The Wenlock Group (Wenlockian), in
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 Ear ...
, is the middle series of
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
in the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
(Upper Silurian) of
Great Britain
Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It is ...
. This group in the typical area in the
Welsh
Welsh may refer to:
Related to Wales
* Welsh, referring or related to Wales
* Welsh language, a Brittonic Celtic language spoken in Wales
* Welsh people
People
* Welsh (surname)
* Sometimes used as a synonym for the ancient Britons (Celtic peop ...
border counties contains the following formations:
Much Wenlock Limestone Formation
The Much Wenlock Limestone Formation is a series of Silurian limestone beds that date back to the Homerian age, the later part of the Wenlock epoch.{{cite web , url=http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=WEL , title=Much Wenlock Limeston ...
, 90–300 ft.; Wenlock Shale, up to 1900 ft.; Woolhope or Barr Limestone and shale, 150 ft.
Stratigraphy
Woolhope Beds
The Woolhope Beds consist mainly of
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 which are generally calcareous and pass frequently into irregular nodular and lenticular limestone. In the
Malvern Hills
The Malvern Hills are 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. The highest summit affo ...
there is much shale at the base, and in places the limestone may be absent. These beds are best developed in
Herefordshire
Herefordshire () is a county in the West Midlands of England, governed by Herefordshire Council. It is bordered by Shropshire to the north, Worcestershire to the east, Gloucestershire to the south-east, and the Welsh counties of Monmouthshire ...
; they appear also at May Hill in
Gloucestershire
Gloucestershire ( abbreviated Glos) 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.
The county town is the city of Gl ...
and in
Radnorshire
, HQ = Presteigne
, Government = Radnorshire County Council (1889–1974) Radnorshire District Council (1974–1996)
, Origin =
, Status = historic county, administrative county
, Start ...
. Common
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s are ''
Phacops
''Phacops'' is a genus of trilobites in the order Phacopida, family Phacopidae, that lived in Europe, northwestern Africa, North and South America and China from the Late Ordovician until the very end of the Devonian, with a broader time range d ...
caudatus'', ''
Encrinurus
''Encrinurus'' is a long-lived genus of phacopid trilobites that lived in what are now Africa, Asia, Australia, Europe, North America, and South America from the middle Ordovician to the early Devonian from 472 to 412.3 mya, existing for appr ...
tuberculatus'', ''
Orthis calligramma'', ''
Atrypa
''Atrypa'' is a genus of brachiopod with shells round to short egg-shaped, covered with many fine radial ridges (or costae), that split further out and growth lines perpendicular to the costae and 2-3 times wider spaced. The pedunculate valve is ...
reticularis'', and ''
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 ...
annulatum''.
Wenlock Shales
The Wenlock Shales are pale or dark-grey shales which extend through
Coalbrookdale
Coalbrookdale is a village in the Ironbridge Gorge in Shropshire, England, containing a settlement of great significance in the history of iron ore smelting. It lies within the civil parish called the Gorge.
This is where iron ore was first s ...
in
Shropshire
Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to th ...
, through Radnorshire into
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 ...
. They appear again southward in the Silurian patches in Gloucestershire, Herefordshire and
Monmouthshire
Monmouthshire ( cy, Sir Fynwy) is a county in the south-east of Wales. The name derives from the historic county of the same name; the modern county covers the eastern three-fifths of the historic county. The largest town is Abergavenny, with ...
. 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
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s are comparatively rare in the shales, while
graptolite
Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and ...
s are abundant. Six graptolite zones were recognized by
Gertrude Elles
Gertrude Lilian Elles MBE (8 October 1872 – 18 November 1960) was a British geologist, known for her work on graptolites.
Personal life
Gertrude Elles was born on October 8, 1872. She was the youngest of six children. Every year the E ...
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
concretion
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
ary. It is typically developed at
Wenlock Edge
Wenlock Edge is a limestone escarpment near Much Wenlock, Shropshire, England and a site of special scientific interest because of its geology. It is over long, running southwest to northeast between Craven Arms and Much Wenlock, and is roughl ...
, where it forms a striking feature for some 20 mi. It appears very well exposed in a sharp
anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
at
Dudley
Dudley is a large market town and administrative centre in the county of West Midlands, England, southeast of Wolverhampton and northwest of Birmingham. Historically an exclave of Worcestershire, the town is the administrative centre of the ...
