sedimentary
Graywacke (Sandstone)
Lithic Sandstone (Graywacke)
Hardness: 6-7 (Mohs); Color: Dark gray to greenish-gray; Luster: Dull; Texture: Fine to medium-grained, poorly sorted, angular grains; Structure: Competent, often heavily fractured or folded.
- Hardness
- 6-7 (Mohs)
- Color
- Dark gray to greenish-gray
- Luster
- Dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray to greenish-gray; Luster: Dull; Texture: Fine to medium-grained, poorly sorted, angular grains; Structure: Competent, often heavily fractured or folded.
Formation & geological history
Formed in high-energy marine environments through turbidity currents (turbidites) that deposit sediment rapidly in deep-ocean trenches. Common in accretionary prisms along convergent plate boundaries.
Uses & applications
Primarily used as aggregate in road construction, railroad ballast, and high-strength concrete because of its extreme hardness and resistance to weathering.
Geological facts
Graywacke is often called the 'dirty sandstone' because of its high clay matrix content. It is a major component of the Franciscan Complex in California.
Field identification & locations
Identify by its characteristic 'slatey' gray color, extreme toughness, and common occurrence in coastal cliffs showing intense folding or quartz veining.
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