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In chronostratigraphy, a stage is a Geologic record laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.
Rock series are divided into stages, just as geological epochs are divided into ages. Stages are divided into smaller stratigraphic units called or substages, and added together into superstages.
The term faunal stage is sometimes used, referring to the fact that the same fauna (animals) are found throughout the layer (by definition).
Thus, for example in the local North American subdivision, a paleontologist finding fragments of the trilobite Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan Stage.
Stages were important in the 19th and early 20th centuries as they were the major tool available for dating and correlating rock units prior to the development of seismology and radioactive dating in the second half of the 20th century. Microscopic analysis of the rock (petrology) is also sometimes useful in confirming that a given segment of rock is from a particular age.
Originally, faunal stages were only defined regionally. As additional stratigraphic and geochronologic tools were developed, they were defined over ever broader areas. More recently, the adjective "faunal" has been dropped as regional and global correlations of rock sequences have become relatively certain and there is less need for faunal labels to define the age of formations. A tendency developed to use European and, to a lesser extent, Asian stage names for the same time period worldwide, even though the faunas in other regions often had little in common with the stage as originally defined.
In many regions local subdivisions and classification criteria are still used along with the newer internationally coordinated uniform system, but once the research establishes a more complete international system, it is expected that local systems will be abandoned.
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