Siltstone

Physical Properties

Hardness: 3-5 Mohs; Color: Tan to reddish-brown (indicated by iron oxides); Luster: Dull/Earthy; Crystal structure: Clastic (cannot see grains with naked eye); Cleavage: None; Specific Gravity: 2.1-2.6

Formation & Geological History

Formed through the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lagoons, or deep sea basins. These sediments are typically millions of years old, often ranging from the Paleozoic to the Cenozoic eras.

Uses & Applications

Used primarily in construction as fill material or low-grade building stone. In some cases, smooth siltstones are used for flagstone or interior decorative panels. Its value to collectors is mostly educational.

Geological Facts

Siltstone is the 'middle child' of sedimentary rocks: it has particles larger than clay (shale) but smaller than sand (sandstone). Its fine texture often preserves trace fossils such as animal tracks or worm burrows.

Field Identification & Locations

Identify by its gritty feel on the teeth (a common field test) and lack of visible grains. It differs from shale by its lack of fissility (it doesn't split into thin plates easily). Found in sedimentary basins worldwide, often near water sources or ancient lake beds.

Identified on: 4/20/2026

Mode: Standard