Siltstone is a common sedimentary rock defined by the size of the particles it contains. Like sandstone or shale, its classification depends on the diameter of mineral fragments that settled out of ancient water or air currents. Siltstone forms when loose silt sediment is compacted and cemented, a process called lithification. The specific grain size distinguishes it from coarser or finer-grained rocks, providing geologists with clues about the environment where the rock was deposited.
Defining Silt and Siltstone Grain Size
The grain size range for silt is precisely defined to differentiate it from sand and clay. Silt grains span from \(1/256\) of a millimeter (mm) at the fine end up to \(1/16\) mm at the coarse end. This narrow range is the defining characteristic of the rock.
Expressed in decimals, the particles within siltstone are between \(0.0039\) mm and \(0.0625\) mm in diameter. Translating this into micrometers (\(\mu\)m), the range is approximately \(4\) \(\mu\)m to \(62.5\) \(\mu\)m. For a rock to be correctly identified as siltstone, more than two-thirds of its constituent grains must fall within this specific size category.
The Wentworth Classification System
The precise numerical boundaries for siltstone are set by a standardized geological framework. Geologists utilize the Udden-Wentworth scale to classify clastic sediments based on particle diameter. This system uses a geometric progression that doubles or halves the size at each major boundary, which is why the limits are expressed as fractions like \(1/16\) and \(1/256\).
The scale organizes sediments into classes such as clay, silt, sand, and gravel. To handle the small numbers associated with fine-grained materials, geologists commonly use the Phi (\(\phi\)) scale notation. This logarithmic transformation converts millimeter measurements into simple, positive whole numbers for easier mathematical analysis. A higher Phi value corresponds to a smaller particle size.
Formation and Physical Characteristics
Siltstone forms in environments where the energy of the water or wind is moderate, allowing silt particles to settle out of suspension. Typical low-energy settings include deep-sea basins, quiet lakes, or expansive river floodplains. The particles are carried further than heavier sand grains but are too large to remain suspended indefinitely like clay particles.
The resulting rock texture is intermediate between sand and clay. A piece of siltstone will feel gritty or powdery when rubbed, a quality used to distinguish it from the smooth feel of shale. Its mineral composition is a mix, frequently containing quartz and feldspar, cemented by clay minerals or silica. Siltstone lacks the tendency of shale to easily split into thin layers, a property known as fissility.
Distinguishing Siltstone from Related Sedimentary Rocks
The specific grain size range of siltstone makes it distinct from related sedimentary rocks: sandstone and mudrock. The upper boundary of siltstone, \(0.0625\) mm, is the precise point where the classification shifts to sandstone. Sandstone particles are large enough that individual grains are visible to the naked eye or a hand lens.
Conversely, the lower boundary of \(0.0039\) mm separates siltstone from mudrock, which includes shale and claystone. Mudrock is composed of particles finer than silt, meaning the grains are too small to impart grittiness when rubbed. A field identification test often involves pressing the rock against a tooth: siltstone feels gritty, while the finer-grained shale feels smooth.