sedimentary

Siltstone with Iron Staining

Siltstone

Hardness: 3-4 (Mohs); Color: Grayish-green to tan with orange/red iron oxide (limonite) staining; Luster: Dull, earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.1-2.6

Hardness: 3-4 (Mohs)
Luster: Dull, earthy

Identified June 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of silt-sized particles in low-energy environments such as river floodplains, lagoons, or deep-water basins. Often Paleozoic to Cenozoic in age.

Uses & applications

Used primarily as low-grade construction fill, as a source for soil when weathered, or occasionally as a building stone if well-indurated.

Geological facts

Siltstone is differentiated from shale by its lack of fissility (it doesn't split into thin layers) and from sandstone by its smaller grain size, which feels smooth to the touch rather than gritty.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty on the teeth but smooth to the fingers. Common in sedimentary basins worldwide. This specific specimen shows chemical weathering (oxidation).