WENLOCK

See also:

• GROUP

GROUP (Wenlockian)  , in geology, the middle series of strata in the Silurian (Upper Silurian) of Great Britain . This group in the typical area in the Welsh border counties contains the following formations: Wenlock or Dudley lime-stone, 90-300 ft.; Wenlock shale, up to 190o ft.; Woolhope or Barr limestone and shale, 150 ft . The Woolhope beds consist mainly of shales which are generally calcareous and pass frequently into irregular nodular and lenticular limestone . In the Malvern Hills there is much shale at the base, and in places the limestone may be absent . These beds are best developed in Herefordshire; they appear also at May Hill in Gloucestershire and in Radnorshire . Common fossils are Phacops caudatus, Encrinurus punctatus, Orthis calligramma, Atrypa reticularis, Orthoceras annulatum . The Wenlock Shales are pale or dark-grey shales which extend through Coalbrookdale in Shropshire, through Radnorshire into Carmarthenshire . They appear again southward in the Silurian patches in Gloucestershire, Herefordshire and Monmouthshire . 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 corals are comparatively rare in the shales, while graptolites are abundant . Six graptolite zones have been recognized by Miss G . L .

Elles in this formation . The Wenlock limestone occurs either as a series of thin limestones with thin shales or as thick massive beds; it is sometimes hard and crystalline and sometimes soft, earthy or concretionary . It is typically developed in Wenlock Edge, where it forms a striking feature for some 20 m . It appears very well exposed in a

sharp anticline at Dudley, whence it is sometimes called the " Dudley limestone "; it occurs also at Aymestry, Ludlow, Woolhope, May Hill, Usk and Malvern . The fossils include corals in great variety (Halysites caienularis, Favosites aspera, Heliolites interstinctus), crinoids (Crotalocrinus, Marsupiocrinus, Periechocrinus), often very beautiful specimens, and trilobites (Calymene Blumenbachii, the " Dudley locust," Phacops caudatus, Illaenus (Bumbastes)barriensis, Homolonotus delphinocephalus) . Merostomatous crustaceans make their first appearance here(Eurypterus punctatus, Hemias pis horridus) . Brachiopods are abundant (Atrypa reticularis, Spirifer plicatilis, Rhynchonella cuneata, Orthis, Leptaena, Pentamerus) ; lamellibranchs include the genera Avicula, Cardiola, Grammysia; Murchisonia, Bellerophon, Omphalotrochus are common gasteropod genera . Conularia Sowerbyi is by no means rare, and there are several common cephalopod genera (Orthoceras, Phragmoceras, Trochoceras) . 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 brachiopods from 7 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 are, from above downwards: Monograptus testis, Cyrtograptus Linnarssoni, Cyrtograptus Murchisoni . When traced northward into Denbighshire and Merionethshire the rocks change their character and become more slaty or arenaceous; they are represented in this area by the " Moel Ferna Slates," the " Pen-y-glog Grit," and " Pen-y-glog Slates," all of which belong to the lower part of a great series (3000 ft.) of slates and grits known as the " Denbighshire Grits." Similar deposits occur on this horizon still farther north, in the Lake district, where the Wenlock rocks are represented by the " Brathay Flags " (lower part of the Coniston Flags series), and in southern Scotland, where their place is taken by the variable " Riccarton beds " of Kirkcudbright Shore, Dumfries-shire, Riccarton and the Cheviots; by greywackes and shales in Lanarkshire; by mudstones, shales and grits in the Pentland Hills, and in the Girvan area by the " Blair " and " Straiton beds." In Ireland the " Ferriters Cove beds," a thick series of shales, slates and sandstones with lavas and tuffs in the Dingle promontory; the" Mweelrea beds " and others in Tipperary and Mayo are of Wenlock age . Lime and flagstones are the most important economic products of the British Wenlock rocks . See the article SILURIAN, and for recent papers, Geological Literature, Geol . Soc., London, annual, and the Q.J . Geol . Soc., London . (J . A .