Bedford Audubon Society

The hills of Upper Westchester are part of a large geologic land mass known as the “New England Province.” Locally, our part of the New England Province is known as the “Manhattan Prong,” and is called a “prong” because of its knife-like shape. These hills which run from nearby Fairfield County, Connecticut, to Manhattan Island, New York City, share distinct geologic similarities. Just to the north, in Putnam County, is the Redding Prong, which runs across the Hudson southwestward, and terminates in Redding, Pennsylvania. Collectively, these prongs and ridgelines are part of the Taconic Range and the larger Appalachian chain of eastern North America.

As evidenced by the rocky outcroppings on the Katonah Ridge Trail (Blue), the metamorphic rock found here is comprised of Fordham gneiss, schist, and amphibolite. These rocks are dated at about one billion years of age. The rocks were originally sediments, clay, sand, and mud. Then were compressed when the continent of Europe or perhaps Africa slammed up against North America’s east coast. Eventually, the pressure of the two colliding continental plates thrust these former sediments upwards, forming what we know today as the Appalachian Mountains. After these mountains formed, they underwent a succession of erosive periods. The most recent of which were the glaciers that this region experienced 15,000 years ago. Many of the boulders scattered around the sanctuary were transported by these glaciers to the sanctuary and are, in fact, fallout from those glaciers. These randomly scattered boulders are known as “glacial erratics” or “glacial boulders.”

A steep side facing south and a shallower side facing north typify many of the ridges in our region, including the ones in the Hunt Sanctuary. This classic form is known in geologic terms as “Roche Moutonnees.” The steep cliff faces of the ridge’s south side have many boulders and rock strewn about at the base of these cliffs. These areas are known as “talus slopes,” and are created by the extensive erosion that took place at the top of the ridge from glaciers, which pushed debris in its way over the steep south side. Finally, even the casual observer will notice the many vertical white bands in the dark gray Fordham composite. These bands of quartz and other minerals were formed due to the great horizontal pressure that was applied to these hills. The minerals realigned in a perpendicular fashion to the horizontal pressure in order to reduce stress. Other minerals that may be found in rocky outcroppings include potassium feldspar, biotite, muscovite, garnet, and hornblende.

Click for photos of a geology walk at the Hunt-Parker Sanctuary

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