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May 31, 2006
Geology with McPhee
"The towers of midtown, as one might imagine, were emplaced in substantial rock - rock that once had been heated near the point of melting, had recrystallized, had been heated again, had recrystallized, and, while not particularly competent, was more than adequate to hold up those buildings. Most important, it was right at the surface. You could see it, it all its micaeous glitter, shining like silver in the outcrops of Central Park. Four hundred and fifty million years in age, it was called Manhattan schist. All through midtown, it was at or near the surface, but in the region south of Thirtieth Street it began to fall away, and at Washington Square it descended abruptly. The whole saddle between midtown and Wall Street would be underwater, if it not filled with many tens of fathoms of glacial till. So there sat Greenwhich Village, SoHo, Chinatown, on material that could not hold up a great deal more than a golf tee - on the ground-up wreckage of the Ramapos, on crushed Catskill, on odd bits of Nyack and Tenafly. In the Wall Street area, the bedrock does not return to the surface, but it comes within forty feet and is accessible for the footings of the tallest things in town."
"The Wisconsinan ice sheet, arriving from the north, had come over the city not from New England, as one might guess, but primarily from New Jersey, whose Hudson River counties lie due north of Manhattan. Big boulders from New Jersey Palisades are strewn about in Central Park, and more of the same diabase is scattered through Brooklyn. The ice wholly covered the Bronx and Manhattan, and its broad snout moved across Astoria, Maspeth, Williamsburg, and Bedford-Stuyvesant before sliding to a stop in Flatbush. Flatbush was the end of the line, the point of no return for the Ice Age, the locus of the terminal moraine. Water poured in the white tumult from the melting ice, carrying and sorting its freight of sands and gravels, building the outwash plain: Bensonhurst, Canarsie, the Flatlands, Coney Island."
"Most of Coney Island is New Jersey diabase, Fordham gneiss, Inwood marble, Manhattan schist. The individual grains are characteristically angular and sharp, because the source rock was so recently crushed by the glacier. To make a well-rounded grain, you need a lot more time. Weather and waves had been working on this sand for fifteen thousand years."
selections from the most excellent book,
In Suspect Terrain, by John McPhee.
Posted by jmarston at May 31, 2006 09:15 PM