sedimentary
Greywacke (River Siltstone)
Lithic Wacke (Sandstone subfamily)
Hardness: 6-7 Mohs; Color: Grey, greenish-grey, or dark charcoal; Luster: Dull/Earthy; Structure: Clastic, fine-grained with visible layering (lamination); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Color
- Grey, greenish-grey, or dark charcoal
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Grey, greenish-grey, or dark charcoal; Luster: Dull/Earthy; Structure: Clastic, fine-grained with visible layering (lamination); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the rapid deposition of muddy sediment and poorly sorted sand in deep ocean trenches or continental slopes. This specimen has been rounded and polished by fluvial (river) or glacial transport processes.
Uses & applications
Commonly used as aggregate in road construction, railroad ballast, and concrete. Smooth river specimens are often used for decorative landscaping or stone massage therapy.
Geological facts
Greywacke is often referred to as 'dirty sandstone' because of its high clay and silt content. It is a key rock for understanding tectonic movement and ancient underwater landslides called turbidity currents.
Field identification & locations
Identify in the field by its distinct grey color, hardness (will not be scratched by a steel nail easily), and fine, parallel bands or laminations. Look for it along riverbeds and pebble beaches near mountain ranges.
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