sedimentary
Glacial Erratic (Graywacke/Sandstone)
Clastic Sedimentary Rock (Lithic Sandstone)
Hardness: 6-7 Mohs; Color: Light gray to tan with weathering rinds; Luster: Dull/Earthy; Structure: Clastic, fine to medium-grained; Cleavage: None
- Hardness
- 6-7 Mohs
- Color
- Light gray to tan with weathering rinds
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Light gray to tan with weathering rinds; Luster: Dull/Earthy; Structure: Clastic, fine to medium-grained; Cleavage: None
Formation & geological history
Formed from the lithification of sand, silt, and clay in marine or river environments, then transported and smoothed by glacial movement during the Pleistocene epoch.
Uses & applications
Commonly used as landscaping stones, fill material in construction, or riprap for erosion control.
Geological facts
These rounded boulders are often called 'erratics' if they differ in composition from the local bedrock, having been carried hundreds of miles by ice sheets.
Field identification & locations
Identify by its rounded, sub-angular shape and lack of visible layering compared to local shale. Common in glacial till deposits across North America and Europe.
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