sedimentary

Siltstone

Siltstone (Clastic Sedimentary Rock)

Hardness: 3-5 (Mohs scale); Color: Brown, red, or grey; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None (may show bedding planes); Specific Gravity: 2.3-2.6

Hardness: 3-5 (Mohs scale)
Color: Brown, red, or grey
Luster: Dull/Earthy

Identified June 24, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs scale); Color: Brown, red, or grey; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None (may show bedding planes); Specific Gravity: 2.3-2.6

Formation & geological history

Formed through the accumulation and cementation of silt-sized particles (0.0039 to 0.0625 mm). Usually found in river floodplains, lakes, or marine lagoons where low-energy water allows fine particles to settle.

Uses & applications

Used primarily as low-quality fill material in construction or as a decorative stone in landscaping. Some varieties are used as flagstones for paving.

Geological facts

Siltstone is intermediate between sandstone (coarser) and shale (finer). Unlike shale, siltstone typically lacks the property of fissility (the ability to split into thin layers).

Field identification & locations

Identify in the field by its fine-grained texture that feels gritty against the teeth but smoother than sandstone. Commonly found in sedimentary basins and riverbeds.