sedimentary
Greywacke (River Siltstone)
Lithic Sandstone / Greywacke
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 More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary