LLANDEILAN SERIES

the fourth of six divisions (in ascending order) comprising the Ordovician System; it represents all those rocks on a global basis deposited during the Llandeilan Age, between 458 and 468 million years ago. The name is derived from the town of Llandeilo in Dyfed, Wales, which serves as the traditional British type area. At its maximum, the series locally attains a thickness of approximately 760 m (2,500 feet) and consists of shales, limestones, and sandy beds. The base of the type succession, however, is unconformable with underlying Llanvirnian strata and thus unsuitable as a global stratotype section. No global stratotype section and point (GSSP) has yet been defined by the International Commission on Stratigraphy, under the authority of the International Union of Geological Sciences. Nearby, above the base of the British type section, is a graptolite fauna within the Glyptograptus teretiusculus biozone. The base of this biozone has been taken as the base of the Llandeilan Series, pending a more precise definition. In principle, the top of the Llandeilan Series is defined by the base of the overlying Caradocian Series.