The informal naming of a stratigraphic unit occurs when (a) the unit-term is referred to as an ordinary noun, and not in the context of a proper name, e.g. ‘the geologic periods’; and (b) when a unit-term (such as zone, or formation) is referred to without having been specifically established in classification, e.g. ‘a sandstone formation’, ‘a mineralized zone’. Terms used informally are not capitalized. Compare formal. See stratigraphic nomenclature.