sedimentary

Greywacke

Lithic Sandstone (Greywacke - (SiO2+Al2O3+Fe2O3))

Hardness: 6-7 on Mohs scale; Color: Dark grey, greenish-grey, or black; Luster: Dull/Earthy; Structure: Clastic, poorly sorted with angular grains; Cleavage: None, breaks with an irregular or subconchoidal fracture.

Hardness: 6-7 on Mohs scale
Color: Dark grey, greenish-grey, or black
Luster: Dull/Earthy

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the rapid deposition of muddy sediment in deep-ocean environments, often via turbidity currents (underwater landslides) along continental margins. Most specimens date from the Paleozoic to Mesozoic eras.

Uses & applications

Primarily used as aggregate in road construction, concrete production, and as railroad ballast due to its hardness and durability. Occasionally used for paving stones.

Geological facts

Greywacke is often referred to as 'dirty sandstone' because of its high clay matrix content. It is a key indicator for geologists identifying ancient submarine sediment flows called turbidites.

Field identification & locations

Identify in the field by its 'salt and pepper' appearance under a lens, its extreme hardness (will often scratch steel), and its lack of visible layering compared to shale. Common in tectonic mountain belts like the Appalachians or the Alps.

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