So, picture this. You're with your family for a semi-annual outing to the Big City and are heading down to the Time Square area to check out some stores and your husband suggests a side trip. He says there's this neat rock in Central Park called Umpire Rock. It's other name is Rat Rock, supposedly because of all the rats that used to swarm there at night. He wants some photographs.
My wife, bless her soul, is used to these types of requests and typically humors me. My brother-in-law, the high finance computer whiz, tags along good naturedly (he knows me well enough by now to not even question such requests). So off we go on a pilgrimage to Rat Rock in Manhattan.
If you've ever walked in Central Park, you'll know that it's is studded with rock outcrops. They belong to a rock unit named the Manhattan Formation. Much of the Manhattan Formation is a metamorphic rock called schist and is quite pretty since it's primarily composed of flakes of mica which reflect sunlight creating a sparkly rock. The Manhattan schist a highly-deformed rock with much folding:
(Yes, I carry a knife and whip it out to use as a scale bar when photographing rocks, even in Manhattan) It's also crosscut in places by granitic-type intrusions as the rock fractured and magma was injected. These are often folded as well.
The real reason I wanted to see Umpire Rock, however, is because it has these massive grooves in it which were carved by glaciers during the last ice age about 18,000 years ago.
Aren't those awesome? I can just picture the glacial ice, studded with big boulders, slowly sliding and grinding up this outcrop witnessed, perhaps, by mastodons out on the surrounding tundra. (My 10-year-old son is standing up there but I cut off half his head to focus on the glacial grooves - I'm a terrible parent)
These rocks formed, by the way, about 450 million years ago during the Ordovician Period of geologic time. They began their lives as interlayered graywacke sandstones and shales, sediments layered by successive submarine landslides in a deep basin between the North American continent (called Laurentia by geologists) and a chain of volcanic islands offshore (we were south of the equator and in a subtropical climate zone back then). When the chain of volcanic islands collided, it resulted in a mountain building event called the Taconic Orogeny and these sedimentary rocks were metamorphosed in the New York City area into schists of the Manhattan Formation. If you want to see what the original sedimentary rocks looked like, drive up north of Newburgh and look at the rocks on the side of the Hudson in places like Poughkeepsie and Kingston.
If you find yourself in midtown Manhattan anytime, the outcrop is easy to find. From Central Park West and West 63rd Street, you enter the park and walk toward the Heckscher Ballfields right in front of you. On the south side of the ballfield is a massive rock (typically crawling with people). It's called Umpire Rock because it overlooks the baseball fields. You can see the rock in the Google Maps satellite image below just south of the ballfields (those funny looking round rings just west of the rock are part of a playground next to the rock - they're rubbery, fake-grass type areas for poor little city kids to play on).
I'd rather climb on rocks in Central Park than visit Times Square any day.