geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

, the Llandeilo Group is the middle subdivision of the British

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

rocks. It was first described and named by Sir Roderick Murchison from the neighborhood of

Llandeilo Llandeilo () is a town and community in Carmarthenshire, Wales, situated at the crossing of the River Towy by the A483 on a 19th-century stone bridge. Its population was 1,795 at the 2011 Census. It is adjacent to the westernmost point of the ...

Carmarthenshire Carmarthenshire ( cy, Sir Gaerfyrddin; or informally ') is a county in the south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. The county is known as ...

. In the type area it consists of a series of slaty rocks, shales, calcareous

flagstone Flagstone (flag) is a generic flat stone, sometimes cut in regular rectangular or square shape and usually used for paving slabs or walkways, patios, flooring, fences and roofing. It may be used for memorials, headstones, facades and other co ...

s and

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s; the calcareous middle portion is sometimes termed the Llandeilo limestone; and in the upper portion

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

rocks are intercalated.

Features

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

North Wales North Wales ( cy, Gogledd Cymru) is a region of Wales, encompassing its northernmost areas. It borders Mid Wales to the south, England to the east, and the Irish Sea to the north and west. The area is highly mountainous and rural, with Snowdonia N ...

and Cumberland, was the outbreak of volcanic action; vast piles of Llandeilo

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

and ashes form such hills as

Cadair Idris Cadair Idris or Cader Idris is a mountain in the Meirionnydd area of Gwynedd, Wales. It lies at the southern end of the Snowdonia National Park near the town of Dolgellau. The peak, which is one of the most popular in Wales for walkers and h ...

, and the Arenigs in

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

, and

Helvellyn Helvellyn (; possible meaning: ''pale yellow moorland'') is a mountain in the English Lake District, the highest point of the Helvellyn range, a north–south line of mountains to the north of Ambleside, between the lakes of Thirlmere and Ul ...

and

Scafell Scafell ( or ; also spelled Sca Fell, previously Scawfell) is a mountain in the English Lake District, part of the Southern Fells. Its height of makes it the second-highest mountain in England after its neighbour Scafell Pike, from which ...

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 Cumberland. 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 of ...

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 In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval. History The term was first used by Adam Sedgwick in 1847 with reference to the "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

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s, '' 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

trilobite Trilobites (; meaning "three lobes") are extinct marine 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 ...

s are found in these rocks, e.g. '' Ogygiocarella debuchii'', '' Asaphus tyrannus'', '' Calymene cambrensis'', and '' Cheirurus''. Among the

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s are '' Crania'', '' Leptaena'', '' Lingula'', and '' Strophomena''. '' Cardiola'' and '' Modiolopsis'' occur among the

pelecypod Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

s; '' Euomphalus'', '' Bellerophon'', '' Murchisonia'' among the gastropods; and '' Conularia'' and ''

Hyolithes Hyoliths are animals with small conical shells, known as fossils from the Palaeozoic era. They are at least considered as lophotrochozoan, and possibly being lophophorates, a group which includes the brachiopods, while others consider them as be ...

'' among the pteropods. The cephalopods are represented by ''

Orthoceras ''Orthoceras'' ("straight horn") is a genus of extinct nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This genus is sometimes called ''Orthoceratites''. Note it is sometimes misspelle ...

'' and '' Cyrtoceras''.

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