Siltstone Facts

Siltstone Facts
Siltstone is a sedimentary rock mainly composed of silt sized particles. It is finer than sandstone and coarser than claystone. Siltstone is found in a variety of colors. While it is usually gray, brown, or reddish brown, it can also be white, yellow, red, purple, green, black, orange and other colors. The colors are a response to the composition of the cement, the composition of the grains, or the stains from subsurface waters. Although it is often mistaken as shale, it lacks the fissility and laminations which are typical of shale. It forms where wind, ice or water deposit silt, and the silt is then compacted and cemented into a rock.
Interesting Siltstone Facts:
Silt does not have a definite composition. It usually has a mixture of micas, feldspars, quartz, and clay minerals. The small fraction of silt is mostly clay. The coarse-size fraction is mostly grains of quartz and feldspar.
Mudstone and shale are rocks that contain mud, which is material that has a range of clay and silt. Siltstone is different because it is mostly silt and not clay.
Siltstone is much less common than shale and sandstone. The rocks are usually thinner and less extensive.
Silt accumulates in sedimentary basins all over the world. It represents a level of current, wind or wave energy between where mud and sand accumulate. These include aeolian, fluvial, tidal, lacustrine, coastal, glacial, paludal, deltaic, and shelf environments. Sedimentary structures such as layering, ripple marks, erosional contacts, cross-bedding, and fossils provide evidence of these environments.
The silt-sized particles range between 0.00015 and 0.0025 inches in diameter, or between 0.0039 and 0.063 millimeters in diameter. They are intermediate in size between coarse clay on the small side and fine sand on the large side.
The grains of coarse silt are large enough that most people can see them without using a magnifying glass.
It can be difficult to identify without close examination and requires breaking off a small piece to observe the grain size. Scraping the surface with a knife blade or nail will dislodge tiny silt grains instead of dislodging sand grains.
Experienced geologists and soil scientists have been known to detect a few grains of silt by biting them gently between their front teeth, though this test is not recommended.
Siltstone has very few uses. If used for building, it is only because of availability, not quality. Its main use is as a low-quality fill when better materials are not locally available.


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