Rocks are often deposited in layers or strata, and the sequence of these strata can be correlated from place to place. These sequences of different rocks are used to establish the changing geological conditions or geological history of the area through time. The description, definition and naming of the rock units is termed lithostratigraphy (rock stratigraphy). The strata can also be described in other ways depending on the types of information available; for example in biostratigraphy (life stratigraphy) fossils are used and in chronostratigraphy (time stratigraphy) the age is used.
Lithostratigraphy is fundamental to most geological studies. Rock units are described using their gross lithological characteristics and named according to their perceived rank in a formal hierarchy. The main lithostratigraphic ranks in this hierarchy are: Supergroup, Group, Formation, Member and Bed. The units are usually named after a geographical locality, typically the place where exposures were first described.
These formal ranks are often appended to names in the Lexicon. The formation is the fundamental rock unit for mapping purposes. A group is an assemblage of related and adjacent formations and a supergroup an assemblage of groups. A member is a sub-division of a formation and a bed is the smallest formal unit. Some possible lithostratigraphic relationships are shown schematically below.
|Supergroup D||Group G||Formation V||Member X|
|Group F||Formation S|
Formations need not be subdivided nor grouped together. In this hierarchical scheme each unit may have parent and child relationships with other units of greater and lesser rank respectively. Thus Formation V has Group G as its parent and is itself the parent of Members X and W.