sedimentary

Graywacke (River Cobble)

Lithic Sandstone (Graywacke)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Color: Dark gray to bluish-gray; Luster: Dull/Earthy; Structure: Massive, fine-grained with visible lithic fragments; Cleavage: None.

Hardness: 6-7 (Mohs)
Color: Dark gray to bluish-gray
Luster: Dull/Earthy

Identified July 5, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs); Color: Dark gray to bluish-gray; Luster: Dull/Earthy; Structure: Massive, fine-grained with visible lithic fragments; Cleavage: None.

Formation & geological history

Formed in deep-water marine environments where submarine landslides (turbidity currents) deposit poorly sorted sand and silt. These deposits are typically from the Paleozoic or Mesozoic eras.

Uses & applications

Commonly used in heavy construction as road base, railroad ballast, and high-quality concrete aggregate due to its durability.

Geological facts

Graywacke is often called 'dirty sandstone' because of its poorly sorted mixture of sand-sized grains and a clay matrix. It is a signature rock of active tectonic margins.

Field identification & locations

Identify by its characteristic 'slatey' gray color, extreme toughness compared to regular sandstone, and lack of distinct layering in individual cobbles. Commonly found in riverbeds near mountain ranges.