Greywacke (River Siltstone)

Physical Properties

Hardness: 6-7 Mohs; Color: Grey, greenish-grey, or dark charcoal; Luster: Dull/Earthy; Structure: Clastic, fine-grained with visible layering (lamination); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the rapid deposition of muddy sediment and poorly sorted sand in deep ocean trenches or continental slopes. This specimen has been rounded and polished by fluvial (river) or glacial transport processes.

Uses & Applications

Commonly used as aggregate in road construction, railroad ballast, and concrete. Smooth river specimens are often used for decorative landscaping or stone massage therapy.

Geological Facts

Greywacke is often referred to as 'dirty sandstone' because of its high clay and silt content. It is a key rock for understanding tectonic movement and ancient underwater landslides called turbidity currents.

Field Identification & Locations

Identify in the field by its distinct grey color, hardness (will not be scratched by a steel nail easily), and fine, parallel bands or laminations. Look for it along riverbeds and pebble beaches near mountain ranges.

Identified on: 4/22/2026

Mode: Standard