Manhattan can seem like one of the most manufactured environments in the world. Yet its bones are often visible. In Central Park, many huge rocks stand out from the groomed landscape, some bearing the deep scoring of glacial striation, some shining with glacial polish. The ice of the glaciers did not carve these rocks, the massive debris they carried did. Boulderers, urban climbers, practice their skills on many of Manhattan’s glacial erratics, especially on Rat Rock and Cat Rock, at the southern end of Central Park.
Inwood Hill Park, at the north end of the island, displays sweeps of Inwood marble, metamorphic rock recrystalized from limestone built of sea creatures’ shells. There are many small glacial potholes in Inwood, bored by ice and water in a process that produces pits oddly similar to the ones in city roadways.
Near the south end of the island, in the excavations for rebuilding at Ground Zero, workers discovered a hidden glacial terrain, with a 40 foot deep pothole carved in bedrock. Such holes are carved by raging glacial meltwaters, which use huge cobbles as scouring powder. The pothole is filled in now, to provide a stable foundation for the rebuilding.
Inwood Park extends to the Hudson River, which a geologist might describe as a fjord, because upriver it is deeper than at its mouth. Depending who you ask, the Hudson may have been ground deep by long ago glaciers. Other experts say that lowering of the sea level by an ice age may have allowed the young river to scour down through the highlands near its source. The mouth of the Hudson is a deep estuary, where salt waters mix with fresh, and the ships of the world come safe to harbor. Nevertheless, the river is deeper still upstream: 175 feet at West Point.
Across the Hudson from Manhattan are the Palisades. They formed underground, during the period when Africa and America first divided, 200 million years ago. At that time, magma liquefied as the pressures on it decreased, and it flowed upward to intrude into sedimentary rocks. It solidified, and through the centuries was uplifted as the softer sedimentary rocks eroded away.
The main street of Harlem follows the line of a geologic fault, the 125th Street/Manhattanville fault that divides Manhattan on a line from the Hudson to the East River. It makes a narrow valley at the end, deep enough that the subway crosses it on a trestle. The other fault of importance to Manhattan is Cameron’s Line, named for USGS geologist Eugene N. Cameron, who mapped its path and recognized its significance.
Ancient rocks of the Manhattan Prong, marked off by Cameron’s Line, are the bedrock that the island’s skyscrapers rest upon. West of the line, which follows the path of the East River along much of Manhattan, is the rock of the Manhattan Formation, made of Manhattan schist and Fordham gneiss, twisted and folded together. Fordham gneiss is a metamorphic rock formed when old sediments were recrystallized into a swirl of black and white bands by the tremendous pressures of mountain building in the pre-Cambrian era. Manhattan schist is old volcanic ash, compressed into shale and then recrystallized into schist.
Where bedrock is near the surface, the buildings stand tall, but where bedrock is absent or too deep, buildings are shorter and humbler. From north of Canal Street to midtown is a basin filled with sedimentary rock. The buildings in this area are, by New York City standards, short.
The geology of Manhattan can be seen in outcroppings of many local parks. It is also apparent in the way it underlies the neighborhoods of skyscrapers, which only rise where the bedrock of the Manhattan formation stands firm beneath them.