sedimentary
River Rock (Greywacke/Sandstone)
Lithic Wacke
Hardness: 6-7 Mohs; Color: Gray to brownish-gray; Luster: Dull/Earthy; Crystal structure: Clastic (sand-sized grains in a finer matrix); Cleavage: None
- Hardness
- 6-7 Mohs
- Color
- Gray to brownish-gray
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Gray to brownish-gray; Luster: Dull/Earthy; Crystal structure: Clastic (sand-sized grains in a finer matrix); Cleavage: None
Formation & geological history
Formed from the rapid deposition of poorly sorted sand, mud, and rock fragments in marine environments, often shaped into a smooth cobble by river or glacial erosion.
Uses & applications
Used primarily in landscaping, decorative river rock beds, construction aggregate, and as garden borders.
Geological facts
Greywacke is often called 'dirty sandstone' because of its high clay content and variety of mineral fragments. It is very common in mountain belts.
Field identification & locations
Identify by its matte gray appearance, smooth rounded texture (if river-worn), and hardness; it will not crumble easily like siltstone. Found worldwide in riverbeds and coastal regions.
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