sedimentary
Banded River Cobble (Argillite or Shale)
Laminated Mudstone / Argillite
Hardness: 3-5 (Mohs scale); Color: Grey, brown, and dark charcoal bands; Luster: Dull/Earthy; Structure: Fine-grained, laminated beds; Cleavage: Poor, splits along bedding planes.
- Hardness
- 3-5 (Mohs scale)
- Color
- Grey, brown, and dark charcoal bands
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-5 (Mohs scale); Color: Grey, brown, and dark charcoal bands; Luster: Dull/Earthy; Structure: Fine-grained, laminated beds; Cleavage: Poor, splits along bedding planes.
Formation & geological history
Formed through the lithification of silt and clay-sized particles in low-energy environments like lake beds or deep oceans. The visible banding represents seasonal or episodic sediment pulses. The rounded shape indicates extensive transport in a river or stream bed.
Uses & applications
Common river stones are used in landscaping, garden decoration, and as aquarium substrate. Historically, fine-grained argillites have been used for carving or sharpening tools.
Geological facts
The rhythmic layering in such rocks is often called 'varves' if they are annual deposits. These stones are excellent records of paleo-climates and ancient hydrological systems.
Field identification & locations
Identify by its clear horizontal layering and fine-grained texture. It feels smoother than sandstone but grittier than pure chert. Commonly found in riverbeds, glacial tills, and alluvial plains.
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