Wenlock Group
Encyclopedia
The Wenlock Group in geology
, is the middle series of strata
in the Silurian
(Upper Silurian) of Great Britain
. This group in the typical area in the Welsh
border counties contains the following formations: Much Wenlock
Limestone
Formation, 90–300 ft.; Wenlock Shale
, up to 1900 ft.; Woolhope or Barr Limestone and shale, 150 ft.
there is much shale at the base, and in places the limestone may be absent. These beds are best developed in Herefordshire
; they appear also at May Hill in Gloucestershire
and in Radnorshire
. Common fossil
s are Phacops
caudatus, Encrinurus tuberculatus, Orthis calligramma, Atrypa reticularis, and Orthoceras
annulatum.
in Shropshire
, through Radnorshire into Carmarthenshire
. They appear again southward in the Silurian patches in Gloucestershire, Herefordshire and Monmouthshire
. They thicken from the south northward. The fossils are on the whole closely similar to those in the limestones above with the natural difference that coral
s are comparatively rare in the shales, while graptolite
s are abundant. Six graptolite zones have been recognized by Miss G. L. Elles in this formation.
. It is typically developed at Wenlock Edge
, where it forms a striking feature for some 20 mi. It appears very well exposed in a sharp anticline
at Dudley
, whence it is sometimes called the Dudley Limestone; it occurs also at Aymestry, Ludlow
, Woolhope
, May Hill, Usk
and Malvern.
The fossils include corals in great variety (Halysites catenularis, Favosites
aspera, Heliolites interstinctus), crinoid
s (Crotalocrinus, Marsupiocrinus
, Periechocrinus), often very beautiful specimens, and trilobite
s (Calymene blumenbachii, the Dudley locust, Phacops caudatus and Illaenus (Bumastus) barriensis. Brachiopod
s are abundant (Atrypa reticularis, Spirifer plicatilis, Rhynchonella cuneata, Orthis, Leptaena, Pentamerus
). Lamellibranchs include the genera Avicula, Cardiola and Grammysia whilst Murchisonia, Bellerophon and Omphalotrochus are common gastropod genera. Common cephalopod
genera include Orthoceras, Phragmoceras and Trochoceras.
s from seven tons of the shale. Not only are there many different genera and species but individually certain forms are very numerous. The three principal zonal graptolites are, from above downwards: Monograptus testis, Cyrtograptus linnarssoni, and Cyrtogra murchisoni.
and Merionethshire
the rocks change their character and become more slaty or arenaceous; they are represented in this area by the Moel Ferna Slates, the Pen-y-glog Grit, and Pen-y-glog Slates, all of which belong to the lower part of a great series (3000 ft.) of slates and grits known as the Denbighshire Grits. Similar deposits occur on this horizon still farther north, in the Lake District
, where the Wenlock rocks are represented by the Brathay Flags (lower part of the Coniston Flags series), and in southern Scotland
, where their place is taken by the variable Riccarton Beds of Kirkcudbright
shore, Dumfriesshire
, Riccarton
and the Cheviots; by greywacke
s and shales in Lanarkshire
; by mudstones, shales and grits in the Pentland Hills
, and in the Girvan
area by the Blair and Straiton Beds. In Ireland
the Ferriters Cove Beds, a thick series of shales, slates and sandstones with lava
s and tuff
s in the Dingle Peninsula
; the Mweelrea Beds and others in County Tipperary
and County Mayo
are of Wenlock age.
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...
, is the middle series of strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
in the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
(Upper Silurian) of Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
. This group in the typical area in the Welsh
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...
border counties contains the following formations: Much Wenlock
Much Wenlock
Much Wenlock, earlier known as Wenlock, is a small town in central Shropshire, England. It is situated on the A458 road between Shrewsbury and Bridgnorth. Nearby, to the northeast, is the Ironbridge Gorge, and the new town of Telford...
Limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
Formation, 90–300 ft.; Wenlock 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...
, up to 1900 ft.; Woolhope or Barr Limestone and shale, 150 ft.
Woolhope Beds
The Woolhope Beds consist mainly of shales which are generally calcareous and pass frequently into irregular nodular and lenticular limestone. In the Malvern HillsMalvern Hills
The Malvern Hills are a range of hills in the English counties of Worcestershire, Herefordshire and a small area of northern Gloucestershire, dominating the surrounding countryside and the towns and villages of the district of Malvern...
there is much shale at the base, and in places the limestone may be absent. These beds are best developed in Herefordshire
Herefordshire
Herefordshire is a historic and ceremonial county in the West Midlands region of England. For Eurostat purposes it is a NUTS 3 region and is one of three counties that comprise the "Herefordshire, Worcestershire and Gloucestershire" NUTS 2 region. It also forms a unitary district known as the...
