sedimentary

Siltstone

Siltstone (primarily SiO2 with Al2O3)

Hardness: 3-4 mohs; Color: Gray, brown, or reddish-brown; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (none-equidimensional grains); Specific Gravity: 2.3-2.6

Hardness: 3-4 mohs
Color: Gray, brown, or reddish-brown
Luster: Dull or earthy

Identified June 8, 2026More sedimentary →

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Physical properties

Hardness: 3-4 mohs; Color: Gray, brown, or reddish-brown; Luster: Dull or earthy; Structure: Clastic, fine-grained; Cleavage: None (none-equidimensional grains); Specific Gravity: 2.3-2.6

Formation & geological history

Formed from the accumulation of silt-sized particles (smaller than sand, larger than clay) in low-energy environments like river floodplains, lakes, or deep marine basins. Grains are compacted and cemented over geologic time.

Uses & applications

Used primarily as low-quality construction fill or road base. Occasionally used for decorative landscaping or interior flagstone if sufficiently durable and colorful.

Geological facts

Siltstone is often mistaken for shale, but it lacks the fissility (the ability to split into thin layers) that characterizes true shale. It often preserves ripple marks or fossils from its original depositional environment.

Field identification & locations

Identify in the field by its gritty feel (rubbing it against your teeth will feel gritty, unlike smooth claystone). It is commonly found in sedimentary basins worldwide, often sandwiched between sandstone and shale layers.