The Ordovician Reedsville Formation is a mapped surficial bedrock unit in Pennsylvania, Maryland, Virginia, West Virginia, and Tennessee, that extends into the subsurface of Ohio. This rock is a slope-former adjacent to (and stratigraphically below) the prominent ridge-forming Bald Eagle sandstone unit in the Appalachian Mountains . It is often abbreviated Or on geologic maps.
10-537: The Reedsville Formation is an olive-gray to dark-gray siltstone , shale , and fine-grained sandstone . In Central Pennsylvania along the Nittany Arch , and extending into the subsurface of northern West Virginia , the base of the Reedsville formation includes the black calcareous Antes Shale formation. The type locality is at Reedsville, Pennsylvania . Relative age dating of the Reedsville places it in
20-542: Is likely biogenic, but most quartz sediments come from granitic rocks in which quartz grains are much larger than quartz silt. Highly energetic processes are required to break these grains down to silt size. Among proposed mechanism are glacial grinding; weathering in cold, tectonically active mountain ranges; normal weathering, particularly in tropical regions; and formation in hot desert environments by salt weathering. Siltstones form in relatively quiet depositional environments where fine particles can settle out of
30-426: Is sometimes a tight gas reservoir rock, an unconventional reservoir for natural gas that requires hydraulic fracturing for economic gas production. Siltstone was prized in ancient Egypt for manufacturing statuary and cosmetic palettes . The siltstone quarried at Wadi Hammamat was a hard, fine-grained siltstone that resisted flaking and was almost ideal for such uses. There is not complete agreement on
40-427: The field by chewing a small sample; claystone feels smooth while siltstone feels gritty. Siltstones differ significantly from sandstones due to their smaller pores and a higher propensity for containing a significant clay fraction. Although often mistaken for a shale , siltstone lacks the laminations and fissility along horizontal lines which are typical of shale. Siltstones may contain concretions . Unless
50-542: The rock types that will be formed after lithification , if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available (e.g. banded iron formations ). Continental Transitional Marine Others Depositional environments in ancient sediments are recognised using
60-465: The United States is a stub . You can help Misplaced Pages by expanding it . Siltstone Siltstone , also known as aleurolite , is a clastic sedimentary rock that is composed mostly of silt . It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility . Although its permeability and porosity is relatively low, siltstone
70-775: The Upper Ordovician. It rests conformably atop the Upper Ordovician Coburn Formation at the top of the Trenton Group limestone and conformably below the Bald Eagle Formation . Isotopic dating of shale mylonite in Pennsylvania reveals a K-Ar age of 372+/-8 Ma. The Reedsville is quarried locally in borrow pits for road material and fill. This article about a specific stratigraphic formation in
80-546: The definition of siltstone. One definition is that siltstone is mudrock ( clastic sedimentary rock containing at least 50% clay and silt) in which at least 2/3 of the clay and silt fraction is composed of silt-sized particles. Silt is defined as grains 2–62 μm in diameter, or 4 to 8 on the Krumbein phi (φ) scale . An alternate definition is that siltstone is any sedimentary rock containing 50% or more of silt-sized particles. Siltstones can be distinguished from claystone in
90-405: The siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding. Siltstone is an unusual rock, in which most of the silt grains are made of quartz . The origin of quartz silt has been a topic of much research and debate. Some quartz silt likely has its origin in fine-grained foliated metamorphic rock, while much marine silt
100-444: The transporting medium (air or water) and accumulate on the surface. They are found in turbidite sequences, in deltas, in glacial deposits, and in miogeosynclinal settings. Depositional environment In geology , depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore,
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