, whence it is sometimes called the Dudley Limestone; it occurs also at
Aymestrey,
Ludlow
Ludlow () is a market town in Shropshire, England. The town is significant in the history of the Welsh Marches and in relation to Wales. It is located south of Shrewsbury and north of Hereford, on the A49 road which bypasses the town. The t ...
,
Woolhope
Woolhope is a village and civil parish in the English county of Herefordshire. The population of the civil parish was 486 at the 2011 census. The civil parish includes the hamlet of Broadmoor Common to the west of the village (and a nature rese ...
, May Hill,
Usk
Usk ( cy, Brynbuga) is a town and community in Monmouthshire, Wales, northeast of Newport. It is located on the River Usk, which is spanned by an arched stone bridge at the western entrance to the town. Usk Castle, above the town, overlooks th ...
and Malvern.
The fossils include corals in great variety (''
Halysites
''Halysites'' (meaning ''chain coral'') is an extinct genus of tabulate coral. Colonies range from less than one to tens of centimeters in diameter, and they fed upon plankton.
These tabulate corals lived from the Ordovician to the Devonian (f ...
catenularis'', ''
Favosites
''Favosites'' is an extinct genus of tabulate coral characterized by polygonal closely packed corallites (giving it the common name "honeycomb coral"). The walls between corallites are pierced by pores known as mural pores which allowed transfer ...
aspera'', ''
Heliolites
''Heliolites'' is a large and heterogenous genus of extinct tabulate corals in the family Heliolitidae. Specimens have been found in Ordovician to Devonian beds in North America, Europe, Africa, Asia, and Australia. The genus is particularly ab ...
interstinctus''),
crinoid
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...
s (''
Crotalocrinus'', ''
Marsupiocrinus'', ''
Periechocrinus''), often very beautiful specimens, and
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 At ...
s (''
Calymene
''Calymene'' Brongniart, 1822, is a genus of trilobites in the order Phacopida, suborder Calymenina, that are found throughout North America, North Africa, and Europe in primarily Silurian outcrops. ''Calymene'' is closely related to ''Flexicaly ...
blumenbachii'', the Dudley locust, ''
Phacops
''Phacops'' is a genus of trilobites in the order Phacopida, family Phacopidae, that lived in Europe, northwestern Africa, North and South America and China from the Late Ordovician until the very end of the Devonian, with a broader time range d ...
caudatus'' and ''
Illaenus
''Illaenus'' is a genus of trilobites from Russia and Morocco, from the middle 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 ...
(
Bumastus) barriensis'').
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 abundant (''
Atrypa
''Atrypa'' is a genus of brachiopod with shells round to short egg-shaped, covered with many fine radial ridges (or costae), that split further out and growth lines perpendicular to the costae and 2-3 times wider spaced. The pedunculate valve is ...
reticularis'', ''
Spirifer plicatilis'', ''
Rhynchonella
''Rhynchonella'' is an extinct genus of brachiopod found in Ordovician to Eocene strata worldwide. It was a stationary epifaunal suspension feeder.
Description
These 1.75 to 3.75 cm long articulate brachiopods are characterized by a tria ...
cuneata'', ''
Orthis'', ''
Leptaena'', ''
Pentamerus
:''The gall mite genus ''Pentamerus'', established by Roivainen in 1951, is invalid and needs to be renamed. Until this happens, use '' Pentamerus (mite)''.''
''Pentamerus'' (meaning "five thighs") is a prehistoric genus of brachiopods that live ...
'').
Lamellibranch
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...
s include the genera ''
Avicula'', ''
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 ''
Grammysia'' whilst ''
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 ...
'', ''
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 ...
'' and ''
Omphalotrochus'' are common gastropod genera. Common
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 ...
genera include ''
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 ...
'', ''
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
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 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
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 the L ...
are, from above downwards: ''
Monograptus
''Monograptus'' is a genus of graptolites in the Order Graptoloidea. This particular genus is the last stage of the graptoloid evolution before its extinction in the early Devonian. A characteristic of the genus includes one uniserial stipes ...
testis'', ''
Cyrtograptus linnarssoni'', and ''
Cyrtogra murchisoni''.
Other areas
When traced northward into
Denbighshire
Denbighshire ( ; cy, Sir Ddinbych; ) is a county in the north-east of Wales. Its borders differ from the historic county of the same name. This part of Wales contains the country's oldest known evidence of habitation – Pontnewydd (Bontnewy ...
and
Merionethshire
, HQ= Dolgellau
, Government= Merionethshire County Council (1889-1974)
, Origin=
, Status=
, Start= 1284
, End=
, Code= MER
, CodeName= ...
