a subdivision of the general stratigraphic scale, subordinate to a geological series. A stage is subdivided into stratigraphic zones. It includes the stratum of rocks that formed in one geological age and corresponds to a definite phase in the geological development of the earth. The chief criterion for the establishment of the size and boundaries of a stage is the findings of biostratigraphic analysis. Most stages identified on the basis of open marine basins correspond to terrestrial subdivisions.