The Geologic Time package contains elements used to describe the classification of geologic time: time periods, time boundaries, and the relationships between them as defined by the International Commission on Stratigraphy. A boundary between two geochronologic time periods Several StratigraphicPoints may be associated with a boundary, but only one may have GSSP ratified status. The others are proposed equivalent. The association of an era with a stratotype is optional. In the GSSP approach recommended by ICS for the Global Geologic Timescale, Unit Stratotypes are not used. Rather, the association of an Era with geologic units and sections is indirect, via the association of an era with Boundaries, which are in turn tied to Stratotype Points, which occur within host Stratotype Sections. Note that the "German School" defines stratigraphic eras conceptually, without reference to stratotypes. A term from a controlled vocabulary describing the rank of the time period (eg, eon, era, period, stage) gsmlgt:GeochronologicEraRank GeochronologicEraRank Type section that names the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. A unit stratotype is the agreed reference point for a particular stratigraphic unit and a boundary stratotype the reference for a particular boundary between strata (Wikipedia) The classic "Geologic Timescale" comprising an ordered, hierarchical set of named "eras" is an example of an Ordinal Temporal Reference System. It may be calibrated with reference to a numeric Temporal Coordinate System, but is in principle defined independently. A type of stratigraphic point used to define a globally agreed point in geologic time A type of stratigraphic section used to define a globally agreed standard period of geologic time Used for pre-Ediacaran and Pleistocene / Holocene boundaries in the standard timescale A standard numeric age point (a numeric analogue to a 'golden spike') applicable to the formal subdivision of the Precambrian, and perhaps the Pleistocene/Holocene boundary (Walshe, Gradstein and Ogg, 2004) A point in the stratigraphic record used to define a geochronologic boundary or point in geologic time A description of the primary criterion used to establish this stratigraphic point Any additional criteria used to establish this stratigraphic point A description of the status of stratigraphic point (eg, formally accepted, etc) A sampled section of the stratigraphic record used to define a period in geologic time A description of the geologic setting of the stratigraphic section A description of the geology of the stratigraphic section (eg, lithology, paleontology, paleogeography, etc) A description of the ability to access the stratigraphic section A description of measures to conserve the stratigraphic section The association of an era with a stratotype is optional. In the GSSP approach recommended by ICS for the Global Geologic Timescale, Unit Stratotypes are not used. Rather, the association of an Era with geologic units and sections is indirect, via the association of an era with Boundaries, which are in turn tied to Stratotype Points, which occur within host Stratotype Sections. Note that the "German School" defines stratigraphic eras conceptually, without reference to stratotypes. Older time boundary of an era gsmlgt:nextEra Younger time boundary of an era gsmlgt:previousEra Subdivisions of TimeOrdinalEra A point in Earth's history which bounds a TimeOrdinalEra. A point in time corresponding to the era boundary Observation supporting the existence of the boundary (geochronology, paleontology, etc.) A measure of the uncertainty in the estimate of the point in time of the era boundary Succeeding era gsmlgt:start Preceding era gsmlgt:end A time reference system comprised of an ordered set of time periods (time ordinal eras). TimeOrdinalEra composing the TimeOrdinalReferenceSystem Two reference points defining the extent of the system