By Claire Mathieson
|Glacial striations in Central Park|
Winter is on its way in and the city is getting progressively chillier, but you should be thankful it’s not much worse; Manhattan is nowhere near as cold now as it once was. Just 20,000 years ago (mere seconds in geologic time), Manhattan was covered in a sheet of ice a mile thick. During the most recent Ice Age, which lasted from about 95,000 years ago to 20,000 years ago, the Laurentide ice sheet covered over 5 million square miles of North America, spreading from the Arctic over much of Canada and the northern United States. New York City was right on its margin and, while fortunately for us the glacier itself has long since melted, there is still a great deal of evidence of its presence right here in Manhattan.
|Glacial erratics in Central Park (a.k.a. those awesome
climbing rocks when you were younger)
Today, Central Park is rife with joggers, tourists, and yappy dogs, but the large rocks that protrude all over the park date way back to when Manhattan was under a glacier. For those of you who never took earth science, glaciers are vast ice sheets that form when more snow falls during the winter, then melts during the summer over a period of many years. Glaciers move, slowly but surely, and as they do, they “pluck” rocks from the Earth, dragging them along with them. These rocks scrape against the ground, creating striations like the ones seen in Central Park. Sometimes, these rocks are left behind by the glacier and stand in place where they are left. They are then known as glacial erratics and are often composed of a type of rock that is not common to the region, as they have been moved long distances by the glacier. Central Park has some extremely impressive striations and glacial erratics that are obvious enough to see on your morning walk. To see evidence even closer to home, head over to Morningside Park, which has its own share of striations.
Many of the striations show up in Manhattan schist, a type of rock extremely common to the area.Manhattan schist was created millions of years ago, when underwater landslides and the shifting of plate tectonics caused land masses in North America to compress like an accordion, shunting it both upwards into tall mountains and downwards into the Earth. This caused the formation of a kind of reverse mountain range, which was unseen, but massive. Due to intense temperature and pressure brought on by being jammed deep into the Earth, schist gradually formed from specific types of sedimentary rock.
Over millions of years the schist grew closer to the surface as the land over it eroded, and it became bare to the world by the time the glacier came along. The glaciers dragged boulders over the schist, which shapes much of the topography we know today. Even Long Island is a product of the glacier, known as a terminal moraine, formed when the glacier began to retreat and left a pile of sediments in its wake.
It’s easy to think of Manhattan as a formation of steel, glass, and neon, but in reality the island has spent longer beneath an icy monolith than it has playing host to the American civilization. You don’t have to worry about a glacier plowing through the city any time soon, but as the gentle reminder of winter rolls in, take a moment to think on the region’s past, when nature reigned and the only thing in vogue was polar white.