Ludlow Group
Encyclopedia
The Ludlow Group are rocks deposited during the Ludlow period of the Silurian
in Great Britain
. This group contains the following formations in descending order:
The Ludlow group is essentially shaly
in character, except towards the top, where the beds become more sand
y and pass gradually into the Old Red Sandstone. The Aymestry limestone
, which is irregular in thickness, is sometimes absent, and where the underlying Wenlock limestones are absent the shales of the Ludlow group graduate, downwards into the Wenlock shales. The group is typically developed between Ludlow
and Aymestry, and it occurs also in the detached Silurian areas between Dudley
and the mouth of the Severn.
The Lower Ludlow rocks are mainly grey, greenish and brown mudstone
s and sandy and calcareous
shales. They contain an abundance of fossil
s. The series has been zoned by means of the Graptolites by E. M. R. Wood; the following in ascending order, are the zonal forms:
Cyathaspis ludensis, the earliest British vertebrate
fossil, was found in these rocks at Leintwardine
in Herefordshire
, a noted fossil locality. Trilobites are numerous (Phacops caudatus, Lichas anglicus, Homolonotus delphinocephalus, Calymene Blumenbachii); brachiopod
s (Leptaena rhomboidalis, Rhynchonella Wilsoni, Atrypa reticularis}, pelecypods (Cardiola interrupts, Ctenodonta sulcata) and gasteropods and cephalopod
s (many species of Orthoceras and also Gomphoceras, Trochoceras) are well represented. Other fossils are Ceratiocaris
, Pterygotus, Protaster, Palaeocoma and Palaeodiscus.
The Upper Ludlow rocks are mainly soft mudstones and shales with some harder sandy beds capable of being worked as building-stones. These sandy beds are often found covered with ripple-marks and annelid
tracks; one of the uppermost sandy layers is known as the " Fucoid bed " from the abundance of the seaweed-like impressions it bears. At the top of this sub-group, near Ludlow, a brown layer occurs, from a quarter of an inch to 4 in. (63 mm to 100 mm) in thickness, full of the fragmentary remains of fish associated with those of Pterygotus
and mollusca
. This layer, known as the " Ludlow Bone bed," has been traced over a very large area (see Bone Bed
). The common fossils include plants (Actinophyllum, Chondrites), ostracods, phyllocarids, eurypterids, trilobites (less common than in the older groups), numerous brachiopods (Lingula minima, Chonetes striatella), gasteropods, pelecypods and cephalopods (Orthoceras bullatum). Fish include Cephalaspis, Cyathaspis, Auchenaspis. The Tilestones, Downton Castle Sandstone and Ledbury shales are occasionally grouped together under the term Downtonian. They are in reality passage beds between the Silurian and Old Red Sandstone, and were originally placed in the latter system by Sir R. I. Murchison
. They are mostly grey, yellow or red micaceous, shaly sandstones. Lingula cornea, Platyschisma helicites and numerous phyllocarids and ostracods occur among the fossils.
In Denbighshire
and Merionethshire
the upper portion of the Denbighshire Grits belongs to this horizon: viz. those from below upwards, the Nantglyn Flags, the Upper Grit beds, the Monograptus leintwardinensis beds and the Dinas Bran beds. In the Silurian area of the Lake district the Coldwell beds, forming the upper part of the Coniston Flags, are the equivalents of the Lower Ludlow; they are succeeded by the Coniston Grits (4,000 ft./1,230 m), the Bannisdale Slates (5200 ft./1,600 m) and the Kirkby Moor Flags (2,000 ft./615 m).
In the Silurian areas of southern Scotland
, the Ludlow rocks are represented in the Kirkcudbright Shore and Riccarton district by the Raeberry Castle beds and Balmae Grits (500-750 ft.). In the northern belt Lanarkshire and the Pentland Hillsthe lower portion (or Ludlovian) consists of mudstones, flaggy shales and greywackes; but the upper (or Downtonian) part is made up principally of thick red and yellow sandstones and conglomerates with green mudstones. The Ludlow rocks of Ireland include the " Salrock beds " of County Galway
and the "Croagmarhin beds" of Dingle promontory.
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...
in 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 contains the following formations in descending order:
- Cilestones, Downton Castle sandstones (90 ft./27.7 m),
- Ledbury shales 270 ft./83 m),
- Upper Ludlow rocks (140 ft./43 m),
- Aymestry limestone (up to 40 ft./12.3 m),
- Lower Ludlow rocks (350 to 780 ft./108 m-240 m).
The Ludlow group is essentially shaly
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...
in character, except towards the top, where the beds become more sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
y and pass gradually into the Old Red Sandstone. The Aymestry 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....