; they appear also at May Hill in Gloucestershire
Gloucestershire
Gloucestershire is a county in South West England. The county comprises part of the Cotswold Hills, part of the flat fertile valley of the River Severn, and the entire Forest of Dean....
and in Radnorshire
Radnorshire
Radnorshire is one of thirteen historic and former administrative counties of Wales. It is represented by the Radnorshire area of Powys, which according to the 2001 census, had a population of 24,805...
. Common fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are Phacops
Phacops
Phacops is a genus of trilobite in the order Phacopida, family Phacopidae that lived in Europe and North American in the Silurian and Devonian periods. It was a rounded animal, with a globosa head and large eyes, and probably fed on detritus...
caudatus, Encrinurus tuberculatus, Orthis calligramma, Atrypa reticularis, and 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 ....
annulatum.
Wenlock Shales
The Wenlock Shales are pale or dark-grey shales which extend through CoalbrookdaleCoalbrookdale
Coalbrookdale is a village in the Ironbridge Gorge in Shropshire, England, containing a settlement of great significance in the history of iron ore smelting. This is where iron ore was first smelted by Abraham Darby using easily mined "coking coal". The coal was drawn from drift mines in the sides...
in Shropshire
Shropshire
Shropshire is a county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. It borders Wales to the west...
, through Radnorshire into 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...
. They appear again southward in the Silurian patches in Gloucestershire, Herefordshire and Monmouthshire
Monmouthshire
Monmouthshire is a county in south east Wales. The name derives from the historic county of Monmouthshire which covered a much larger area. The largest town is Abergavenny. There are many castles in Monmouthshire .-Historic county:...
. They thicken from the south northward. The fossils are on the whole closely similar to those in the limestones above with the natural difference that coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s are comparatively rare in the shales, while graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
s are abundant. Six graptolite zones have been recognized by Miss G. L. Elles in this formation.
Wenlock Limestone
The Wenlock Limestone occurs either as a series of thin limestones within shales or as thick massive beds; it is sometimes hard and crystalline and sometimes soft, earthy or concretionaryConcretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
. It is typically developed at Wenlock Edge
Wenlock Edge
Wenlock Edge is a limestone escarpment near Much Wenlock, Shropshire, England. It is long and runs from South West to North East between Craven Arms and Much Wenlock. It is roughly 330 metres high...
, where it forms a striking feature for some 20 mi. It appears very well exposed in a sharp anticline
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
at Dudley
Dudley
Dudley is a large town in the West Midlands county of England. At the 2001 census , the Dudley Urban Sub Area had a population of 194,919, making it the 26th largest settlement in England, the second largest town in the United Kingdom behind Reading, and the largest settlement in the UK without...
, whence it is sometimes called the Dudley Limestone; it occurs also at Aymestry, Ludlow
Ludlow
Ludlow is a market town in Shropshire, England close to the Welsh border and in the Welsh Marches. It lies within a bend of the River Teme, on its eastern bank, forming an area of and centred on a small hill. Atop this hill is the site of Ludlow Castle and the market place...
, Woolhope
Woolhope
- Location :Woolhope is located about 7 miles east of Hereford.- History :The manor of Woolhope in Herefordshire, along with three others, was given to the cathedral at Hereford before the Norman Conquest by the benefactresses Wulviva and Godiva, local Anglo-Saxon landowners before the Norman...
, May Hill, Usk
Usk
Usk is a small town in Monmouthshire, Wales, situated 10 miles northeast of Newport.The River Usk flows through the town and is spanned by an ancient, arched stone bridge at the western entrance to the town. A castle above the town overlooks the ancient Anglo-Welsh border crossing - the river can...
and Malvern.
The fossils include corals in great variety (Halysites catenularis, Favosites
Favosites
Favosites is an extinct genus of tabulate coral characterized by polygonal closely packed corallites . The walls between corallites are pierced by pores which allowed transfer of nutrients between polyps....
aspera, Heliolites interstinctus), crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
s (Crotalocrinus, Marsupiocrinus
Marsupiocrinus
Marsupiocrinus is an extinct genus of crinoid that lived from the Silurian to the Early Devonian in North America.-External links:* in the Paleobiology Database...
, Periechocrinus), often very beautiful specimens, and 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 (Calymene blumenbachii, the Dudley locust, Phacops caudatus and Illaenus (Bumastus) barriensis. 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 abundant (Atrypa reticularis, Spirifer plicatilis, Rhynchonella cuneata, Orthis, Leptaena, Pentamerus
Pentamerus
Pentamerus is a prehistoric genus of brachiopod that lived from the Silurian to the Middle Devonian in Asia, Europe, and North America.-External links:* in the Paleobiology Database...
