LLANDEILO

GROUP  , in geology, the middle subdivision of the British Ordovician rocks . It was first described and named by Sir . R . I . Murchison from the neighbourhood of Llandeilo in Carmarthenshire . In the type area it consists of a series of slaty rocks, shales, calcareous flagstones and sandstones; the calcareous middle portion is sometimes termed the " Llandeilo limestone "; and in the upper portion volcanic rocks are intercalated . A remarkable feature in the history of the Llandeilo rocks in Britain, more especially in North Wales and Cumberland,was the outbreak of volcanic action; vast piles of Llandeilo lava and ashes form such hills as Cader Idris, and the Arenigs in Wales, and Helvellyn and Scafell in Westmorland and Cumberland . The series is also found at Builth and in Pembroke-shire . The average thickness in Wales is about 2000 ft . The group is usually divided in this area into three sub-divisions . In the Corndon district of Shropshire the Middleton Series represents the Llandeilo group; it includes, in descending order, the Rorrington black shales, the Meadowtown limestones and flags, and the western grits and shales . In the Lake District the great volcanic series of Borrowdale, green slates and porphyries, 8000 to 9000 ft. in thickness, lies on this horizon; and in the Cross Fell area the Milburn beds of the Skiddaw slates (see ARENIG) appear to be of the same age .

In

Scotland the Llandeilo group is represented by the Glenkiln shales, black shales and yellowish mudstones with radiolarian cherts and volcanic tuffs; by the Barr Series, including the Benan conglomerates, Stinchar lime-stone and Kirkland sandstones; and by the Glenapp conglomerates and Tappins'mudstones and grits south of Stinchar . Graptolitic shales, similar to those of southern Scotland, are traceable into the north-east of Ireland . The fossils of the Llandeilo group include numerous graptolites, Coenograplus gracilis being taken as the zonal fossil of the upper portion, Didymograptus Murchisoni of the lower . Other forms are Climacograptus Scharenbergi and Diplograplus foliaceus . Many trilobites are found in these rocks, e.g . Ogygia Buchi, Asaphus tyrannus, Calymene cambrensis, Cheirurus Sedgwickii . Among the brachiopods are Crania, Leptaena, Lingula, Strophomena; Cardiola and Modiolopsis occur among the Pelecypods; Euomphalus, Bellerophon, Murchisonia among the Gasteropods; Conularia and Hyolithes among the Pteropods; the Cephalopods are represented by Orthoceras and Cyrtoceras . The green roofing slates and plumbago (graphite) of the Lake District are obtained from this group of rocks . (see ORDOVICIAN) .