Editing Greywacke

{{Short description|Sandstone with angular grains in a clay-fine matrix}}

[[File:Greywacke-f-and-l.jpg|thumb|right|Photomicrographs of feldspathic (L) and lithic (R) greywacke. The top images are in [[Plane polarized light|plane-polarized light]]; the bottom images are in [[cross-polarized light]]. Cements fill the pore spaces.]]
[[File:Menkaura Bust Closeup.jpg|thumb|Closeup of Pharaoh [[Menkaure]]'s greywacke statue, 25th century BCE, from the [[Egyptian Museum]] in [[Cairo]]]]

'''Greywacke''' or '''graywacke''' ({{langx|de|Grauwacke}} {{gloss|a grey, earthy rock}}) is a variety of [[sandstone]] generally characterized by its [[hardness]] (6–7 on [[Mohs scale]]), dark color, and [[Sorting (sediment)|poorly sorted]] angular grains of [[quartz]], [[feldspar]], and small rock fragments or sand-size [[Lithic fragment (geology)|lithic fragments]] set in a compact, [[clay]]-fine matrix. It is a texturally immature [[sedimentary rock]] generally found in [[Paleozoic]] [[Stratum|strata]]. The larger [[Particle size (grain size)|grain]]s can be sand- to gravel-sized, and [[Matrix (geology)|matrix]] materials generally constitute more than 15% of the rock by volume.

== Formation ==
The origin of greywacke was unknown until [[turbidity current]]s and [[turbidite]]s were understood, since, according to the normal laws of [[sedimentation]], [[gravel]], [[sand]] and [[mud]] should not be laid down together. [[Geologist]]s now attribute its formation to submarine [[avalanche]]s or strong turbidity currents. These actions churn [[sediment]] and cause mixed-sediment slurries, in which the resulting deposits may exhibit a variety of sedimentary features. Supporting the turbidity origin theory is the fact that deposits of greywacke are found on the edges of the [[Continental shelf|continental shelves]], at the bottoms of [[oceanic trench]]es, and at the bases of mountain formational areas. They also occur in association with black [[shale]]s of deep-sea origin.

As a rule, greywackes do not contain [[fossil]]s, but organic remains may be common in the finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably [[indurated]] by [[Recrystallization (geology)|recrystallization]], such as the introduction of interstitial [[silica]]. In some districts, the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates.{{sfn|Flett|1911}}

Although the group is so diverse that it is difficult to characterize mineralogically, it has a well-established place in [[Petrology|petrographical]] classifications because these peculiar composite [[arenite|arenaceous]] deposits are very frequent among [[Silurian]] and [[Cambrian]] rocks, and are less common in [[Mesozoic]] or [[Cenozoic]] strata.{{Citation needed|date=August 2007}} Their essential features are their gritty character and their complex composition. By increasing [[metamorphism]], greywackes frequently pass into mica-[[schist]]s, chloritic schists and sedimentary [[gneiss]]es.{{sfn|Flett|1911}}

=== Varieties ===
The term "greywacke" can be confusing, since it can refer to either the immature (rock fragment) aspect of the rock or its fine-grained (clay) component.

Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with [[shale]]s and [[limestone]]s. Some varieties include ''feldspathic greywacke'', rich in [[feldspar]], and ''lithic greywacke'', rich in other tiny rock fragments. They can contain a very great variety of [[mineral]]s, the principal ones being quartz, [[orthoclase]] and [[plagioclase]] feldspars, [[calcite]], [[iron oxide]]s and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as [[felsite]], [[chert]], [[slate]], [[gneiss]], various [[schist]]s, and [[quartzite]]. Among other minerals found in them are [[biotite]], [[Chlorite group|chlorite]], [[tourmaline]], [[epidote]], [[apatite]], [[garnet]], [[hornblende]], [[augite]], [[sphene]] and [[pyrite]]s. The cementing material may be [[siliceous]] or [[argillaceous minerals|argillaceous]] and is sometimes [[calcareous]].{{sfn|Flett|1911}}

