sedimentary
Graywacke (River Cobble)
Lithic Sandstone (Graywacke)
Hardness: 6-7 (Mohs); Color: Dark gray to bluish-gray; Luster: Dull/Earthy; Structure: Massive, fine-grained with visible lithic fragments; Cleavage: None.
- Hardness
- 6-7 (Mohs)
- Color
- Dark gray to bluish-gray
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray to bluish-gray; Luster: Dull/Earthy; Structure: Massive, fine-grained with visible lithic fragments; Cleavage: None.
Formation & geological history
Formed in deep-water marine environments where submarine landslides (turbidity currents) deposit poorly sorted sand and silt. These deposits are typically from the Paleozoic or Mesozoic eras.
Uses & applications
Commonly used in heavy construction as road base, railroad ballast, and high-quality concrete aggregate due to its durability.
Geological facts
Graywacke is often called 'dirty sandstone' because of its poorly sorted mixture of sand-sized grains and a clay matrix. It is a signature rock of active tectonic margins.
Field identification & locations
Identify by its characteristic 'slatey' gray color, extreme toughness compared to regular sandstone, and lack of distinct layering in individual cobbles. Commonly found in riverbeds near mountain ranges.
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