill. Having seen rocks strewn about Central Park on earlier visits to the city, I suppose it shouldn’t have been all that surprising, but rock was holding up a building. New York City Public School 36, to be exact.  A place of learning for 634 elementary school children. At some point, I’d read that Manhattan’s bedrock was what made it possible to build the two clusters of massive skyscrapers further down the island. Yet, here it was — aboveground, an example of the very basis of the New York City Skyline.

Large masses of gray stone are not uncommon around Manhattan, but until I confronted Morningside’s rocky outcropping, I had never wondered about what lurked deep below the city’s busy sidewalks and roadways. In Central Park, the fingers of dark gray stone strewn about beneath carefully pruned trees and manicured lawns seemed to be there by design. But that scratched, blasted piece of rock sticking out from beneath a school in Morningside served as a cutaway — a window into the island’s geologic history. I’d seen the tip of the iceberg, so to speak, and felt inclined to do a little digging to see what the rest of it looked like.

Hundreds of millions of years before Manhattan’s shining, glass- and steel-fronted skyscrapers were built, the mostly buried rock that now supports their weight dominated the skyline as a mountain ridge. Over the ensuing eons, tectonic plate movements shifted everything around, new layers were formed and worn down, and things were again shifted, intruded upon, and worn. Much more recently — only tens of thousands of years ago — a few ice ages did their work on New York City’s geology, with ponderous ice sheets carving great grooves and troughs in the rock. When the ice sheets receded, they left behind debris like silt and boulders, covering most of the three layers of metamorphic rock that once formed the ridge. Today, the only telltale signs of how the layers below the visible surface came to be are occasional protrusions (the iceberg tips) and the odd boulder, balanced where it’s been since the last ice melted and set it gently into place about 12,000 years ago.

The layer of rock that supports the often-overlooked Bronx also forms the geological foundation for perennially limelighted Manhattan. In fact, it forms the deep-down basis for all five boroughs of New York City. Known as Fordham gneiss, the approximately 1 billion-year-old formation reaches down from Connecticut and Westchester County, dipping below the much younger Inwood dolomite and Manhattan schist that form Manhattan Island.

Although it pops up along the Harlem River and on Marble Hill at Manhattan’s very northern end, Inwood dolomite makes a cursory appearance on New York City’s geologic stage. The star of the show is the approximately 380 million-year-old Manhattan schist — layers of metaphoric rock — which geologists believe began life as sand and clay on the bottom of the ocean. Movement, heat, and pressure over millions of years created the schist that exists today — a hard, durable rock able to support the weight of towering buildings. Tiny bits of white mica give exposed schist a glittery look, which has caught many a person’s eye as they strolled through Central Park on a sunny day. Over the last 100 or so years of subway and water tunnel construction, work crews occasionally unearthed garnet and other sparkly gems as they honeycombed the Manhattan schist creating the city’s subterranean passageways.

From north to south, Manhattan schist tends to slope downhill. There are a few spots where it dips pretty far below the even surface of soil and organic material — the terminal moraine — left behind by the 1,000 foot-thick Wisconsin Ice Sheet when it melted back into Canada 12,000 years ago. That explains why the city’s heaviest skyscrapers are only found in clusters in Midtown and in the Financial District: In those spots, building engineers had to dig only 20 or 30 feet to reach bedrock. In other areas of Manhattan — such as Greenwich Village — the hard stuff is submerged nearly 100 feet below the blacktop. That’s deep enough to make building large structures impractical.

Humans were not around to see the drastic changes that occurred over the milennia that have passed since what is now New York City was part of a stark mountain range. Nor were they present here during the Pleistocene period, when — over 80,000 years ago — glacial ice sheets must have come and gone four or five times. When you stack that next to the history of human civilization — maybe 10 or 15,000 years — or the 400 years since Europeans first discovered what they then called New Amsterdam, whatever humans have accomplished in this booming metropolis seems slightly less significant.

In a city where everyone rushes from place to place — where buildings come and go, and are built taller the next time a few years later — the relentless, plodding progression of the ground beneath our feet is something that’s difficult to consider. But sometimes, a portal opens. Mine was a dark, sparkly slab of rock propping up a public school. As I scurried on my way someplace else, it struck me that this city, this dense cluster of huge buildings, is no more than the head of a pin in the grand scheme of things.

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