The GCR sites selected for this GCR Block represent the British geological record of Earth history from around 460 to 458 million years ago (Ma). Originally afforded ‘Series’ status at the time of GCR site selection, the Llandeilo rocks now comprise the Llandeilian Stage of the Llanvirn Series. Rocks formed during the Llanvirn Epoch comprise the Llanvirn Series. The Llanvirn Epoch is the third of the Ordovician Period (495–440 Ma), which is the second period of the Palaeozoic Era (545 to 251 Ma). Rocks that formed during the Ordovician Period constitute the Ordovician System. Arenig–Llanvirn (excluding ‘Llandeilo’) sites have their own GCR Block. See ARE-LLV.
The Cambrian and Ordovician rocks of Britain are essentially of marine origin; freshwater and subaerial deposits have been identified only in settings where volcanic edifices were raised above sea level. Within the marine setting there are rocks of the widest variety, from sandstones and limestones of the shoreline and shallow shelves to beds that accumulated in deep basins or on the continental slopes. Transects from shelf to basin are seen in both Cambrian and Ordovician rocks, and examples of shallow- and deep-water deposits are recognized in both Anglo-Welsh and Scottish areas.
The traditional series of the Ordovician System are Tremadoc (oldest), Arenig, Llanvirn, Caradoc and Ashgill (youngest).
For details of Ordovician stratigraphy, palaeogeography, palaeontology and GCR site selection, see CAM.