sedimentary
Siltstone
Siltstone (fined-grained clastic sedimentary rock)
Hardness: 3-4 on Mohs scale; Color: Grayish-green or tan; Luster: Dull/Earthy; Crystal structure: Non-crystalline clastic; Grain size: Intermediate between sandstone and shale.
- Hardness
- 3-4 on Mohs scale
- Color
- Grayish-green or tan
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Grayish-green or tan; Luster: Dull/Earthy; Crystal structure: Non-crystalline clastic; Grain size: Intermediate between sandstone and shale.
Formation & geological history
Formed from the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lakes, or deep-sea basins. These deposits typically occur over millions of years.
Uses & applications
Commonly used as fill material in construction, as a base for roads, and occasionally in decorative landscaping. Fine-grained varieties are used for sharpening stones.
Geological facts
Siltstone is distinct from shale because it lacks 'fissility'—it doesn't easily split into thin plates. It often contains micro-fossils due to the calm waters in which it settles.
Field identification & locations
Identify in the field by its gritty 'toothy' feel against the teeth (unlike clay-rich shale) and its lack of visible grains (unlike sandstone). Commonly found in sedimentary basins worldwide.
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