). Lamellibranchs include the genera Avicula, Cardiola and Grammysia whilst Murchisonia, Bellerophon and Omphalotrochus are common gastropod genera. Common cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
genera include Orthoceras, Phragmoceras and Trochoceras.
Silurian fauna
The greater part of the known Silurian fauna of Britain comes from Wenlock rocks; J. Davidson and G. Maw obtained no fewer than 25,000 specimens of brachiopodBrachiopod
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 from seven tons of the shale. Not only are there many different genera and species but individually certain forms are very numerous. The three principal zonal graptolites are, from above downwards: Monograptus testis, Cyrtograptus linnarssoni, and Cyrtogra murchisoni.
Other areas
When traced northward into DenbighshireDenbighshire
Denbighshire is a county in north-east Wales. It is named after the historic county of Denbighshire, but has substantially different borders. Denbighshire has the distinction of being the oldest inhabited part of Wales. Pontnewydd Palaeolithic site has remains of Neanderthals from 225,000 years...
and Merionethshire
Merionethshire
Merionethshire is one of thirteen historic counties of Wales, a vice county and a former administrative county.The administrative county of Merioneth, created under the Local Government Act 1888, was abolished under the Local Government Act 1972 on April 1, 1974...
the rocks change their character and become more slaty or arenaceous; they are represented in this area by the Moel Ferna Slates, the Pen-y-glog Grit, and Pen-y-glog Slates, all of which belong to the lower part of a great series (3000 ft.) of slates and grits known as the Denbighshire Grits. Similar deposits occur on this horizon still farther north, in the Lake District
Lake District
The Lake District, also commonly known as The Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous not only for its lakes and its mountains but also for its associations with the early 19th century poetry and writings of William Wordsworth...
, where the Wenlock rocks are represented by the Brathay Flags (lower part of the Coniston Flags series), and in southern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, where their place is taken by the variable Riccarton Beds of Kirkcudbright
Kirkcudbright
Kirkcudbright, is a town in Dumfries and Galloway, Scotland.The town lies south of Castle Douglas and Dalbeattie, in the part of Dumfries and Galloway known as the Stewartry, at the mouth of the River Dee, some six miles from the sea...
shore, Dumfriesshire
Dumfriesshire
Dumfriesshire or the County of Dumfries is a registration county of Scotland. The lieutenancy area of Dumfries has similar boundaries.Until 1975 it was a county. Its county town was Dumfries...
, Riccarton
Riccarton
The place-name of Riccarton may refer to:* in New Zealand** Riccarton, New Zealand, a suburb of Christchurch*** Riccarton , the electorate named after it*** The location of Riccarton Race Course...
and the Cheviots; by greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s and shales in Lanarkshire
Lanarkshire
Lanarkshire or the County of Lanark ) is a Lieutenancy area, registration county and former local government county in the central Lowlands of Scotland...
; by mudstones, shales and grits in the Pentland Hills
Pentland Hills
The Pentland Hills are a range of hills to the south-west of Edinburgh, Scotland. The range is around 20 miles in length, and runs south west from Edinburgh towards Biggar and the upper Clydesdale.Some of the peaks include:* Scald Law...
, and in the Girvan
Girvan
Girvan is a burgh in Carrick, South Ayrshire, Scotland, with a population of about 8000 people. Originally a fishing port, it is now also a seaside resort with beaches and cliffs. Girvan dates back to 1668 when is became a municipal burgh incorporated by by charter...
area by the Blair and Straiton Beds. In Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
the Ferriters Cove Beds, a thick series of shales, slates and sandstones with 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...
s and tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s in the Dingle Peninsula
Dingle Peninsula
The Dingle Peninsula is the northernmost of the major peninsulae in County Kerry. Its ends beyond the town of Dingle at Dunmore Head, the westernmost point of Ireland.-Name:...
; the Mweelrea Beds and others in County Tipperary
County Tipperary
County Tipperary is a county of Ireland. It is located in the province of Munster and is named after the town of Tipperary. The area of the county does not have a single local authority; local government is split between two authorities. In North Tipperary, part of the Mid-West Region, local...
and County Mayo
County Mayo
County Mayo is a county in Ireland. It is located in the West Region and is also part of the province of Connacht. It is named after the village of Mayo, which is now generally known as Mayo Abbey. Mayo County Council is the local authority for the county. The population of the county is 130,552...
are of Wenlock age.