File:2021_Revised_NPS_Geologic_Stratigraphic_Column_of_the_Grand_Canyon.jpg · Wikimedia Commons · See Wikimedia Commons
geological formation
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body of rock identified by lithic characteristics and stratigraphic position; it is usually but not necessarily tabular and is mappable at the Earth's surface or traceable in the subsurface. Smaller than a subgroup
~8 min read
Encyclopedic overview
Figure 1. A geologic cross section of the Grand Canyon.
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the stratigraphic column). It is the fundamental unit of lithostratigraphy, the study of strata or rock layers.
Excerpted from Wikipedia’s “geological formation” article, available under the CC BY-SA 4.0 licence.