Definitions

rhynchonella

Wenlock Group

Wenlock Group (Wenlockian), in geology, is 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: Much Wenlock Limestone Formation, 90-300 ft.; Wenlock shale, up to 1900 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 tuberculatus, Orthis calligramma, Atrypa reticularis, and 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 within 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 mi. 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 catenularis, Favosites aspera, Heliolites interstinctus), crinoids (Crotalocrinus, Marsupiocrinus, Periechocrinus), often very beautiful specimens, and trilobites (Calymene blumenbachii, the Dudley locust, Phacops caudatus, and Illaenus (Bumastus) barriensis. Brachiopods are abundant (Atrypa reticularis, Spirifer plicatilis, Rhynchonella cuneata, Orthis, Leptaena, Pentamerus); lamellibranchs include the genera Avicula, Cardiola, Grammysia; Murchisonia, Bellerophon, and Omphalotrochus are common gastropod genera. Common cephalopod genera include Orthoceras, Phragmoceras, and 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 seven 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: Monognaptus testis, Cyrtograptus linnarssoni, and Cyrtogra 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, Dumfriesshire, 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.

References

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