Llandeilo Group

Last updated April 20, 2019

In geology, the Llandeilo Group is the middle subdivision of the British Ordovician rocks. It was first described and named by Sir Roderick Murchison from the neighborhood 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.

Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology significantly overlaps all other earth sciences, including hydrology and the atmospheric sciences, and so is treated as one major aspect of integrated earth system science and planetary science.

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.

Sir Roderick Impey Murchison, 1st Baronet, was a Scottish geologist who first described and investigated the Silurian system.

Features

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 Cadair Idris, and the Arenigs in Wales, and Helvellyn and Scafell in Westmorland and Cumberland. The series is also found at Builth and in Pembrokeshire. 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 ooooft. 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 limestone and Kirkland sandstones; and by the Glenapp conglomerates and Tappinsrnudstones and grits south of Stinchar. Graptolitic shales, similar to those of southern Scotland, are traceable into the north-east of Ireland.

North Wales is a region of Wales. Retail, transport and educational infrastructure are centred on Wrexham, Rhyl, Colwyn Bay, Llandudno and Bangor. It is bordered to the rest of Wales with the counties of Ceredigion and Powys, and to the east by the English counties of Shropshire, Merseyside, and Cheshire.

Cumberland is a historic county of North West England that had an administrative function from the 12th century until 1974. It was bordered by Northumberland to the east, County Durham to the southeast, Westmorland and Lancashire to the south, and the Scottish counties of Dumfriesshire and Roxburghshire to the north. It formed an administrative county from 1889 to 1974 and now forms part of Cumbria.

Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1,200 °C. The structures resulting from subsequent solidification and cooling are also sometimes described as lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites, though such material located below the crust is referred to by other terms.

Fossils

The fossils of the Liandeilo group include numerous graptolites, Coenograpus gracilis being taken as the zonal fossil of the upper portion, Didymograptus murchisoni of the lower. Other forms are Climacograptus scharenbergi and Diplograptus foliaceus . Many trilobites are found in these rocks, e.g. Ogygiocarella debuchii , Asaphus tyrannus , Calymene cambrensis , and Cheirurus . Among the brachiopods are Crania , Leptaena , Lingula , and Strophomena . Cardiola and Modiolopsis occur among the pelecypods; Euomphalus , Bellerophon , Murchisonia among the gastropods; and Conularia and Hyolithes among the pteropods. The cephalopods are represented by Orthoceras and Cyrtoceras .

Didymograptus is an extinct genus of graptolites with four rows of cups. They lived during the Middle Ordovician, to Late Ordovician.

Trilobites are a group of extinct marine arachnomorph arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period, and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out. Trilobites disappeared in the mass extinction at the end of the Permian about 252 million years ago. The trilobites were among the most successful of all early animals, existing in oceans for over 300 million years.

Ogygiocarella is a genus of asaphid trilobites. It occurred during the Middle Ordovician.

Quarrying

The green roofing slates and plumbago (graphite) of the Lake District are obtained from this group of rocks.

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References

This article incorporates text from a publication now in the public domain : Chisholm, Hugh, ed. (1911). "Llandeilo Group". Encyclopædia Britannica . 16 (11th ed.). Cambridge University Press. p. 828.

The public domain consists of all the creative works to which no exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly waived, or may be inapplicable.

The Encyclopædia Britannica, Eleventh Edition (1910–11) is a 29-volume reference work, an edition of the Encyclopædia Britannica. It was developed during the encyclopaedia's transition from a British to an American publication. Some of its articles were written by the best-known scholars of the time. This edition of the encyclopedia, containing 40,000 entries, is now in the public domain, and many of its articles have been used as a basis for articles in Wikipedia. However, the outdated nature of some of its content makes its use as a source for modern scholarship problematic. Some articles have special value and interest to modern scholars as cultural artifacts of the 19th and early 20th centuries.

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