Urban Geology Part 1: Manhattan

The character of any city will to a great degree be shaped by its underlying geology. Chicago's even, straight grid is possible thanks to its flat terrain. San Francisco's steep hills underlie much of its mystique. London's relationship with the Thames has driven so much of its history. New York has an especially fascinating geology, which shouldn't be a surprise. New York doesn't do boring, ever. (We're going to be focusing primarily on Manhattan's geology, since New York's geology in general is way too much for a single article.)

An aerial view of Manhattan. Note the lack of skyscrapers in the center of the island- that'll be important later. [Image source]

The first thing we've got to confront in talking about Manhattan's geology is its bedrock. Manhattan's bedrock is primarily composed of a rock known as Manhattan Schist. Schist is a type of metamorphic rock known for having its grains oriented in flat, sheetlike shapes- this is partially thanks to its protolith (the rock type it was before metamorphosis), which tended to be shales and similar sedimentary rocks. (Rare igneous protolith schists do occur as well, but the Manhattan Schist had a shale protolith.) The distinctive texture of schist (also known as its foliation) is also thanks to its formation, which involves the application of immense heat and pressure. To be more specific, Manhattan Schist is a garnet mica schist- there are large quantities of mica and garnets that formed in it. (It's a pretty literal name.)

The Manhattan Schist was formed around 450 million years ago when the continents collided to form the super-continent Pangaea. The present day site of New York was right at that collision zone, and the landscape there was forced upwards into a massive mountain range comparable in size to the Himalayas named the Tectonic Mountain Range- which is a bit too on the nose if you ask me. As part of this orogeny (mountain building event) the rocks weren't just metamorphosed- they were twisted and folded into massive curves and irregular shapes, meaning that the bedrock isn't an even distance below the surface. In the center of Manhattan, the bedrock sinks much deeper than at either end.

An outcropping of Manhattan Schist in Central Park.* [Image source]

That dip in the depth of the Manhattan Schist bedrock layer beneath central Manhattan is behind one of the biggest urban geological controversies out there- why there are no skyscrapers in the center of Manhattan island. In 1968, geologist Christopher J. Schuberth claimed the much deeper bedrock was responsible for the lack of skyscrapers in central Manhattan- that without being able to easily reach the bedrock to root skyscrapers, it was cost prohibitive to do so.

Unfortunately, the bedrock/skyscraper connection, as cool as it sounds, probably isn't true. In 2012, economist Jason Barr, along with some colleagues, showed that there was probably no actual correlation. Many of the tallest buildings in the earlier days of Manhattan skyscrapers were actually built in areas with deep bedrock. On top of that, it would have only added 7% to the cost of a skyscraper to build it in an area with deep, rather than accessible, bedrock. The reason the Manhattan skyline looks the way it does? Just a quirk of the history of the city, and the result of people moving around in it. These days, advances in construction and architectural techniques mean that you don't have to worry about where the bedrock is.

The Palisades, a series of basaltic cliffs found on the West Side of the lower Hudson River. [Image source]

Manhattan Schist is far from the only type of rock under the city, though it is the dominant one. Also present in large amounts are the billion year old Fordham Gneiss, the Inwood Marble, and even some basalt. The Fordham Gneiss is the most interesting of the lot- this metamorphic rock (with a granite protolith)- is a close relative of some of the rocks found in the Canadian Shield, the oldest portion of the North American continent.

The aforementioned basalt forms the Palisades, a series of cliffs along the West Side of the Hudson river. They were formed around 200 million years ago during the Upper Triassic. (Upper in this context means the end of the Triassic- rocks get older the deeper you go, so the Lower part of a geological era is its beginning, and the Upper part is its ending.) Rather than being a flood basalt like is found in the Pacific Northwest or other areas, it's intrusive basalt- it leaked into sandstone from underground. The sandstone eventually eroded away, leaving the more competent (erosion resistant) basalt behind. Interestingly, while the Palisades weren't themselves flood basalts, they seemed to have erupted contemporaneously to the flood basalts that formed the Siberian Traps and triggered the Upper Triassic extinction event, the second most severe mass extinction in geological history. (The most severe was the Permian Extinction, which led into the Triassic.)

The only public domain photo of Fordham Gneiss I could find on the internet. Gneiss is a metamorphic rock known for its alternating dark and light bands. I highly recommend you check out more photos of Fordham Gneiss, it's cool stuff. [Image source]

There are, however, much more recent geological influences on Manhattan- namely, the Ice Age. During the last ice age, a two mile thick ice sheet advanced southwards across much of north America. The southernmost advance of that ice sheet during its last phases can be found right in the middle of New York. The massive ice sheet covered most of Manhattan, but left Brooklyn, Queens, and Staten Island largely alone, except for leaving a massive terminal moraine- a ridge composed of glacial debris in the form of rocks ranging in size from sand and pebbles to semi truck sized boulders. Forest and Highland Parks in Queens are found on the moraine, as is Prospect Park in Brooklyn.

While Manhattan itself doesn't have any of the terminal moraine ridge running through it, it's got plenty of glacial erratics- massive rocks abandoned by the melting glacier that were too big to move during later human constructions. Many of the native rocks found in Manhattan have also been carved and shaped by the glaciers. Interestingly, thanks to the terminal moraine's close proximity to the city, New York became a hub of geological research on the Ice Age.

Umpire Rock in Central Park. Umpire rock's smooth surfaces and grooves were carved into it by glacial action during the Ice Age. It's also known as Rat Rock, because, well, rats like to hang out on it. [Image source]

This is by no means an exhaustive survey of Manhattan's geological features- there are a number of faults crossing the city as well, for instance, including Cameron's Line, which runs through Central Park. (You don't need to really worry about quakes, though- Manhattan is remarkably earthquake resistant, which is one of the reasons it was a great place for skyscrapers.) If you're interested in learning more about the city's geology- especially if you live there- I highly, highly recommend it. Learning about geology on a local scale is one of the most fascinating activities in science, and it's seldom something most people get a taste of when learning about geology in high school.

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In interesting/ terrifying science news, a massive fire tornado killed six people in California recently. It was the single biggest recorded tornado of any sort to ever form in California. If you're interested in learning more about fire tornadoes, you should check out my Steemit article on them. This recent fire tornado might force me to revise my danger assessment for them upwards a little bit.