With the intense urbanization of Manhattan, one may not expect to find much to look at in regards to natural history or geology. Of course, the fantastic Museum of Natural History displays many natural items brought in from all around the world. Or, one can cross the street into Central Park and see a good example of geologic history exposed right in the middle of Manhattan.
Amongst the landscaped masterpiece of Central Park, a few exposures of bedrock poke out under the trees and though the grass. While the intensely foliated Manhattan Schist may capture the eye of some, what’s carved into the schist certainly brings attention of its own.
Take a look at this photo I took last week:
You may immediately notice the swirly foliation pattern of the schist. But notice anything different? Here is an annotated photo from another angle:
Yep, those grooves that cut across the foliation are glacial striae. I didn’t have a compass with me (well, I suppose I could have used my phone but I wasn’t that crafty), but the striae trend in a North-South direction. Nearby exposures have similar striae oriented in the same North-South direction.
The striae record the movement of the Laurentide ice sheet as it dragged along the bedrock of Manhattan on the way to the Atlantic ocean. Smaller rocks called “tools” trapped under the weight of the ice sheet were dragged along with the ice, carving the striae in the bedrock below. The direction of the striae is a record of the direction of ice movement.
Around 25,000 years ago, during the Last Glacial Maximum, a mile-thick lobe of the Laurentide Ice Sheet was flowing south down what is now the Hudson River valley, calving icebergs into the Atlantic Ocean. Much of the modern topography of the New York City area and Long Island can be attributed to glacial activity.