A geologic epoch is the fourth largest unit of time for the geologic time scale (Figure 1). Geologic epochs are also referred to as "series" (the chronostratigraphic name) or simply "epochs". Epochs tend to be 13 to 35 million of years in length. Epochs are subsections of longer periods and consist of groups of shorter ages. Epoch names are often "Upper", "Middle", or "Lower". If they are formally named they usually end in the suffix "-ian" or"-an".