stratigraphic nomenclature The naming of stratigraphic and geologic-time units according to established practices and principles. Formal naming of a stratigraphic unit occurs when the unit is first proposed and described from a type section (see STRATOTYPE), which acts thereafter as the standard reference for that unit. Ideally, the name given is binomial, and in the case of chronostratigraphic and lithostratigraphic units consists of a preceding geographic name taken from the type locality (plus lithological description where appropriate), followed by the name of the unit, e.g. Ludlow Series and Elk Point Group. The names of biostratigraphic units consist of the name of the characteristic fossil plus the relevant unit term, e.g. Monograptus uniformis Range zone. The name chosen for a stratigraphic unit should be unique to that unit. When used as a proper name, as above, the initial letters are capitalized. Except in very special circumstances the first formal name given has priority and is adhered to. In practice many well-known units, e.g. Coal Measures, Millstone Grit, were named long before the present conventions were established, and to avoid confusion these names are preserved in their original form. Geologic-time units generally take their preceding name from that of the corresponding chronostratigraphic unit, plus the name of the unit (period, epoch, age, etc.), e.g. the Jurassic Period, from the Jurassic System (named after the type area in the Jura Mountains). The names of eons and eras (e.g. Phanerozoic Eon, Mesozoic Era) were proposed independently, so that the names for the corresponding eonothems and erathems are derived from the time units. See also INFORMAL.