What is Geological formation?
[[Image:Grand_Canyon_geologic_column.jpg|thumb|right|230px|A geologic cross section of the [[Grand Canyon]]. Black numbers correspond to groups of formations and white numbers correspond to formations (click on picture for more information).]] [[File:Grand Canyon.jpg|thumb|right|230px|Strata of the Grand Canyon]] A '''formation''', or [[Rock (geology)|rock]] formation, is the fundamental unit of litho[[stratigraphy]]. A formation consists of a certain number of rock [[strata]]. They have similar lithology (rocks), [[sedimentary]] facies (appearance) or other properties. Formations are not defined on the thickness of the rock strata, and the thickness of different formations can therefore vary widely. The concept of formally defined layers or strata is central to [[stratigraphy]]. A formation can be divided into 'members' and are themselves packed together in 'groups'. Formations were initially described as time markers, based on [[relative dating]] and the law of superposition. The divisions of the [[history of the Earth]] were the formations described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. Rock formations are formed by [[sedimentary]] [[Deposition (geology)|deposition]] in [[environment]]s which may persist for hundreds of millions of years. For example, the Hammersley Basin in [[Pilbara]], [[Western Australia]], is a [[Proterozoic]] sedimentary [[Basin (geology)|basin]] where up to 1200 million years of sedimentation is preserved intact. Here, up to 300 million years is represented by a single unit of [[banded iron formation]] and [[shale]]. == In order == *Stratigraphic '''groups''' **'''Formations''' ***'''Members''' ****'''[[Stratum]]''' (the smallest unit) [[Category:Geological formations| ]] [[Category:Sedimentary rocks]] [[Category:Stratigraphy]]