Goldenville strata in a quarry in 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 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 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, 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 "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.

<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

• Stratigraphy
• Polystrate fossil
• Important publications in stratigraphy
• Archaeology
• Geologic map
• Geologic unit
• Geologic formation
• Archaeological horizon

External links

• GeoWhen Database

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