RocksdayMudstone vs Siltstone
How to tell them apart
Hardened mud that breaks into blocks rather than thin sheets.
Photo: James St. John, CC BY 2.0
Rock of silt — grains finer than sand but coarser than clay.
Photo: Mike Davis, CC0
Quick tests
- Hardness: Siltstone is clearly harder: Mohs 3–5 versus 2–3.
Key differences
- Siltstone feels gritty on the teeth.
Side by side
| Property | Mudstone | Siltstone |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 2–3 | 3–5 |
| Luster | Dull | Dull |
| Colors | Gray, Red, Black, Brown | Gray, Brown, Red |
| Crystal system / structure | Clastic, clay and silt, blocky | Clastic, silt-sized grains |
| Specific gravity | 2.3–2.7 | 2.4–2.7 |
| Found in | Sedimentary basins worldwide; Gale crater, Mars | Sedimentary basins worldwide; Grand Canyon (Supai Group) |
Properties that are the same for both are grayed out.
How to identify mudstone
- Smooth, very fine-grained.
- Breaks into blocky chunks.
- Soft; may smell earthy when wet.
Structure: Clastic, clay and silt, blocky
Full Mudstone guide →How to identify siltstone
- Grains too small to see but gritty against your teeth.
- Thin layers, not as flaky as shale.
- Dull and earthy.
Structure: Clastic, silt-sized grains
Full Siltstone guide →More comparisons
Could you tell them apart from a blurry close-up?
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