Siltstone

Physical Properties

Hardness (Mohs): 6-7, Color: Gray, brown, or reddish, Luster: Dull, Crystal structure: Clastic (microscopic grains), Cleavage: None (may have bedding planes), Specific gravity: 2.5-2.8

Formation & Geological History

Formed from the lithification of silt-sized particles (larger than clay but smaller than sand) deposited in slow-moving water environments like floodplains, deltas, or lakes. Geological age varies by outcrop.

Uses & Applications

Used primarily as a soil conditioner, fill material, and occasionally as a low-grade building stone or flagstone in landscaping.

Geological Facts

Siltstone is intermediate between sandstone and shale. While it resembles shale, it lacks the thin bedding planes (fissility) that cause shale to split into thin sheets.

Field Identification & Locations

Identify in the field by its fine-grained texture that feels gritty against the teeth but is too fine to seeindividual grains with the naked eye. Commonly found in coastal or fluvial sedimentary basins.

Identified on: 5/3/2026

Mode: Standard