Siltstone

Physical Properties

Hardness: 3-4 (Mohs scale); Color: Brown, reddish-brown, or grey; Luster: Dull or earthy; Crystal structure: Non-crystalline (clastic texture); Cleavage: None, often shows fissility; Specific gravity: 2.1-2.6

Formation & Geological History

Formed from the lithification of silt-sized particles (fine-grained clastic material) deposited in low-energy environments like river floodplains, lakes, or deep marine basins. Geological age can range from Precambrian to Holocene.

Uses & Applications

Used primarily as low-grade construction fill, as a source for soil when weathered, and occasionally for decorative stone or paving if sufficiently hard.

Geological Facts

Siltstone is differentiated from sandstone by its smaller grain size and from shale by its lack of laminations and higher silt content. It often preserves delicate trace fossils like worm burrows.

Field Identification & Locations

Identify in the field by its fine-grained texture that feels slightly gritty on the teeth but smoother than sandstone. Commonly found in sedimentary basins worldwide. Look for flat-lying beds and thin layers.

Identified on: 5/2/2026

Mode: Standard