sedimentary

Siltstone (Iron-rich)

Ferruginous Siltstone

Hardness: 3-5 (Mohs); Color: Reddish-brown, yellow-ochre, tan; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None (may show fissility)

Hardness: 3-5 (Mohs)
Color: Reddish-brown, yellow-ochre, tan
Luster: Dull/Earthy

Identified July 2, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs); Color: Reddish-brown, yellow-ochre, tan; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None (may show fissility)

Formation & geological history

Formed by the lithification of silt-sized particles deposited in low-energy environments like river floodplains or lake beds. The reddish color indicates the presence of iron oxides like hematite or goethite.

Uses & applications

Informally used as construction fill or flagstone; historically used as a source of pigment (ochre) or for carving items like small tools or ornaments.

Geological facts

Siltstones occupy the middle ground between sandstone and shale. If they split easily into thin layers, they are referred to as silt-shale. Iron-rich versions were often used by ancient cultures to create pigments for cave paintings.

Field identification & locations

Identify in the field by its fine-grained texture (grittier than clay/shale but smoother than sandstone) and its distinct layering. Common in sedimentary basins worldwide. Check for a red/brown streak when rubbed on ceramic.