The rock is called Rat Rock by neighborhood residents, who were plagued for decades by rats nesting in it. It is an example of how geology has quietly dictated New York City's building patterns. City officials, urban planners and developers may think that they make the decisions, but often the rock that lies beneath the ground, whether its bedrock exposures or its fault lines, has the final word.
Rat Rock looms 30 feet above the sidewalk and stretches 100 feet away from 114th Street, behind an iron fence Columbia University placed there to discourage vandals. The lot and development rights are incredibly valuable, but removing the rock could cost hundreds of thousands of dollars.
"When the row houses were built around it in 1896, leaving the rock was no big deal because there were hundreds of acres to develop," said William Scott, vice president for institutional real estate at Columbia. "But now, it's just this huge ugly rock in the middle of our neighborhood."
Rat Rock's beauty, or lack thereof, may be in the eye of the beholder. From a distance, its hulking mass looks dark and foreboding. But up close, its minerals catch sunlight and sparkle. The glittery layers are made of mica, common in this kind of rock, which is known as a schist. Most of Manhattan's bedrock is made of the rock, so recognizable that it was called Manhattan Schist, even though it is found throughout Long Island and Westchester County. About 15 years ago, however, local geologists reclassified and renamed many of the rock formations in the area.
Similar kinds of rock, a group of schists now called the Hartland Formation, burst through the earth's surface in Central Park, Riverside Park and Morningside Park.
"The rocks helped to define the borders of the parks," said Jed Lackritz, a historian at the City Department of Parks and Recreation. "If land was rocky, it wasn't as usable for commercial development."
But the presence of Manhattan's bedrock near the surface, not poking through it, was a necessity for early skyscrapers. The bedrock is crumpled into great waves underneath its thin veneer of soil. It sits close to the surface in the Wall Street area, but then dips 150 feet below the surface, covered with soft sand and gravel, until 34th Street, where it stays near the surface through Central Park. "If you look at photographs of the city at the turn of the century, you see the very tall buildings grouped at the south tip of the island and north of 34th Street, with no tall buildings between," said Robert Fickies, an engineering geologist at the New York State Geological Survey. "The skyscrapers needed bedrock support, and the technology to put pilings down for support in other areas wasn't developed yet. So the buildings went where the bedrock was near the surface."
Much farther north, geology again forced city developers to bend to its will. The 125th Street subway station on the 1 and 9 IRT line is not an underground subway at all — it is an elevated station. Because of a geological fault, a viaduct rather than a buried tunnel had to be built over the valley in that area.
An ancient fault line, where a block of rock thrusts over another, runs parallel to 125th Street.
The movement of the rocks along the fault hundreds of millions of years ago crushed much of the surrounding rock, which has since been eroded, leaving behind the steep Manhattan Valley.
So when the Metropolitan Transportation Authority wanted to build a 125th Street Station as part of its first subway line, the IRT from Times Square to 145th Street, the shape of the land forced the MTA to turn the station into a viaduct over the valley.
"If they had run the train on a straight line from the Columbia University stop, it would pop out of the ground at the level of the traffic," said Sidney Horenstein, the coordinator for environmental programs at the American Museum of Natural History who is known for his walking tours of New York City's geology. "They didn't want to dig deeply into the fault area, so they built the elevated station."
A subway rider could pass that station every day and never know about the fault line that called for its elevated status. A person could work on the 25th floor of the Standard Oil skyscraper and never know why it was built at 26 Broadway near Wall Street rather than in Union Square or Greenwich Village. A person could live for decades on 114th Street and never know why Rat Rock persisted in a neighborhood with chronic housing shortages.
The answer, said Andrew S. Dolkart, a professor of architectural preservation at Columbia University, is that Rat Rock is "an extraordinary survivor" because it "hints at the geology of the city."