sedimentary
Siltstone / Shale River Rock
Siltstone (primarily SiO2 with Al2O3)
Hardness: 3-4 (Mohs); Color: Dark brown to chocolate; Luster: Dull/Earthy; Crystal structure: Microcrystalline/Clastic; Cleavage: None, but breaks in flat planes; Specific Gravity: 2.3-2.6
- Hardness
- 3-4 (Mohs)
- Color
- Dark brown to chocolate
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Dark brown to chocolate; Luster: Dull/Earthy; Crystal structure: Microcrystalline/Clastic; Cleavage: None, but breaks in flat planes; Specific Gravity: 2.3-2.6
Formation & geological history
Formed by the compaction of silt-sized particles in low-energy environments like river beds, lake bottoms, or floodplains. Over millions of years, pressure from overlying layers lithifies the mud/silt into solid rock.
Uses & applications
Commonly used as gravel, landscape decoration, or in the manufacturing of cement. Polished versions are sometimes used for decorative garden stones.
Geological facts
Siltstone is finer than sandstone but coarser than shale. It lacks the 'fissility' or ease of splitting into thin sheets that is characteristic of true shale.
Field identification & locations
Identified by its fine-grained texture that feels slightly gritty on the teeth (compared to smooth clay) and its flat, water-worn shape. Found in various riverbeds and sedimentary basins worldwide.
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