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Encyclopedia Britannica - Main :: LEO-LOB |
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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
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
average
district
District
great
Cross
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
Scotland , are traceable into the north-east
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). End of Article: LLANDEILO GROUP If you wish, you can link directly to this article.
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