120. Portage Escarpment Rhinestree Shale?

The Portage Escarpment

Escarpment to Escarpment to Escarpment: Three Vistas Reflecting the Subsurface Structure of Western New York,

The Portage Escarpment forms the highest step on land along the edge of the Appalachian Plateau. In some places, it rises 200 to 300 feet above the Lake Erie Plain.

Farther north beneath the lake, the Dunkirk Escarpment forms another step, where the lake floor drops by roughly 30 to 50 feet into deeper water.

Then comes the Long Point Escarpment near the boundary between the Central and Eastern Basins, where the lake floor descends by more than 100 feet toward Erie’s deepest waters.

Brian CalleyLake Erie’s deepest basin is broken into giant geological steps. The Eastern Basin formed in layers of harder and softer rock that dip southward beneath the region. Over time, softer layers eroded...

This rock is a piece of Upper Devonian marine shale, most likely from the West Falls Group (specifically the Rhinestreet Shale or Angola Shale), which forms the bedrock of the Portage Escarpment throughout Eden and southern Erie County, NY.

Key Identifying Features

Core vs. Weathered Surface: The fresh, exposed edges show the characteristic dark gray to near-black interior, while the broad flat faces display an ochre/tan and rusty-brown patina. This discoloration is caused by the oxidation of microscopic iron sulfides (pyrite/marcasite) and organic matter exposed to air and groundwater.
Fissility and Texture: The rock exhibits classic shale fissility—splitting into thin, smooth, planar sheets along original sedimentary bedding planes deposited in quiet, low-energy waters.

Geological Background & Context

Age: Late Devonian period (approximately 370 to 380 million years old).
Formation Environment: Deposited in the deep, oxygen-depleted Appalachian Basin (Catskill Delta system) when Western New York was covered by a warm inland sea south of the equator.
Local Stratigraphic Candidates in Eden, NY:
- Rhinestreet Shale (West Falls Group): The most likely match. A dark gray to bituminous black, organic-rich, fissile shale that commonly weathers to a distinct yellowish-brown rind.
- Angola Shale (West Falls Group): Overlies the Rhinestreet; composed of medium-to-dark gray silty shale with occasional thin calcareous bands.
- Cashaqua Shale (Sonyea Group): Sits directly beneath the Rhinestreet and is exposed in lower creek cuts (such as branches of Eighteenmile Creek), consisting of softer, greenish-to-medium gray shale.