sedimentary

Banded Siltstone

Siltstone (primarily SiO2 with Al2O3)

Hardness: 3-5 Mohs; Color: Gray, tan, buff, and brown; Luster: Dull or earthy; Crystal structure: Clastic (microscopic grains); Cleavage: None (may split along bedding planes); Specific gravity: 2.3-2.6.

Hardness: 3-5 Mohs
Color: Gray, tan, buff, and brown
Luster: Dull or earthy

Identified June 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy aquatic environments such as river floodplains, deltas, or lakes. The banding represents seasonal cycles or changing environmental conditions over thousands of years.

Uses & applications

Used as fill material in construction, for making cement, and occasionally as a decorative paving stone (flagstone) or for landscape design. Small, attractively banded pieces are popular with rock collectors.

Geological facts

Siltstone is often mistaken for shale, but it lacks the fissility (thin splitting) of shale and has a grittier feel on the teeth. The banding seen here is known as rhythmic bedding.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty compared to smooth claystone. To test, rub it against your teeth; if it feels gritty, it is siltstone. Commonly found in sedimentary basins worldwide.

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