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LUDLOW GROUP, or LUDLOVIAN

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Originally appearing in Volume V17, Page 113 of the 1911 Encyclopedia Britannica.
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LUDLOW See also:GROUP, or LUDLOVIAN , in See also:geology, the uppermost subdivision of the See also:Silurian rocks in See also:Great See also:Britain. This group contains the following formations in descending See also:order: Tilestones, Downton See also:Castle sandstones (90 ft.), See also:Ledbury shales (270 ft.), Upper Ludlow rocks (140 ft.), Aymestry See also:limestone (up to 40 ft.), See also:Lower Ludlow rocks (350 to 780 ft.). The Ludlow group is essentially shaly in See also:character, except towards the See also:top, where the beds become more sandy and pass gradually into the See also:base of the Old Red See also:Sandstone. The Aymestry limestone, which is irregular in thickness, is sometimes absent, and where the underlying See also:Wenlock limestones are absent the shales of the Ludlow group See also:graduate, downwards into the Wenlock shales. The group is typically See also:developed between Ludlow and Aymestry, and it occurs also in the detached Silurian areas between See also:Dudley and the mouth of the See also:Severn. The Lower Ludlow rocks are mainly See also:grey, greenish and See also:brown mudstones and sandy and calcareous shales. They contain an abundance of fossils. The See also:series has been zoned by means of the See also:graptolites by E. M. R. See also: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 See also:

British vertebrate fossil, was found in these rocks at Leintwardine in See also:Shropshire, a noted fossil locality. See also:Trilobites are numerous (Phacops caudatus, Lichas anglicus, Homolonotus delphinocephalus, Calymene Blumenbachii) ; brachiopods (Leptaena rhomboidalis, Rhynchonella Wilsoni, Atrypa reticularis), pelecypods (Cardiola interrupts, Ctenodonta sulcata), and gasteropods and cephalopods (many See also: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 See also: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 See also: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 See also:quarter of an See also:inch to 4 in. in thickness, full of the fragmentary remains of See also:fish associated with those of Pterygotus and See also:mollusca. This layer, known as the " Ludlow See also:Bone bed," has been traced over a very large See also:area (see BONE BED). The See also:common fossils include See also:plants (Actinophyllum, Chondrites), ostracods, phyllocarids, eurypterids, trilobites (less common than in the older See also: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 See also:term Downtonian. They are in reality passage beds between the Silurian and Old Red Sandstone, and were originally placed in the latter See also:system by See also:Sir R. I.

See also:

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 See also:horizon: viz. those from below upwards, the Nantglyn Flags, the Upper Grit beds, the Monograptus leintwardinensis beds and the Dinas See also:Bran beds. In the Silurian area of the See also:Lake See also:district the Coldwell beds, forming the upper See also:part of the Coniston Flags, are the equivalents of the Lower Ludlow; they are succeeded by the Coniston Grits (4000 ft.), the Bannisdale Slates (5200 ft.) and the See also:Kirkby See also:Moor Flags (2000 ft.). In the Silurian areas of See also:southern See also:Scotland, the Ludlow rocks are represented in the See also:Kirkcudbright See also:Shore and Riccarton district by the Raeberry Castle beds and Balmae Grits (500-750 ft.). In the See also:northern See also:belt—See also:Lanarkshire and the Pentland Hills—the 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 See also:green mudstones. The Ludlow rocks of See also:Ireland include the " Salrock beds " of See also:County See also:Galway and the " Croagmarhin beds " of See also:Dingle promontory. See SILURIAN, and, for See also:recent papers, the Q.J. Geol. See also:Soc. (See also:London) and See also:Geological Literature (Geol.

Soc., London) See also:

annual.

End of Article: LUDLOW GROUP, or LUDLOVIAN

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