sedimentary

Graywacke (River Cobble)

Graywacke (Lithic Sandstone)

Hardness: 6-7 on Mohs scale; Color: Dark gray, greenish-gray, or brownish-gray; Luster: Dull/Earthy; Structure: Clastic with poorly sorted grains (quartz, feldspar, lithic fragments); Specific Gravity: 2.6-2.7

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

Identified May 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the rapid deposition of muddy sediment in deep marine environments (turbidity currents). This specific specimen has been rounded and smoothed by fluvial or coastal water erosion.

Uses & applications

Commonly used in construction as crushed stone for road base, railway ballast, and in some decorative landscaping due to its durability. Smooth cobbles are often used for aesthetic river rock gardens.

Geological facts

Graywackes are often referred to as 'dirty sandstones' because of their high proportion of fine-grained matrix (clay and silt) mixed with larger sand grains. They often show graded bedding within a geological formation.

Field identification & locations

Identify in the field by its dark, 'gritty' appearance and hardness (it will not be easily scratched by a steel blade). Commonly found in mountainous regions or along riverbeds and beaches near tectonic plate boundaries.

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