Editing Geological formation (section)

==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}}