Rock
Siltstone
Silttaşı
A rock finer than sand and coarser than clay: it feels gritty between the teeth, and its hard types were carved into statues and palettes in ancient Egypt.
Also known as: Aleurolite
- Popularly: Common stone
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In short
Siltstone is a clastic sedimentary rock made mostly of silt-sized particles, finer than sand and coarser than clay. Its grains cannot be seen with the naked eye, but a small piece crushed between the teeth feels slightly gritty, whereas claystone feels smooth. It forms when fine sediment settling in quiet water, on floodplains, in lakes and on deltas is buried and hardens. It is mostly used as fill and low-grade aggregate; in ancient Egypt a hard, fine-grained type was worked into statues and palettes.
Photos
How does it form?
Silt is made when rocks and minerals are ground down: glaciers grind rock, rocks break down in cold mountains and in the tropics, and salt shatters rock in deserts. Most silt grains are quartz. Rivers or wind carry the silt, and it settles where currents are weak: on floodplains, lake floors, in front of deltas and on shallow sea floors.
As new sediment piles up the silt is buried and compacted, and silica, calcite or iron oxide precipitated from the water between the grains glues them together to make siltstone. It is a two-stage process: first the silt accumulates, then it is buried and cemented.
What is it used for?
- Fill and aggregate: Most siltstone is used as low-grade aggregate or fill.
- Building stone: Hard, silica-cemented siltstone can be cut and dressed for walls, paving and flagstones.
- Gas reservoir: Because of its low permeability it can be a “tight gas” reservoir, but the gas can only be produced by hydraulic fracturing.
Egypt’s stone
The quarry at Wadi Hammamat supplied ancient Egypt’s hard, fine-grained “bekhen stone”. From Predynastic times it was made into cosmetic palettes and vessels, and later into statues, stelae and sarcophagi. Quarrying expeditions sometimes involved thousands of workers. The stone’s fine grain and resistance to flaking made it suited to detailed work.
What determines its value?
Siltstone is an abundant, low-value rock, most often used as low-grade aggregate and fill. Hard, silica-cemented types can be cut and dressed for walls, paving and flagstones, and then their value depends on the hardness of the cement, the evenness of the beds, the size of the blocks and the nearness of the quarry. The hard, fine-grained types sought in ancient Egypt carry historical and archaeological value.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
Its grains cannot be seen with the naked eye and are hard to make out even with a hand lens; the surface looks smooth and dull.
- 2
A small piece crushed between the teeth feels slightly gritty, whereas claystone feels slippery and smooth.
- 3
It does not split into thin sheets like shale; it breaks in blocks or looks finely bedded (laminated).
- 4
Its hardness depends on its cement: clay-cemented types scratch easily with a knife, while silica-cemented types become very hard.
Fakes and imitations
What shops usually don’t tell you:
- Siltstone, claystone and mudstone look much alike: if silt dominates it is siltstone, if clay it is claystone, and if both are mixed it is mudstone. Rubbing a bit between the teeth or fingers to test for grittiness helps tell them apart.
- The stone of the Narmer Palette was often wrongly identified in the past as slate or schist; this fine-grained, hard stone is now called greywacke (a kind of siltstone).
Myths and misconceptions
MythSiltstone is a fine-grained kind of sandstone.
FactSiltstone is finer-grained than sandstone: silt grains are about 0.004–0.063 mm across, smaller than sand grains and invisible to the naked eye. Both are clastic rocks, but they are separate rocks.
MythSiltstone and shale are the same rock.
FactBoth belong to the mudrock family, but shale splits into thin sheets and siltstone does not; siltstone also has little clay and feels gritty between the teeth.
Did you know?
Egypt's famous Narmer Palette (about the 31st century BCE) was carved from a dark grey-green stone from the Wadi Hammamat quarry; Wikipedia describes the stone as greywacke, a fine-grained, hard kind of siltstone.
Most silt grains are quartz, made by glaciers grinding rock, by weathering in cold mountains and in the tropics, and by salt weathering in deserts.
Siltstone has very low permeability, so gas locked in it can only be produced economically by hydraulic fracturing.
