sedimentary

Greywacke (River Siltstone)

Lithic Sandstone / Greywacke

AI-generated identification · may be incorrect

Hardness: 6-7 on Mohs scale; Color: Dull grey to greenish-grey; Luster: Earthy to matte; Structure: Fine-grained, poorly sorted matrix; Cleavage: None; Specific Gravity: 2.6-2.7

Hardness: 6-7 on Mohs scale
Color: Dull grey to greenish-grey
Luster: Earthy to matte

Identified February 17, 2026More sedimentary →

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Physical properties

Hardness: 6-7 on Mohs scale; Color: Dull grey to greenish-grey; Luster: Earthy to matte; Structure: Fine-grained, poorly sorted matrix; Cleavage: None; Specific Gravity: 2.6-2.7

Formation & geological history

Formed from the rapid deposition of muddy sediment in deep marine environments (turbidity currents). Often dates from the Paleozoic or Mesozoic eras. This specimen shows significant rounding due to fluvial (river) erosion.

Uses & applications

Commonly used in construction as road aggregate, railroad ballast, and high-quality concrete. Sometimes used in landscape gardening.

Geological facts

Greywacke is often called 'dirty sandstone' because it contains a high percentage of clay and silt along with sand grains. It is a signature rock of the oceanic subduction zones.

Field identification & locations

Identify by its dull grey color, extreme hardness (it won't scratch easily with a steel knife), and lack of visible layering compared to shale. Found abundantly in riverbeds and mountainous coastal regions like New Zealand, the UK, and the Pacific Northwest.