, which is irregular in thickness, is sometimes absent, and where the underlying Wenlock limestones are absent the shales of the Ludlow group graduate, downwards into the Wenlock shales. The group is typically developed between 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...
and Aymestry, and it occurs also in the detached Silurian areas between 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...
and the mouth of the Severn.
The Lower Ludlow rocks are mainly grey, greenish and brown mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s and sandy and calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
shales. They contain an abundance of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s. The series has been zoned by means of the Graptolites by E. M. R. Wood; the following in ascending order, are the zonal forms:
- Monograptus vulgaris,
- M. Nilssoni,
- M. scanicus,
- M. tumescens and
- M. leintwardinensis.
Cyathaspis ludensis, the earliest British vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
fossil, was found in these rocks at Leintwardine
Leintwardine
Leintwardine is a large village and civil parish in north Herefordshire, England, close to the border with Shropshire.- Roman Leintwardine :...
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...
, a noted fossil locality. Trilobites are numerous (Phacops caudatus, Lichas anglicus, Homolonotus delphinocephalus, Calymene Blumenbachii); 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 (Leptaena rhomboidalis, Rhynchonella Wilsoni, Atrypa reticularis}, pelecypods (Cardiola interrupts, Ctenodonta sulcata) and gasteropods and 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...
s (many species of Orthoceras and also Gomphoceras, Trochoceras) are well represented. Other fossils are Ceratiocaris
, Pterygotus, Protaster, Palaeocoma and Palaeodiscus.
The Upper Ludlow rocks are mainly soft mudstones and shales with some harder sandy beds capable of being worked as building-stones. These sandy beds are often found covered with ripple-marks and annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
tracks; one of the uppermost sandy layers is known as the " Fucoid bed " from the abundance of the seaweed-like impressions it bears. At the top of this sub-group, near Ludlow, a brown layer occurs, from a quarter of an inch to 4 in. (63 mm to 100 mm) in thickness, full of the fragmentary remains of fish associated with those of Pterygotus
Pterygotus
Pterygotus is the second-largest known eurypterid, or sea scorpion and one of the largest arthropods of all time.-Description:...
and mollusca
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
. This layer, known as the " Ludlow Bone bed," has been traced over a very large area (see Bone Bed
Bone bed
A bone bed is any geological stratum or deposit that contains bones of whatever kind. Inevitably, such deposits are sedimentary in nature. Not a formal term, it tends to be used more to describe especially dense collections...
). The common fossils include plants (Actinophyllum, Chondrites), ostracods, phyllocarids, eurypterids, trilobites (less common than in the older groups), numerous brachiopods (Lingula minima, Chonetes striatella), gasteropods, pelecypods and cephalopods (Orthoceras bullatum). Fish include Cephalaspis, Cyathaspis, Auchenaspis. The Tilestones, Downton Castle Sandstone and Ledbury shales are occasionally grouped together under the term Downtonian. They are in reality passage beds between the Silurian and Old Red Sandstone, and were originally placed in the latter system by Sir R. I. Murchison
Roderick Murchison
Sir Roderick Impey Murchison, 1st Baronet KCB DCL FRS FRSE FLS PRGS PBA MRIA was a Scottish geologist who first described and investigated the Silurian system.-Early life and work:...
. They are mostly grey, yellow or red micaceous, shaly sandstones. Lingula cornea, Platyschisma helicites and numerous phyllocarids and ostracods occur among the fossils.
In Denbighshire
Denbighshire
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 upper portion of the Denbighshire Grits belongs to this horizon: viz. those from below upwards, the Nantglyn Flags, the Upper Grit beds, the Monograptus leintwardinensis beds and the Dinas Bran beds. In the Silurian area of the Lake district the Coldwell beds, forming the upper part of the Coniston Flags, are the equivalents of the Lower Ludlow; they are succeeded by the Coniston Grits (4,000 ft./1,230 m), the Bannisdale Slates (5200 ft./1,600 m) and the Kirkby Moor Flags (2,000 ft./615 m).
In the Silurian areas of 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...
, the Ludlow rocks are represented in the Kirkcudbright Shore and Riccarton district by the Raeberry Castle beds and Balmae Grits (500-750 ft.). In the northern belt Lanarkshire and the Pentland Hillsthe lower portion (or Ludlovian) consists of mudstones, flaggy shales and greywackes; but the upper (or Downtonian) part is made up principally of thick red and yellow sandstones and conglomerates with green mudstones. The Ludlow rocks of Ireland include the " Salrock beds " of County Galway
Galway
Galway or City of Galway is a city in County Galway, Republic of Ireland. It is the sixth largest and the fastest-growing city in Ireland. It is also the third largest city within the Republic and the only city in the Province of Connacht. Located on the west coast of Ireland, it sits on the...
and the "Croagmarhin beds" of Dingle promontory.