Bedrocks in the city that never sleeps

This past weekend I made the short trip from Washington DC up to New York. I went to visit friends and to spend some time enjoying the city itself. But as I sat on the bus and departed New York on Sunday evening there were tears in my eyes. You see, for the past three and a half years while my body has been based here in DC, my heart has lain firmly in the grasp of Manhattan. As it has since the first time I visited New York back in the summer of 2008. On that trip I was fortunate enough to stay in my friend's south-facing Harlem high-rise apartment, which had this for a view:

View south from Harlem.

On my first evening in the city we made a short foray into the Bronx and relaxed on a rooftop with some delicious mojitos. My love affair with New York City had truly begun. Since that week, it has been my ambition—sometimes a nagging one at the back of my mind and sometimes the overriding purpose of my day—to live and work in the crazy confines of the amazing metropolis.

In the next few weeks, however, I will be moving back across the Atlantic Ocean to London, my home. As much as I am looking forward to living again in my first city-love and to starting the next chapter of my life, I am sad that I will no longer be within arm's reach of New York. I don't know when I'll next be able to walk its streets, relish its anonymity, and savour the familiarity of all its sights, smells, and sounds. My ambition remains unfulfilled. Those were the reasons for my tears.

After that rather sappy introduction, I should explain how my feelings for New York are relevant to this postcard from planet Earth.

In July 2010 I had a week-long New York visit, primarily for the the Meteoritical Society's annual meeting. Before the conference started, however, I was spending some time with my friend (whose Harlem apartment I stayed in, and with whom many of my New York stories are entangled)—we'll refer to her as 'Mrs Cowboy' (I'm repaying the favour here after I was featured on her blog a while back)—in Brooklyn. As we sat on a park bench in Brooklyn Heights gazing over the East River to Manhattan our conversation meandered around. At one point, I posed the question as to why the Manhattan skyscrapers have a bimodal distribution. Both the Financial District downtown and the landscape of midtown are dominated by buildings reaching up for the heavens, but the architecture between them is distinctly challenged in the vertical dimension.

The twin peaks of downtown and midtown Manhattan. Credit: Jason Barr

Mrs Cowboy, a seasoned New Yorker, didn't have a particularly good answer and neither did I at the time. But I had a geologist's hunch that maybe it was something to do with the rocks that lie beneath. After all, those huge buildings must need some almighty foundations. Then one afternoon during the conference I participated in a lovely and informative tour of Central Park with a local geologist, Sidney Horenstein. I learnt all kinds of interesting facts about the city's geology, including an answer to my skyscraper question.

Many of Manhattan's bedrocks consist of original sediments (clays and muds) that underwent major mineralogical changes due to metamorphism. The actions of heat and pressure during the Taconic and Acadian orogenies (mountain-building episodes) changed the original sedimentary rocks to metamorphic rocks. This took place between about 400 and 500 million years ago, during the Devonian and Silurian geologic periods, and was part of the construction of the ancient supercontinent Pangaea. As Earth's tectonic plates shifted and jostled for position, the sedimentary strata were squeezed and squashed. The resulting metamorphic alteration created new rocks, including what is known as the Manhattan Schist. Today, these formations are not nice flat strata, but rather they have been folded to form a series of anticlines and synclines, or peaks and troughs.

A folded outcrop of the Manhattan schist (with a pegmatite intrusion) in Central Park.

One of these synclines dips down below the surface between the approximate city levels of Washington Square Park and Chambers Street, i.e., the gap between the midtown and downtown skyscrapers. This syncline was filled during the Pleistocene ice ages with much softer glacial sediments. So I had been correct with my geologist's hunch. New York's geology really does play a vital role in shaping the city's architecture. The ancient metamorphic bedrocks are sturdy and strong enough to support the huge skyscraper structures, but the younger and less consolidated glacial deposits just can't do the job.

A beautiful old illustration from A Geological History of Manhattan or New York Island by Issachar Cozzens. The geologic profile shown in the middle panel depicts some of the folded rock formations that make up the island of Manhattan.

I definitely believe that one of the most characteristic things about our Earth is its capacity to sustain complex and intelligent life. Great cities are amazing examples of the engineering feats that human civilization can achieve. So I can't think of of a better way to represent the human race, and Earth, to fellow sentient beings than with a piece of Manhattan's bedrock. For it is on these solid foundations that the (perhaps) greatest city of them all has been built.

The view from the Staten Island Ferry on Sunday. Midtown's Empire State building peaks through the urban canyons of downtown, just to the right of the Freedom Tower.