Geological formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties. Formations are not defined on the thickness of the rock strata they consist of and the thickness of different formations can therefore vary widely.
The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. Formations can be divided into members and are themselves frequently parcelled together in groups.
Usefulness of formations
The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata.
Formations were initially described to be the essential geologic time markers based on relative ages and the law of superposition. The divisions of the geological time scale were the formations described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.