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A geological formation, or formation, is a body of rock having a consistent set of physical characteristics (
lithology as seen in southeastern Utah The lithology of a Rock (geology), rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, te ...
) that distinguish 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 A stratigraphic column is a representation used in geology and its subfield of stratigraphy to describe the vertical location of rock units in a particular area. A typical stratigraphic column shows a sequence of sedimentary rocks, with the oldest ...
). It is the fundamental unit of
lithostratigraphy through Jurassic lithostratigraphy of the Colorado Plateau area of southeastern Utah that makes up much of the famous prominent rock formations in protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom ...
, the study of
strata (Argentina). , Canada. These are Middle Cambrian marine sediments. This formation covers over half of Nova Scotia and is recorded as being 8,800 m (29,000 ft) thick in some areas. In geology and related fields, a stratum (plural: strata) is a l ...
or rock layers. 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 Thickness may refer to: * Thickness (graph theory) * Thickness (geology), the distance across a layer of rock * Thickness (meteorology), the difference in height between two atmospheric pressure levels * Thickness planer a woodworking machine * Op ...
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. The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by
Abraham Gottlob Werner Abraham Gottlob Werner (25 September 174930 June 1817) was a German geologist who set out an early theory about the stratification of the Earth's crust and propounded a history of the Earth that came to be known as Neptunism. While most tenets of ...

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. 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.
Geologic map A geologic map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with strike and dip or trend and ...
s showing where various formations are exposed at the surface are fundamental to such fields as
structural geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information ...
, allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources.


Defining formations

is a formation composed almost entirely of a single lithology (sandstone). File:Summerville Formation.jpg, 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 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, fossils 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. 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 geologic mapping normally practiced in the region; the thickness of formations may range from less than a meter to several thousand meters. 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. 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. 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.


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. The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops 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. Geologic formations can be usefully defined for sedimentary rock layers, low-grade metamorphic rocks, and volcanic rocks. Intrusive igneous rocks and highly metamorphosed rocks are generally not considered to be formations, but are described instead as Lithodemic unit, lithodemes.


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 stalactites and stalagmites.


See also

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Footnotes


References

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Further reading

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External links

* {{DEFAULTSORT:Formation (Stratigraphy) Geologic formations,