Ludlow Group
3625156
191425825
2008-02-14T14:55:11Z
Smith609
846901
The '''Ludlow Group''' are rocks deposited during the Ludlow period of the [[Silurian]] in [[Great Britain]].{{Verify source|date=February 2008}} 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 [[shale|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:
#''[[Monograptus vulgaris]]'',
#''[[M. Nilssoni]]'',
#''[[M. scanicus]]'',
#''[[M. tumescens]]'' and
#''[[M. leintwardinensis]]''.
''[[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''}, [[pelecypod]]s (''Cardiola interrupts'', ''Ctenodonta sulcata'') and [[gasteropod]]s 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.
==References==
*{{1911}}
[[Category:Geology of England]]
[[Category:Geological groups]]