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New York and San Francisco Geology

A tale of two favorite cities

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new york city geology

Triassic volcanics lie west of Manhattan's ancient gneisses.

U.S. Geological Survey

Are cities geologically interesting? The two I know best are. Two great American cities, San Francisco and New York, owe their greatness to geologic good fortune. Geology made them rich and makes them beautiful.

You could say that gold made San Francisco and that iron and coal made New York, but that's not what I mean here. I'm thinking of their physical settings and their landscapes. To begin with, both cities are blessed with superb harbors, with sheltered, deep water abutting solid ground at the mouth of large river valleys.

Lucky San Francisco

San Francisco's lucky position is quite recent, geologically speaking, and quite random. The entire western edge of North America has been broken, shoved northwestward along the San Andreas fault zone and other faults, one earthquake at a time, and reassembled like junk falling from a dumptruck for the last dozen million years. The end of the line is Alaska, where dozens of mountain ranges, island arcs, microplates, archipelagoes and other pieces of land are packed together in a sort of continental scrapple.

At the moment, the San Francisco Peninsula finds itself midway in this ride, carried to the Pacific's edge and sitting by the mouth of the Sacramento-San Joaquin River, which has cut a big waterway called the Golden Gate. A million years or so from today—a geologic blink of an eye—the Bay Area will be a completely different shape.

Lucky Manhattan

Manhattan Island, by contrast, has not moved since the Triassic Period, about 180 million years ago, when the Atlantic opened up between it and present-day North Africa. The place where the ocean was born is now the edge of the continental shelf hundreds of kilometers from Manhattan, but some preliminary cracking in New Jersey opened up a volcanic gash nearby (details here). That ancient feature guides the course of the lower Hudson River, which runs straight and deep.

The river runs so deep because much later, practically today, the Ice Age glaciers sucked sea level down as much as 150 meters lower than today. This allowed the Hudson to carve its bed much deeper than it could reach now (all the details are here). In a million years, New York Harbor may be buried in ice (as it was in the last ice age) or drowned in seawater, but the deep structure will be the same as today.

That is how both places got their world-class harbors, which in turn made them prosperous cities. Good bedrock at shallow depth supports large buildings in both cities, too. And Frederick Law Olmsted did his part, making possible Golden Gate Park in one city and Central Park in the other.

But the uniquenesses of New York and San Francisco also lie in their landscapes. The noble Palisades and the glittering schist of Riverside Park or Rat Rock—nothing like that in San Francisco. The craggy Marin Headlands and the twisted chocolate chert of Bernal Hill—fugeddaboutit, New York!

Two Great Geologic Field Guides

Another great San Franciscan was the geologist Clyde Wahrhaftig (1919–1994), who never drove a car. In his later years he worked out a set of geologic field trips in and around the city that relied on public transportation. This irresistible and inexhaustible volume, "A Streetcar to Subduction," can be ordered direct from the American Geophysical Union). One of my favorite parts is in the Marin Headlands trip, where you reach a fenced-off military zone marked "Keep Out": "I have to advise you to go no further. However, if you choose to ignore my advice, do as I do, and go through the hole in the fence; once inside the detonation area, stay on the beaten path."

Frisco, shmisco, you might be saying: what about New York? In fact there is a Manhattan equivalent. Prof. Charles Merguerian, who thankfully is still alive, has written Duke Geological Lab's Geology of New York and New Jersey. You can't buy a copy because he gives it away online. (There's lots more stuff at dukelabs.com.) Is the Web great or what?

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