Editing Geological formation (section)

{{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}}