Greywacke

Physical Properties

Hardness: 6-7 on Mohs scale; Color: typically grey, greenish-grey, or dark brown; Luster: dull to earthy; Texture: poorly sorted, angular grains in a fine clay matrix; Structure: massive or bedded.

Formation & Geological History

Formed from rapid deposition of sediment in deep marine environments, typically through turbidity currents (underwater landslides). Often associated with active plate boundaries and mountain-building events.

Uses & Applications

Primarily used as a durable aggregate in road construction, concrete production, and as railway ballast. Sometimes used for garden walling or paving due to its hardness.

Geological Facts

Greywacke is often called 'dirty sandstone' because of its high clay content and variety of unsorted mineral fragments. It is a key component of many ancient mountain belts like the Appalachians and the Alps.

Field Identification & Locations

Identify by its dark color, hardness (cannot be scratched easily by a knife), and lack of clearly defined layers compared to shale. Found in many coastal regions and mountain ranges worldwide.

Identified on: 5/3/2026

Mode: Standard