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Stratum
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 cliffs, road cuts, quarry|quarries, and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more. Each band represents a specific mode of deposition -- river silt, beach sand, coal swamp, sand dune, lava bed, etc.
Geologists 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 fossiliferous, 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.
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).
See also
- Stratigraphy
- Polystrate fossil
- List of publications in geology#Stratigraphy|Important publications in stratigraphy
- Archaeology
- Geologic map
- Geologic unit
- Geologic formation
- Archaeological horizon
External links
- GeoWhen Database
Category:Stratigraphy
Category:Methods and principles in archaeology
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Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL
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