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Definition of formation

i. A persistent body of igneous, sedimentary, or metamorphic rock, having easily recognizable boundaries that can be traced in the field without recourse to detailed paleontologic or petrologic analysis, and large enough to be represented on a geologic map as a practical or convenient unit for mapping and description; the basic cartographic unit in geologic mapping.

Synonym of: rock formation

ii. A body of sedimentary rock identified by lithic characteristics and stratigraphic position; it is prevailing but not necessarily tabular, and is mappable at the Earth's surface or traceable in the subsurface. The formation is the fundamental unit in lithostratigraphic classification.

Ref: AGI

iii. A general term applied by drillers to a sedimentary rock that can be described by certain drilling or reservoir characteristics; e.g. hard formation, cherty formation, or porous formation.

Ref: AGI

iv. A naturally formed topographic feature, commonly differing conspicuously from adjacent objects or material, or being noteworthy for some other reason; esp. a striking erosional form on the land surface.

Synonym of: geologic formation

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