Subage and Substage
A subage is lower than an age in the hierarchy of regional geochronologic units. It is equivalent to the time span of a substage (regional stratigraphic unit). A substage is the rock that formed during a subage; it is a regional stratigraphic unit that is lower than a stage and is a subdivision of a stage. The nomenclature is similar to that of stages; they are named after a location. A substage can also be labelled as a lower substage, middle substage or upper substage.