Where is it found?
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- Wadi Hammamat, Eastern Desert, Egypt · Historic sourceIn ancient Egypt the hard, fine-grained, flake-resistant stone (“bekhen stone”) prized for statues and cosmetic palettes was quarried in this valley. Wikipedia calls it siltstone or greywacke; quarrying began about 6,000 years ago, in Predynastic times, and continued almost without a break to Roman times.
- Tatacoa Desert, Huila, Colombia · Known localitySiltstones and claystones make up 75% of the Villavieja Formation; these greenish, reddish-brown or bluish-grey beds weather into so-called “cauliflower erosion” forms. This is the La Venta area, famous for its Miocene fossils.
- Badlands National Park, South Dakota, USA · Known localityThe Oligocene Brule Formation is made of claystone, mudstone and siltstone interbedded with freshwater carbonates, volcanic ash and sandstone.
- Petrified Forest National Park, Arizona, USA · Known localityThe Late Triassic Chinle Formation is mostly river-laid mudstone, siltstone and sandstone, and is famed for the Painted Desert and its petrified wood.
- Dinosaur National Monument, Utah–Colorado, USA · Known localityThe Late Jurassic Morrison Formation consists of mudstone, sandstone, siltstone and limestone; most of its fossils occur in green siltstone beds and the lower sandstones.
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is siltstone?
Siltstone is a clastic sedimentary rock made mostly of silt-sized particles, finer than sand and coarser than clay. Its grains cannot be seen with the naked eye, but a small piece crushed between the teeth feels slightly gritty, whereas claystone feels smooth. It forms when fine sediment settling in quiet water, on floodplains, in lakes and on deltas is buried and hardens. It is mostly used as fill and low-grade aggregate; in ancient Egypt a hard, fine-grained type was worked into statues and palettes.
What is siltstone used for?
Fill and aggregate: Most siltstone is used as low-grade aggregate or fill. Building stone: Hard, silica-cemented siltstone can be cut and dressed for walls, paving and flagstones. Gas reservoir: Because of its low permeability it can be a “tight gas” reservoir, but the gas can only be produced by hydraulic fracturing.
How hard is siltstone?
It has a hardness of 3–6 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2–2.7 g/cm³.
Where is siltstone found?
Its best-known localities: Wadi Hammamat, Eastern Desert, Egypt; Tatacoa Desert, Huila, Colombia; Badlands National Park, South Dakota, USA; Petrified Forest National Park, Arizona, USA; Dinosaur National Monument, Utah–Colorado, USA.
How can you tell if siltstone is real?
Its grains cannot be seen with the naked eye and are hard to make out even with a hand lens; the surface looks smooth and dull. A small piece crushed between the teeth feels slightly gritty, whereas claystone feels slippery and smooth. It does not split into thin sheets like shale; it breaks in blocks or looks finely bedded (laminated). Siltstone, claystone and mudstone look much alike: if silt dominates it is siltstone, if clay it is claystone, and if both are mixed it is mudstone. Rubbing a bit between the teeth or fingers to test for grittiness helps tell them apart.
Is siltstone valuable?
On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Siltstone is an abundant, low-value rock, most often used as low-grade aggregate and fill. Hard, silica-cemented types can be cut and dressed for walls, paving and flagstones, and then their value depends on the hardness of the cement, the evenness of the beds, the size of the blocks and the nearness of the quarry. The hard, fine-grained types sought in ancient Egypt carry historical and archaeological value.
Sources
- Siltstone · Wikipedia
- Silttaşı · Vikipedi
- Siltstone: the fine-grained sedimentary rock · Rockhounding.org
- Rock Density Tables (after Tenzer et al. 2011, A digital rock density map of New Zealand, Computers & Geosciences 37: 1181–1191) · Rocscience, RocFall2 documentation
- Wadi Hammamat · Wikipedia
- Narmer Palette · Wikipedia
- Honda Group, Colombia · Wikipedia
- Brule Formation · Wikipedia
- Chinle Formation · Wikipedia
- Morrison Formation · Wikipedia
External links
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