Bed (geology)

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Tilted sedimentary bedding in shales of the Cretaceous Salto del Fraile Formation, Peru.

In geology a bed is the smallest division of a geologic formation or stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below. A bed is the smallest lithostratigraphic unit, usually ranging in thickness from a centimetre to several metres and distinguishable from beds above and below it. Beds can be differentiated in various ways, including rock or mineral type and particle size. The term is generally applied to sedimentary strata, but may also be used for volcanic flows or ash layers.

In a quarry, a bedding is a term used for a structure occurring in granite and similar massive rocks that allows them to split in well-defined planes horizontally or parallel to the land surface.

Engineering considerations

In geotechnical engineering a bedding plane often forms a discontinuity that may have a large influence on the mechanical behaviour (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction.

See also


  • Lamina, Laminaset, Bed and Bedset. Campbell, Charles V. Sedimentology, vol. 8, issue 1, pp. 7-26.