Stratum
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{{dablink|This article is about the geologic use of the term, for other uses see [[Stratum (disambiguation)]]}}
{{dablink|For other uses, see [[strata (novel)]] and [[strata title]].}}
[[Image:IM000671.JPG|thumb|Goldenville strata in a quarry in [[Bedford, Nova Scotia|Bedford]], [[Canada]]. These are [[Middle Cambrian]] marine sediments. This formation covers over half of [[Nova Scotia]] and is recorded as being 29,000 feet thick in some areas.]]
In [[geology]] and related fields, a '''stratum''' (plural: ''strata'') is a layer of [[rock (geology)|rock]] or [[soil]] with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over [[1 E11 m²|hundreds of thousands of square kilometers]] of the [[Earth]]'s surface. Strata are typically seen as bands of different colored or differently structured material exposed in [[cliff]]s, [[road]] cuts, [[quarry|quarries]], and [[river]] banks. Individual bands may vary in thickness from a few [[millimeter]]s to a [[kilometer]] or more. Each band represents a specific mode of [[deposition]] -- river [[silt]], beach [[sand]], coal [[swamp]], [[sand dune]], [[lava]] bed, etc.
[[Geologist]]s study rock strata and categorize them by the material in the beds. Each distinct layer is usually assigned to a "[[geologic formation|formation]]" name usually based on a town, river, mountain, or region where the formation is exposed and available for study. For example, the [[Burgess Shale]] is a thick exposure of dark, occasionally [[fossil]]iferous, [[shale]] exposed high in the [[Canadian Rockies]] near Burgess Pass. Slight distinctions in material in a formation may be described as "members" or sometimes "beds." Formations are collected into "groups." Groups may be collected into "supergroups."
The stratum is the fundamental unit in a [[stratigraphic column]] and forms the basis of the study of [[stratigraphy]].
<gallery widths="180px" perrow=3>
Image:Strata-french-alps.jpg|Strata on a mountain face in the French Alps
Image:Rockstrata3435.JPG|Interstate road cut through [[limestone]] and [[shale]] strata in East [[Tennessee]]
Image:Rock_Strata.jpg|Rock strata at [[Depot Beach]], [[New South Wales]]
Image:Rainbow_Basin.JPG|Rainbow Basin Syncline near [[Barstow, California]]. Folded strata.
Image:OrdOutcropTN.JPG|Outcrop of Upper Ordovician limestone and minor shale, central Tennessee; [[College of Wooster]] students.
Image:Geology_of_Cyprus-Chalk.jpg|Chalk Layers in Cyprus - showing classic layered structure.
Image:HeavyMineralsBeachSand.jpg|Heavy minerals (dark) as thin strata in a quartz beach sand ([[Chennai]], India).
</gallery>
==See also==
*[[Stratification]]
*[[Polystrate fossil]]
*[[List of publications in geology#Stratigraphy|Important publications in stratigraphy]]
*[[Archaeology]]
*[[Geologic map]]
*[[Geologic unit]]
*[[Geologic formation]]
*[[Archaeological horizon]]
==External links==
*[http://www.stratigraphy.org/geowhen/ GeoWhen Database]
[[Category:Stratigraphy]]
[[Category:Methods and principles in archaeology]]
[[ca:Estrat]]
[[de:Bank (Geologie)]]
[[es:Estrato]]
[[eu:Estratu]]
[[fa:چینه]]
[[fr:Strate (géologie)]]
[[gl:Estrato]]
[[ko:지층]]
[[he:שכבה (גאולוגיה)]]
[[it:Giacimento]]
[[nl:Laag (stratigrafie)]]
[[ja:地層]]
[[pl:Warstwa (geologia)]]
[[pt:Estrato geológico]]
[[ru:Пласт (геология)]]
[[sk:Vrstva (geológia)]]
[[vi:Địa tầng]]
[[uk:Надра]]