sedimentary
Greywacke (River Pebble)
Dirty Sandstone (SiO2 with lithic fragments)
Hardness: 6-7 on Mohs scale; Color: Grey to dark greenish-grey; Luster: Dull to earthy; Crystal Structure: Clastic matrix-supported; Cleavage: None; Specific Gravity: 2.6-2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Grey to dark greenish-grey
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey to dark greenish-grey; Luster: Dull to earthy; Crystal Structure: Clastic matrix-supported; Cleavage: None; Specific Gravity: 2.6-2.7
Formation & geological history
Formed by the rapid deposition of poorly sorted sediment in deep ocean basins via turbidity currents (underwater landslides). Usually Paleozoic to Mesozoic in age.
Uses & applications
Primarily used in construction as road base, aggregate, and railroad ballast. Smooth river-worn specimens are popular for rock painting and decorative landscaping.
Geological facts
Greywacke is often called 'dirty sandstone' because it contains a large amount of fine-grained clay matrix and diverse rock fragments, unlike pure quartz sandstone.
Field identification & locations
Identify by its 'salt and pepper' look under a lens, its extreme hardness compared to slate, and its lack of visible layering. Found commonly in mountainous regions and riverbeds.
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