== In geology and geography ==

Greywackes are abundant in [[Wales]], the south of [[Scotland]], the Longford-Down Massif<ref>{{cite web |title=The Geological Heritage of County Longford |url=https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/Longford_Audit.pdf |access-date=25 October 2022 |publisher=The County Longford Geological Heritage Project}}</ref> in [[Ireland]] and the [[Lake District National Park]] of [[England]]; they compose the majority of the main [[Southern Alps]] that make up the backbone of [[New Zealand]]. Both feldspathic and lithic greywacke have been recognized in [[Ecca Group]] in [[South Africa]].<ref>{{Cite journal |last1=Baiyegunhi |first1=Christopher |last2=Liu |first2=Kuiwu |last3=Gwavava |first3=Oswald |date=2017-10-28 |title=Modal composition and tectonic provenance of the sandstones of Ecca Group, Karoo Supergroup in the Eastern Cape Province, South Africa |journal=Open Geosciences |volume=9 |issue=1 |pages=491–508 |bibcode=2017OGeo....9...38B |doi=10.1515/geo-2017-0038 |issn=2391-5447 |doi-access=free}}</ref> Greywackes are also found in parts of the [[Eastern Desert]] east of the [[Nile]].<ref name=":0">{{Cite web |title=Ancient Egyptian Materials: Greywacke (schist) |url=http://www.hallofmaat.com/ancient-egyptian-materials-greywacke-schist/ |access-date=2023-09-01 |website=www.hallofmaat.com}}</ref>

They were an early object of geological study in Britain where the [[Geological Society of London|Geological Society]] was founded in 1807, and excited much public interest in geology.<ref name="BB80">{{cite book |last1=Bryson |first1=Bill |title=A Short History of Nearly Everything |date=October 27, 2009 |publisher=Crown |page=80 |edition=Kindle |access-date=July 24, 2022 |quote=In 1839, when Roderick Murchison published The Silurian System, a plump and ponderous study of a type of rock called greywacke, it was an instant best seller...|url=https://www.penguinrandomhouse.com/books/20549/a-short-history-of-nearly-everything-special-illustrated-edition-by-bill-bryson/}}</ref> Greywacke was interesting because it was found in many places in Britain and its occurrence in particular places was evidence of the pattern of [[Stratum|geological strata]] that had been laid down.<ref>{{cite book |last1=Bryson |first1=Bill |title=Short History of Nearly Everything |publisher=Crown |page=85 |edition=Kindle |quote=Martin J. S. Rudwick’s excellent and somber account of the issue, The Great Devonian Controversy.}}</ref><ref name="Davidson">{{cite book |last1=Davidson |first1=Nick |title=The greywacke: how a priest, a soldier and a schoolteacher uncovered 300 million years of history |date=2021 |publisher=Profile Books |location=London |isbn=1788163788}}</ref>

==Uses==
Greywacke stone has been used as a building material and a sculptural material across many eras and societies. Its oldest known uses date to the early [[3rd millennium BC|third millennium]] [[Common Era|BCE]], in Egypt's [[Early Dynastic Period (Egypt)|early dynastic period]].  Its wide use in sculpture and vessels is thought to have been due to its fine grain size and resistance to fracturing, making it suitable for fine detail and intricate shapes.<ref name=":0" />

Aside from its structural uses, greywacke stone (or [[Molding (process)|molds]] taken from it) is valuable to practitioners of traditional motion picture [[miniature photography]], because due to its unusually mixed nature, it remains looking natural when portraying a wide range of miniature scale ratios, from 1:1 to as high as 1:600.<ref>{{cite video |title=Worldbuilding a "Bigature" for Wētā Workshop Unleashed! |author=[[Adam Savage's Tested]] |url=https://www.youtube.com/watch?v=Tr5JBIiXI88&t=985s |website=youtube.com |date=2022-07-19 |access-date=2023-03-27}}</ref>

==Gallery==
<gallery widths="200px" heights="200px">
Image:Grauwacke AC Rathaus.jpg|A greywacke rock

File:Moehnesee wall 05.JPG|Wall of greywacke at [[Möhne Reservoir]] dam, Germany

</gallery>

==See also==
* [[Greywacke zone]]
* [[Torlesse Greywacke]]

==References==
{{Reflist}}

===Other works cited===
* {{EB1911|wstitle=Greywacke|first=John Smith|volume=12|page=592|last=Flett|author-link=John Flett (geologist)}}

==External links==
{{Commons category|Greywacke}}
* [https://web.archive.org/web/20050417224825/http://www.marin.cc.ca.us/~jim/ring/rgrey.html Franciscan Greywacke/Shales]
* [https://www.nps.gov/prsf/naturescience/graywacke-sandstone.htm National Park Service site Presidio]
{{Rock type}}
{{Authority control}}

[[Category:Sandstone]]
[[Category:Sedimentary rocks]]