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

Roderick Murchison Sir Roderick Impey Murchison, 1st Baronet, (19 February 1792 – 22 October 1871) was a Scotland, Scottish geologist who served as director-general of the British Geological Survey from 1855 until his death in 1871. He is noted for investigat ...

from the neighborhood of Llandeilo in Carmarthenshire. In the type area it consists of a series of slaty rocks,

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

s, calcareous flagstones and sandstones; the calcareous middle portion is sometimes termed the Llandeilo limestone; and in the upper portion volcanic rocks are intercalated.

Features

A remarkable feature in the history of the Llandeilo rocks in Britain, more especially in North Wales and

Cumberland Cumberland ( ) is a historic county in the far North West England. It covers part of the Lake District as well as the north Pennines and Solway Firth coast. Cumberland had an administrative function from the 12th century until 1974. From 19 ...

, 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 Westmorland (, formerly also spelt ''Westmoreland'';R. Wilkinson The British Isles, Sheet The British IslesVision of Britain/ref> is a historic county in North West England spanning the southern Lake District and the northern Dales. It had an ...

and

. The series is also found at

Builth Builth Wells (; cy, Llanfair-ym-Muallt) is a market town and community in the county of Powys and historic county of Brecknockshire (Breconshire), mid Wales, lying at the confluence of rivers Wye and Irfon, in the Welsh (or upper) part ...

and in

Pembrokeshire Pembrokeshire ( ; cy, Sir Benfro ) is a Local government in Wales#Principal areas, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and the rest by sea. The count ...

. 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.

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 ''Leptaena'' is an extinct genus of mid-sized brachiopod that existed from the Dariwilian epoch to the Emsian epoch, though some specimens have been found in strata as late in age as the Tournasian epoch. Like some other Strophomenids, ''Lept ...

'', ''

Lingula Lingula is Latin for "little tongue". It can stand for: * ''Lingula'' (brachiopod), a brachiopod genus of the family Lingulidae, which is among the few brachiopods surviving today but also known from fossils over 500 million years old * Lingala la ...

'', and '' Strophomena''. ''

Cardiola ''Cardiola'' is an extinct genus of saltwater clams, marine bivalve molluscs that lived from the Silurian to the Middle Devonian in Africa, Europe, and North America. References * ''Fossils'' (Smithsonian Handbooks) by David Ward (Page 94 ...

'' and '' Modiolopsis'' occur among the pelecypods; ''

Euomphalus ''Euomphalus'' is a genus of fossil marine gastropods known to have lived from the Silurian to the Middle Permian.J.B.Knight,et al 1960. Treatise on Invertebrate Paleontology, Part I, Mollusca 1, ch on systematic descriptions. Geol Soc of Amer ...

'', ''

Bellerophon Bellerophon (; Ancient Greek: Βελλεροφῶν) or Bellerophontes (), born as Hipponous, was a hero of Greek mythology. He was "the greatest hero and slayer of monsters, alongside Cadmus and Perseus, before the days of Heracles", and his ...

'', ''

Murchisonia ''Murchisonia'' was a genus of perennial herbs in the family Asparagaceae, subfamily Lomandroideae. There were two known species. In 2016, both were recognized as closely related to '' Thysanotus'' species, therefore ''Murchsonia'' was merged in ...

'' among the

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s; and '' Conularia'' and '' Hyolithes'' among the pteropods. The

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s are represented by '' Orthoceras'' and ''

Cyrtoceras ''Cyrtoceras'' is an extinct genus of oncoceridan nautiloids that lived from the middle Ordovician to the middle Devonian, in Africa, Europe, North America and South America South America is a continent entirely in the Western Hemisphe ...

''.

Quarrying

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

References

{{EB1911 , wstitle=Llandeilo Group , volume=16 , page=828 Lithostratigraphy of England Stratigraphy of the United Kingdom