LLANVIRN SERIES

sequence of Ordovician rocks first studied in North Wales (the Ordovician Period lasted from 505 to 438 million years ago). The Llanvirn precedes the Llandeilo Series and follows the Arenig Series. In the type region, the Llanvirn consists of approximately 760 m (2,500 feet) of mudstones and shales, as well as volcanic lavas and tuffs. Two Llanvirn graptolite fossil zones, shorter spans of time, are generally recognized: a lower zone of the species Didymograptus bifudus and an upper zone of D. murchisoni. Rocks correlated with the Llanvirn of Wales have a worldwide distribution. In the Baltic region, shales predominate and zonation has been achieved with both trilobites (extinct marine arthropods) and graptolites. Important Llanvirn sequences are known from eastern Europe, northern Africa, China, Australia and New Zealand, and South America. Different regional names are applied to rocks that have been correlated with the Llanvirn in these areas.