In geology, Caradoc Series is the name introduced by
Roderick Murchison in 1839 for the sandstone series of
Caer Caradoc in
Shropshire
Shropshire (; abbreviated SalopAlso used officially as the name of the county from 1974–1980. The demonym for inhabitants of the county "Salopian" derives from this name.) is a Ceremonial counties of England, ceremonial county in the West M ...
,
England
England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
. It is the fifth of the six subdivisions (in ascending order) of the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
System, comprising all those rocks deposited worldwide during the Caradocian Age (458 to 448 million years ago).
The limits of Murchison's Caradoc series have since been somewhat modified, and through the labours of
Charles Lapworth
Charles Lapworth Fellow of the Royal Society, FRS Fellow of the Geological Society, FGS (20 September 1842 – 13 March 1920) was a headteacher and an English geologist who pioneered faunal analysis using index fossils and identified the Ordov ...
the several members of the series have been precisely defined by means of
graptolitic zones. These zones are identical with those found in the rocks of the same age in North Wales, the
Bala Series, and the terms Bala or Caradoc series are used interchangeably by geologists when referring to the uppermost substage of the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
System.
The Ordovician rocks of the Caradoc district have been subdivided into the following beds, in descending order: the Trinucleus shales, Acton Scott beds, Longville flags, Chatwell and Soudley
sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
s, Harnage
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
s and Hoar Edge
grits
Grits (stylized as GRITS) is an American Christian hip hop group from Nashville, Tennessee. Their name is an acronym, which stands for "Grammatical Revolution In the Spirit". GRITS is made up of Stacey "Coffee" Jones and Teron "Bonafide" Carter ...
and
limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
. In the Corndon district in the same county the Caradoc series is represented by the Marrington group of ashes and shales and the Spy Wood group beneath them; these two groups of strata are sometimes spoken of as the Chirbury series. In the Breidden district are the barren Criggeon shales with ashes and flows of andesite.
In the Lake district the Coniston limestone series represents the Upper Caradocian, the lower portion being taken up by part of the great Borrowdale volcanic series of rocks. The Coniston limestone series contains the following subdivisions Ashgill group (Ashgill shales and Staurocephalus limestone) -
*Kiesley limestone.
*Sleddale group (Applethwaite beds) Upper Coniston limestone
conglomerate; Yarlside
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...
; stye end beds=Lower Coniston limestone.
*Roman Fell group (Corona beds).
*The Dufton shales and Drygill shales are equivalents of the Sleddale group.
Rocks of Caradoc age are well developed in southern Scotland; in the
Girvan
Girvan (, "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, and north of St ...
district they have been described as the Ardmillan series with the Drummock group and Barren Flagstone group in the upper portion, and the Whitehouse, Ardwell and Balclatchie groups in the lower part. Similarly, two divisions, known as the Upper and Lower Hartfell series, are recognized in the southern and central area, in
Peeblesshire
Peeblesshire (), the County of Peebles or Tweeddale is a Counties of Scotland, historic county of Scotland. Its county town is Peebles, and it borders Midlothian to the north, Selkirkshire to the east, Dumfriesshire to the south, and Lanarkshire ...
,
Ayrshire
Ayrshire (, ) is a Counties of Scotland, historic county and registration county, in south-west Scotland, located on the shores of the Firth of Clyde. The lieutenancy areas of Scotland, lieutenancy area of Ayrshire and Arran covers the entirety ...
and
Dumfriesshire
Dumfriesshire or the County of Dumfries or Shire of Dumfries () is a Counties of Scotland, historic county and registration county in southern Scotland. The Dumfries lieutenancy areas of Scotland, lieutenancy area covers a similar area to the hi ...
.
In Ireland the Caradoc or Bala series is represented by the limestones of
Portraine near Dublin and of the Chair of Kildare; by the Ballymoney series of
Wexford
Wexford ( ; archaic Yola dialect, Yola: ''Weiseforthe'') is the county town of County Wexford, Republic of Ireland, Ireland. Wexford lies on the south side of Wexford Harbour, the estuary of the River Slaney near the southeastern corner of the ...
and Carnalea shales of
County Down
County Down () is one of the six counties of Northern Ireland, one of the nine counties of Ulster and one of the traditional thirty-two counties of Ireland. It covers an area of and has a population of 552,261. It borders County Antrim to the ...
. In the Lough Mask district beds of this age are found, as in Wales, interstratified with volcanic lavas and tuffs. Other localities are known in counties
Tyrone,
Meath and
Louth, also in
Lambay Island.
Early descriptions of the Bala series
The Bala Series was described as a group of dark
slate
Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...
s and
sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
s with
beds of
limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
which occurs in the neighborhood of
Bala, Gwynedd, in north
Wales
Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
.
It was first described by
Adam Sedgwick, who considered it to be the upper part of his
Cambrian
The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
System. The Bala limestone is from 20 to thick, and is recognizable over most of North Wales. The series in the type area consists of the
Hirnant limestone, a thin inconstant bed, which is separated by . of slates from the Bala limestone, below this are more slates and volcanic rocks. The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick.
In South Wales the Bala Series contains the following beds in descending order: the Trinucleus seticornis beds (Slade beds, Redhill shales and Sholeshook limestone), the Robeston Wathen beds, and the Dicranograplus shales. The typical
graptolites are, in the upper part, ''Dicellograptus anceps'' and ''D. complanatus''; in the lower part, ''Pleurograptus linearis'' and ''Dicranograptus clingani''.
References
{{reflist
Bibliography
*Nat. Hist., 5th series, vol. vi., 1880; Geol. Mag., 1889; C. Lapworth and W. W. Watts, Proc. Geol. Assoc., xiii., 1894; J. E. Marr, Geol.
Meg., 1892; J. B. Marr and T. Roberts, Q. J. G. S., 1885; B. N.
*Peach and J. Home, Silurian Rocks of Great Britain, vol. I., 1899
Geologic formations of the United Kingdom
Stratigraphy of the United Kingdom