In geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, the Llandeilo Group is the middle subdivision of the British Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

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

Llandeilo

Llandeilo is a town in Carmarthenshire, Wales, situated at the crossing of the River Towy by the A483 on a 19th century stone bridge. Its population is 1,731.The town is served by Llandeilo railway station on the Heart of Wales Line.- Early history :...

 in Carmarthenshire

Carmarthenshire

Carmarthenshire is a unitary authority in the south west of Wales and one of thirteen historic counties. It is the 3rd largest in Wales. Its three largest towns are Llanelli, Carmarthen and Ammanford...

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

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

s, calcareous flagstone

Flagstone

Flagstone, is a generic flat stone, usually used for paving slabs or walkways, patios, fences and roofing. It may be used for memorials, headstones, facades and other constructions. The name derives from Middle English flagge meaning turf, perhaps from Old Norse flaga meaning slab.Flagstone is a...

s and sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s; 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 Liandeilo rocks in Britain, more especially in North Wales

North Wales

North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...

 and Cumberland

Cumberland

Cumberland is a historic county of North West England, on the border with Scotland, from the 12th century until 1974. It formed an administrative county from 1889 to 1974 and now forms part of Cumbria....

, was the outbreak of volcanic action; vast piles of Llandeilo lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 and ashes form such hills as Cadair Idris

Cadair Idris

Cadair Idris or Cader Idris is a mountain in Gwynedd, Wales that lies at the southern end of the Snowdonia National Park. The peak, which is one of the most popular in Wales for walkers and hikers,  is composed largely of Ordovician igneous rocks, with classic glacial erosion features such as...

, and the Arenigs

Arenigs

Arenigs is an informal term for a group of mountains in central Snowdonia, in north Wales. They are not strictly defined, but normally include at least the following peaks:*Arenig Fawr*Moel Llyfnant*Rhobell Fawr*Arenig Fach*Dduallt*Carnedd y Filiast...

 in Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

, and Helvellyn

Helvellyn

Helvellyn is a mountain in the English Lake District, the apex of the Eastern Fells. At above sea level, it is the third highest peak in both the Lake District and England...

 and Scafell in Westmorland

Westmorland

Westmorland is an area of North West England and one of the 39 historic counties of England. It formed an administrative county from 1889 to 1974, after which the entirety of the county was absorbed into the new county of Cumbria.-Early history:...

 and Cumberland

Cumberland

Cumberland is a historic county of North West England, on the border with Scotland, from the 12th century until 1974. It formed an administrative county from 1889 to 1974 and now forms part of Cumbria....

. The series is also found at Builth and in Pembrokeshire

Pembrokeshire

Pembrokeshire is a county in the south west of Wales. It borders Carmarthenshire to the east and Ceredigion to the north east. The county town is Haverfordwest where Pembrokeshire County Council is headquartered....

. 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

Arenig

In geology, the Arenigian refers both to a time interval during the Lower Ordovician period and also to the suite of rocks which were deposited during this interval.-History:...

) 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 and Extracts

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 trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. 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...

s are found in these rocks, e.g. Ogygia buchi (AKA Ogygiacarella debuchii), Asaphus tyrannus, Calymene cambrensis, Cheirurus sedgwick. Among the brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s are Crania

Crania (brachiopod)

Crania is a fossil genus of brachiopods found during the Carboniferous.-See also:* List of brachiopod genera* Taxonomy of commonly fossilised invertebrates* List of extant animal genera represented in the fossil record-References:*...

, Leptaena, Lingula, Strophomena; Cardiola

Cardiola

Cardiola is an extinct genus of bivalve mollusk that lived from the Silurian to the Middle Devonian in Africa, Europe, and North America.-External links:* in the Paleobiology Database...

 and Modiolopsis occur among the Pelecypods; Euomphalus

Euomphalus

Euomphalus is a genus of fossil marine gastropods known to have lived from the Silurian to the Middle Permian. Euomphalus is characterized by a closely coiled shell with a depressed to slightly elevated spire and a channel-bearing angulation on the upper surface of the whorls...

, Bellerophon

Bellerophon

Bellerophon or Bellerophontes is a hero of Greek mythology. He was "the greatest hero and slayer of monsters, alongside of Cadmus and Perseus, before the days of Heracles", and his greatest feat was killing the Chimera, a monster that Homer depicted with a lion's head, a goat's body, and a...

, Murchisonia

Murchisonia

Murchisonia is a genus of perennial herbs in the family family Asparagaceae, subfamily Lomandroideae.There are two species:*Murchisonias fragrans Brittan, endemic to Western Australia...

 among the Gasteropods; Conularia and Hyolithes among the Pteropods; the Cephalopods are represented by Orthoceras

Orthoceras

Orthoceras is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros ....

 and Cyrtoceras

Cyrtoceras

Cyrtoceras is an extinct genus of oncoceridan nautiloid that lived from the middle Ordovician to the middle Devonian, in Africa, Europe, North America, and South America.-Sources:* Fossils by David Ward...

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