Editing Geological formation

{{Short description|Fundamental unit of lithostratigraphy}}
[[File:Grand_Canyon_geologic_column.jpg|thumb|right|A geologic [[cross section (geology)|cross section]] of the [[Geology of the Grand Canyon area|Grand Canyon]]. Black numbers correspond to [[group (stratigraphy)|groups]] of formations and white numbers correspond to formations (click on picture for more information)]]

A '''geological formation''', or simply '''formation''', is a body of rock having a consistent set of physical characteristics ([[lithology]]) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the [[stratigraphic column]]). It is the fundamental unit of [[lithostratigraphy]], the study of [[strata]] or rock layers.{{sfn|Boggs|1987|pp=545-547}}{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1567-1569}}

A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the [[thickness (geology)|thickness]] of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even a heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent bodies of rock.{{sfn|Boggs|1987|pp=545-547}}{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1567-1569}}

The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by [[Abraham Gottlob Werner]] in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817.{{sfn|Frank|1938|pp=221-222}}  The concept became increasingly formalized over time and is now codified in such works as the North American Stratigraphic Code and its counterparts in other regions.{{sfn|North American Commission on Stratigraphic Nomenclature|2005}}

[[Geological map]]s showing where various formations are exposed at the surface are fundamental to such fields as [[structural geology]], allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources.{{sfn|Ghosh|1993}}

==Defining formations==
[[File:SEUtahStrat.JPG|thumb|upright=1.35|The [[Permian]] through [[Jurassic]] strata of the [[Colorado Plateau]] area of southeastern [[Utah]] 
demonstrate the principles of [[stratigraphy]].  These strata make up much of the famous prominent rock formations in widely spaced protected areas such as [[Capitol Reef National Park]] and [[Canyonlands National Park]].  From top to bottom: Rounded tan domes of the [[Navajo Sandstone]], layered red [[Kayenta Formation]], cliff-forming, vertically jointed, red [[Wingate Sandstone]], slope-forming, purplish [[Chinle Formation]], layered, lighter-red [[Moenkopi Formation]], and white, layered [[Cutler Formation]] sandstone.  Picture from [[Glen Canyon National Recreation Area]], Utah.]]

[[File:Uluru (Helicopter view)-crop.jpg|thumb|[[Uluru]] (Ayers Rock) in [[Australia]] is underlain by the Mutitjulu Arkose, a formation composed almost entirely of a single lithology (arkosic sandstone).]]
[[File:Summerville Formation.jpg|thumb|The [[Summerville Formation]] is composed of alternating thin beds of two lithologies, mudstone and sandstone, penetrated by veins of a third lithology, gypsum.]]
The boundaries of a formation are chosen to give it the greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary [[Sedimentary structures|depositional structures]], [[fossil]]s regarded as rock-forming particles, or other organic materials such as [[coal]] or [[kerogen]]. The [[Taxonomy (biology)|taxonomy]] of fossils is ''not'' a valid lithological basis for defining a formation.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1567-1569}}

The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geological mapping normally practiced in the region; the thickness of formations may range from less than a meter to several thousand meters.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1567-1569}}

Geologic formations are typically named after a permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the [[Morrison Formation]], named for the town of [[Morrison, Colorado]], and the [[Kaibab Limestone]], named after the [[Kaibab Plateau]] of Arizona. The names must not duplicate previous formation names, so, for example, a newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first name applied to a particular formation.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1561-1562}}

As with other stratigraphic units, the formal designation of a formation includes a ''[[stratotype]]'' which is usually a ''[[type section]].'' A type section is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional ''reference sections'' may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining the formation is expected to describe the stratotype in sufficient detail that other geologists can unequivocally recognize the formation.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|p=1563}}

Although formations should not be defined by any criteria other than primary lithology, it is often useful to define [[Geologic unit|biostratigraphic units]] on [[paleontology|paleontological]] criteria, [[Geologic unit|chronostratigraphic units]] on the age of the rocks, and [[Geologic unit|chemostratigraphic units]] on geochemical criteria, and these are included in stratigraphic codes.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1553-1554}}

==Usefulness of formations==
The concept of formally defined layers or strata is central to the geologic discipline of [[stratigraphy]], and the formation is the fundamental unit of stratigraphy. Formations may be combined into [[group (stratigraphy)|groups]] of strata or divided into [[Geological unit#Member|members]]. Members differ from formations in that they need not be mappable at the same scale as formations, though they must be lithologically distinctive where present.{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1569}}

The definition and recognition of formations allow [[geologist]]s to correlate geologic strata across wide distances between [[outcrop]]s and exposures of rock [[stratum|strata]]. Formations were at first described as the essential [[geologic time]] markers, based on their relative ages and the [[law of superposition]]. The divisions of the [[geological time scale]] were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.{{sfn|Frank|1938}}

Geologic formations can be usefully defined for [[sedimentary rock]] layers, low-grade [[metamorphic rocks]], and [[volcanic rock]]s. Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as [[Lithodemic unit|lithodemes]].{{sfn|North American Commission on Stratigraphic Nomenclature|2005|pp=1570-1572}}

== Other uses of the term ==

"Formation" is also used informally to describe the odd shapes (forms) that rocks acquire through erosional or depositional processes. Such a formation is abandoned when it is no longer affected by the geologic agent that produced it. Some well-known [[cave]] formations include [[stalactite]]s and [[stalagmite]]s.

==See also==
* {{annotated link|Geochronology}}
* {{annotated link|List of rock formations}}
* {{annotated link|List of fossil sites}}

==Footnotes==
{{Reflist}}

==References==
* {{cite book |last1=Boggs |first1=Sam Jr. |title=Principles of sedimentology and stratigraphy |date=1987 |publisher=Merrill Pub. Co |isbn=0675204879 |edition=1st}}
*{{cite book|last=Frank |first=Adams Dawson |title=The Birth and Development of the Geological Sciences |location=Baltimore |publisher=The Williams & Wilkins Company |year=1938 |url=https://archive.org/details/birthanddevelopm031745mbp }}
* {{cite journal |author=North American Commission on Stratigraphic Nomenclature |title=North American Stratigraphic Code |journal=AAPG Bulletin |date=November 2005 |volume=89 |issue=11 |pages=1547–1591 |doi=10.1306/07050504129 |bibcode=2005BAAPG..89.1547. |url=https://ngmdb.usgs.gov/Geolex/resources/docs/AAPG_Bull-89_NACSN-Code.pdf |access-date=8 August 2020}}
* {{cite book |last1=Ghosh |first1=S.K. |title=Structural geology : fundamentals and modern developments |date=1993 |publisher=Pergamon Press |isbn=0-08-041879-1 |edition=1st}}

==Further reading==
{{Refbegin}}
*{{cite book|last=Brookfield|first=Michael E.|title=Principles of Stratigraphy|year=2008|publisher=John Wiley & Sons|isbn=9780470693223|pages=111&ndash;114, 200}} 
*{{cite book|editor-last=Rey |editor-first=Jacques |editor2=Simone Galeotti |title=Stratigraphy : terminology and practice |year=2008 |publisher=Editions OPHRYS |location=Paris, France |isbn=9782710809104}}
{{Refend}}

== External links ==
* {{Commons category-inline|Geologic formations}}

{{Authority control}}

{{DEFAULTSORT:Formation (Stratigraphy)}}
[[Category:Geologic formations| ]]