The Sheltering Rocks of Manhattan Island EarthCache
The Sheltering Rocks of Manhattan Island
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Waypoint 6 of 10 on Going Coastal’s NY-NJ Harbor Estuary EarthCache Discovery Trail in Inwood Hill Park, caches developed by Going Coastal, Inc. (www.goingcoastal.org) as a special project in affiliation with Groundspeak and support from the NY-NJ Harbor Estuary Program and the New England Interstate Water Pollution Control Commission.
The NY-NJ Harbor Estuary Earthcache Discovery Trail is meant to help visitors develop a better understanding of the Estuary, make connections between earth and environmental science, and foster stewardship.
Looking around you will see outcroppings, cliffs and boulders, a lot of exposed rock. This rock is called bedrock. Some of the huge rocks tumbled together during a landslide to form over-hangings of natural rock shelters.
Native Americans used these cave shelters as a temporary base when they fished in the nearby Hudson River. (visit link) There were still Native Americans living around these caves up until the 1930’s. Princess Naomi operated a museum and shop, gave tours of the caves and demonstrated native arts and crafts.
"It may not have occurred to you in these terms, but as you observe outcrops in New York City, you are doing two contrasting things. First, you are vicariously walking backward in time. And second, you are figuratively descending deep within a former mountain zone."* All of the bedrock in New York City is allochthonous, denoting a sediment or rock that originated at a distance from its present position. During the Paleozoic Era, the New York region was situated in tropical latitudes.
Inwood Hill lies within the Manhattan Prong of the New England physiographic province of the Appalachian mountain belt, extending through Westchester, the Bronx, Manhattan and northern Queens. It is a small belt of ancient bedrock, consisting of sedimentary and igneous rocks. These rocks have been subjected to several episodes of deformation.
Bedrock is the solid basement rock below loose soil. (visit link) Bedrock continues to the earth’s crust over 18 miles deep! There are three main types of bedrock found in Inwood Hill Park -
(1) Fordham Gneiss consisting of bands of minerals. (visit link)
(2) Manhattan Schist foliated with thin layers of micaceous minerals. (visit link)
(3) Inwood Marble (visit link)
Fordham Gneiss is the oldest rock and was formed from a collision of tectonic plates that occurred 1.1 billion years ago. Manhattan Schist and Inwood Marble were both formed 450 million years ago. Inwood Marble is a former coral reef turned to limestone. (visit link) Gneiss in some places under New York City folds like rock taffy with the Schist and Marble.
Manhattan schist is the most prevalent bedrock in the city. The rock contains low value garnet which makes it hard enough to support the highest valued real estate in the world. In Greenwich Village, the schist is 260 feet below ground and can only support the Brownstones favored by artists. The schist lies only 18 feet below the surface in mid-town and lower Manhattan supporting towering skyscrapers.
Why is the bedrock popping up right here in the park? In many places you’d have to dig hundreds of feet down into the ground just to reach the beginning of bedrock. When bedrock appears on the earth’s surface it is called an outcropping. In order to get an outcropping all the loose soil (like sand) that usually covers the bedrock must be removed. So what removed all the loose soil here? Glaciers reshaped and rearranged those rocks. During the last ice age, about 10,000 years ago, massive glaciers (1,000 feet high) slowly slid through this area. (visit link)
In some areas the Earth’s crust erupted to the surface under the weight of the giant ice sheets. The rocks acted much like when a person sits on an air mattress in a pool, the mattress sinks beneath the weight of the person and raises around the sides. The force of the glacier plunged some layers of rock, called strata, deeper while lifting the terrain in other areas. The hills at Inwood are evidence of this process called coastal rebound.
Some of these rocks travelled great distances to be dumped here. The huge boulders of Manhattan schist are glacial erratic, stones transported from as far away as western Connecticut. After the last glacier, known as the Wisconsin Ice Sheet, as the ice finally melted, it lowered into place stones, gravel and soil far from where they were picked up. Glaciers also pushed masses of debris, called till, materials from sand to gigantic boulders, just like a snowplow pushes snow, leaving behind hills of terminal moraine. (visit link)
Logging Tasks:
1. Look closely at the outcropping of rocks around you. Describe the characteristics of the rocks. Based on the traits you observe, what type of rock is it?
2. How would you describe the surface texture of these rocks? ie: rough, smooth, grainy.
3. (Optional) Log a photo at the rock shelters.
To log a find on this earthcache, email the cache owner (DO NOT POST IN YOUR LOG). Use your GPS device to locate the next cache - GC2RWME. (visit link)
Remember, to upload a photo and let us know in your log ways we can improve the trail.
Data Sources:
• Bedrock Geology of NYC: (visit link)
• Hot Rocks: A Geological History of NYC Parks - (visit link)
• Rock cycle: (visit link)
• Sidney Horenstein, geologist American Museum of Natural History
• *Trips on Rocks (visit link)
• U.S. Geological Survey: Geology of the New York Bight: (visit link)
• Wikipedia: Last Glacial Period
Data Collected: September 26 – March 11, 2011
Name and Type of Land
Inwood Hill Park
W 218 Street & Seaman Avenue, Manhattan
Phone: (212) 304-2365
www.nycgovparks.org
Additional Hints
(No hints available.)