metamorphic
Greywacke (Briarcliff Schist/Gneiss)
Manhattan Schist / Fordham Gneiss complex
Hardness: 6-7 on Mohs scale. Color: Grey to dark green. Luster: Dull to pearly. Crystal structure: Foliated or granoblastic. Cleavage: Slaty to schistose. Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey to dark green
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Grey to dark green. Luster: Dull to pearly. Crystal structure: Foliated or granoblastic. Cleavage: Slaty to schistose. Specific gravity: 2.6-2.9.
Formation & geological history
Formed through regional metamorphism of sedimentary protoliths (sandstone and shale) during the Taconic and Acadian orogenies approximately 450 million years ago. Found primarily in high-pressure metamorphic belts.
Uses & applications
Used extensively in local construction for building foundations, walls, and historical structures like the Briarcliff Station (now a library). Also used as crushed stone for railroad ballast seen in the image tracks.
Geological facts
The station in the image is constructed from local stone excavated during the railway expansion. This specific formation is part of the bedrock that supports the skyscrapers of nearby New York City.
Field identification & locations
Identify in the field by its distinct layering (foliation), presence of mica flakes that glint in the sun, and its toughness. Commonly found in the Westchester and Hudson Valley regions of New York.
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