Last Saturday I went on a Geology Tour of Inwood Hill Park, located at the northern end of Manhattan. Urban Park Ranger Jerry Seigler (pictured below) led the tour of rock formations, the impact of the Wisconsin Glacier on shaping the park and the history of the area.
Urban Ranger Jerry Seigler leading geology tour of Inwood Hill Park
The tour started inside the Nature Center, which is right along the only natural salt water marsh in Manhattan:
Nature Center and Salt Water Marsh and Inwood Hill Park
The Nature Center includes exhibits about local rock formations, salt water marshes and other natural features of Inwood Hill Park. The following picture is of Inwood Marble, which is the softest of the three rock layers on which Manhattan rests. You can identify Inwood Marble by its sugar-like crystals and its white or blue-grey color:
Inwood Marble at exhibit in Inwood Hill Park Nature Center
I took this close-up shot of Inwood Marble outcroppings in Isham Park, which is just southeast of Inwood Hill Park:
Inwood Marble outcropping in Isham Park
A picture of Fordham Gneiss in the exhibit at the Nature Center. Gneiss consists of alternating light and colored bands and is the deepest and hardest of the three primary metamorphic rocks that form Manhattan and the surrounding areas:
Fordham Gneiss in exhibit at Inwood Hill Park Nature Center
The Columbia “C” painted on a Fordham Gneiss outcropping in the Bronx, directly across from Inwood Hill Park’s Nature Center. Because of pollution and erosion, it is difficult to see the alternating light and colored bands of the Gneiss:
Columbia C painted on Fordham Gneiss
A picture of Manhattan Schist in the exhibit at Inwood Hill Park Nature Center. Manhattan Schist is the island’s strong bedrock on which skyscrapers are built downtown and in midtown. Almost all of the rocks in Inwood Hill Park are Manhattan Schist. Schist often contains mica specs, a flaky mineral that appears metal-like:
Manhattan Schist in exhibit at Inwood Hill Park Nature Center
During the the last ice age, the Wisconsin Ice Sheet reached a height of 1,000 feet, crushing the lands and forming valleys and riverbeds. The advancing and retreating ice sheets also left their marks with glacial scratches or striations that you can sometimes see in exposed rocks. Urban Park Ranger Jerry Seigler showed us an example of glacial striations on what is known as “whale rock.” These large groves run in a north south direction, following the path of the ice sheets:
Glacial striations on Whale Rock in Inwood Hill Park
This is a close-up of a glacial striation on Whale Rock:
Glacial striations on Whale Rock in Inwood Hill Park - close-up
As ice sheets advanced and retreated, they often dislodged rocks from one location and moved them to another location. This picture, from the exhibit, is an example of the many rocks from the Palisades, across the Hudson in New Jersey, that were moved to Inwood Hill Park. Rocks that are moved by glaciers are called erratics:
Erratic in exhibit at Inwood Hill Park Nature Center
On the Clove Trail that runs through the heart of Inwood Hill Park, there are glacial potholes that were formed as the melting water from an ice sheet mixed with gravel and rock to drill down into the below rock formations. And the result is this glacial pothole:
Glacial Pothole in Inwood Hill Park
On the Clove Trail, you will also come across a number of caves known as the Indian Rock Shelters, which were occupied for centuries by the Lenape. Here’s one of them:
Indian Rock Shelters in Inwood Hill Park
