Oldest (488.3–478.6 Ma ago) of the six stages that comprise the Ordovician period, preceded by the Late Cambrian epoch and followed by the Arenig stage. Mudstones and sandstones of the Tremadoc series occur in N. America, Ireland, Wales, England, and Scandinavia. They mark the continental shelf and slope areas that occur on the southern edge of the northern Iapetus Ocean.