sedimentary

Siltstone

Siltstone (Siliclastic sedimentary rock)

Hardness: 3-4 (Mohs); Color: Brown, tan, or reddish-brown; Luster: Dull/Earthy; Structure: Clastic with very fine grains; Cleavage: None (may exhibit fissility or layering); Texture: Gritty on teeth but smoother than sandstone.

Hardness: 3-4 (Mohs)
Color: Brown, tan, or reddish-brown
Luster: Dull/Earthy

Identified July 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains, lakes, or deep marine basins.

Uses & applications

Used primarily as low-grade construction fill, occasional flagstone for landscaping, and as a source of study for paleoenvironments; generally not durable enough for high-end construction.

Geological facts

Siltstone is often confused with shale, but it lacks the thin, splitting laminations (fissility) characteristic of true shale. It often contains well-preserved fossils and ripple marks from ancient water currents.

Field identification & locations

Identify in the field by its fine-grained texture and visible layering (bedding). It feels slightly gritty to the touch compared to smooth claystone. Common in sedimentary basins worldwide.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Mudstone

Argillite / Siliciclastic sedimentary rock

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Brown Sandstone (River Rock)

Arenite

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary