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chronostratigraphic unit (time-stratigraphic unit, time-rock unit) The sequence of rocks formed during a discrete and specified interval of geologic time. Chronostratigraphic units are ranked, according to the length of time they record, into erathems (the longest), systems, series, stages (the basic working unit), and chronozones (the shortest). Each unit comprises a number of units of lower rank, e.g. a system would consist of a number of series, and, similarly, a number of stages would constitute a series. All the rocks formed anywhere in the world, regardless of lithology or local thickness, would be referred to the chronostratigraphic unit appropriate to their time of formation, e.g. all rocks laid down in the Cambrian Period belong to the Cambrian System. In the traditional stratigraphic scales, however, note that chronostratigraphic units, and the geologic-time units to which they correspond, have been defined on the basis of a type section (see STRATOTYPE), so historically it is the chronostratigraphic unit that has determined the geologic-time unit and not vice versa. See CHRONOSTRATIGRAPHY; and STRATIGRAPHIC NOMENCLATURE.