, the Wenlock-age rocks change their character and become more slaty or
arenaceous
Arenite ( Latin: ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix. The related adjective is ''arenaceous''. The equivalen ...
. They are represented in this area by the
Moel Ferna Slates, the
Pen-y-glog Grit, and
Pen-y-glog Slates. All of those layers belong to the lower part of the
Denbighshire Grits, a great series of slates and grits thick.
Similar deposits of similar age occur on this horizon still farther north. In the
Lake District
The Lake District, also known as the Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous for its lakes, forests, and mountains (or ''fells''), and its associations with William Wordswor ...
, the Wenlock rocks are represented by the
Brathay Flags, the lower part of the
Coniston Flags
Coniston may refer to:
Australia
*Coniston (Northern Territory), a cattle station
**Coniston massacre, 1928
*Coniston, New South Wales
**Coniston railway station, New South Wales
*Coniston, Tasmania, a town in the Derwent Valley Council, Derwent ...
series. In southern
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
, their place is taken by the variable
Riccarton Beds of
Kirkcudbright
Kirkcudbright ( ; sco, Kirkcoubrie; gd, Cille Chùithbeirt) is a town, parish and a Royal Burgh from 1455 in Kirkcudbrightshire, of which it is traditionally the county town, within Dumfries and Galloway, Scotland.
The town lies southwest of C ...
shore,
Dumfriesshire
Dumfriesshire or the County of Dumfries or Shire of Dumfries (''Siorrachd Dhùn Phris'' in Gaelic) is a historic county and registration county in southern Scotland. The Dumfries lieutenancy area covers a similar area to the historic county.
I ...
,
Riccarton Riccarton may refer to:
New Zealand
* Riccarton, New Zealand, a suburb of Christchurch
** Riccarton (New Zealand electorate), the electorate named after it
** The location of Riccarton Race Course
* a locality on the Taieri Plains in Otago
Scotlan ...
, and the
Cheviots
The Cheviot Hills (), or sometimes The Cheviots, are a range of uplands straddling the Anglo-Scottish border between Northumberland and the Scottish Borders. The English section is within the Northumberland National Park. The range includes Th ...
. In
Lanarkshire
Lanarkshire, also called the County of Lanark ( gd, Siorrachd Lannraig; sco, Lanrikshire), is a historic county, lieutenancy area and registration county in the central Lowlands of Scotland.
Lanarkshire is the most populous county in Scotlan ...
, they are represented by sandstone in the form of
greywacke
Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) 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 lit ...
s and by shales. In the
Pentland Hills
The Pentland Hills are a range of hills southwest of Edinburgh, Scotland. The range is around in length, and runs southwest from Edinburgh towards Biggar and the upper Clydesdale.
Etymology
The name is first recorded for the farm of Pentlan ...
, one finds mudstones, shales, and grits. In the
Girvan
Girvan ( gd, Inbhir Gharbhain, "mouth of the River Girvan") is a burgh and harbour town in Carrick, South Ayrshire, Scotland. Girvan is situated on the east coast of the Firth of Clyde, with a population of about 6,450. It lies south of Ayr, ...
area are the Blair and Straiton Beds.
In
Ireland
Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
, there is more rock of Wenlock age. In the
Dingle Peninsula
The Dingle Peninsula ( ga, Corca Dhuibhne; anglicised as Corkaguiny, the name of the corresponding barony) is the northernmost of the major peninsulas in County Kerry. It ends beyond the town of Dingle at Dunmore Head, the westernmost point o ...
one finds the
Ferriters Cove Beds, a thick series of shales, slates, and sandstones with
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 ...
s and
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
s. In
County Tipperary
County Tipperary ( ga, Contae Thiobraid Árann) is a county in Ireland. It is in the province of Munster and the Southern Region. The county is named after the town of Tipperary, and was established in the early 13th century, shortly after th ...
and
County Mayo
County Mayo (; ga, Contae Mhaigh Eo, meaning "Plain of the Taxus baccata, yew trees") is a Counties of Ireland, county in Republic of Ireland, Ireland. In the West Region, Ireland, West of Ireland, in the Provinces of Ireland, province of Conn ...
are the
Mweelrea Beds and others.
Economic geology
Lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
and flagstones are the most important economic products of the British Wenlock rocks.
References
{{Reflist
Lithostratigraphy of England
Geologic formations of the United Kingdom
Lagerstätten