Siltstone

Physical Properties

Hardness: 3-4 (Mohs); Color: Dark brown to grey with visible bedding planes; Luster: Dull or earthy; Crystal structure: Clastic (grained); Cleavage: None/Planar bedding; Specific gravity: 2.1-2.6

Formation & Geological History

Formed from the accumulation and lithification of silt-sized particles (smaller than sand, larger than clay) in low-energy aquatic environments like floodplains or lagoons. Most deposits are Paleozoic to Cenozoic in age.

Uses & Applications

Commonly used as a filler in construction materials, for low-quality flagstone, and occasionally as an abrasive or in the production of cement.

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 represents a intermediate environment between sand-rich beaches and clay-rich deep waters.

Field Identification & Locations

Identify in the field by its gritty feel (less gritty than sandstone) and the presence of fine laminations or bedding planes. It will not usually fizz with acid unless it has calcareous cement.

Identified on: 4/23/2026

Mode: Standard