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LLANDOVERY GROUP

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Originally appearing in Volume V16, Page 829 of the 1911 Encyclopedia Britannica.
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LLANDOVERY See also:GROUP , in See also:geology, the lowest See also:division of the See also:Silurian (Upper Silurian)in See also:Britain. C. See also:Lapworth in 1879 proposed the name Valentian (from the See also:ancient See also:north See also:British See also:province of See also:Valentia) for this group. It includes in the type See also:area the Tarannon Shales 1000-1500 ft., Upper Llandovery and May See also:Hill See also:Sandstone 80o ft., See also:Lower Llandovery, 600-15oo ft. The Lower Llandovery rocks consist of conglomerates, sandstones and slaty beds. Ai Llandovery they See also:rest unconformably upon Ordovician rocks (See also:Bala), but in many other places no unconformity is traceable. These rocks occur with a narrow See also:crop in See also:Pembrokeshire, which curves See also:round through Llandovery, and in the Rhyader See also:district they attain a considerable thickness. Northwards they thin out towards Bala See also:Lake. They occur also in See also:Cardiganshire and See also:Carmarthenshire in many places where they have not been clearly separated from the associated Ordovician rocks. There is a See also:change in the See also:fauna on leaving the Ordovician and entering the Llandovery. Among the See also:graptolites the Diplograptidae begin to be replaced by the Monograptidae. Characteristic graptolite zones, in descending See also:order, are:—Monograptus gregarius, Diplograptusvesiculosus, D. acuminatus.

See also:

Common See also:trilobites are:—Acidaspis, Encrinurus, Phacops, Proetus; among the brachiopods are Orthis elegantula, O. tesludinaria, Meristella crassa and Pentamerus (Stricklandinia) See also:lens (Pentamerus is so characteristic that the Llandovery rocks are frequently described as the " Pentamerus beds"). The Upper Llandovery, including the May Hill Sandstone of May Hill, See also:Gloucestershire, is an arenaceous See also:series generally conglomeratic at the See also:base, with See also:local lenticular developments of shelly See also:limestone (Norbury, Hollies and Pentanerus limestones). It occurs with a narrow outcrop in Carmarthenshire at the base of the Silurian, disappearing beneath the Old Red Sandstone westward to reappear in Pembrokeshire; north-eastward the outcrop extends to the Longmynd, which the See also:conglomerate wraps round. As it is followed along the crop it is found to rest unconformably upon the Lower Llandovery, Caradoc, Llandeilo, See also:Cambrian and pre-Cambrian rocks. The fossils include the trilobites Phacops caudata, Encrinurus punctatus, Calymene Blumenbachis; the brachiopods Pentamerus oblongus, Orthis calligramma, See also:Airy pa reticularis; the See also:corals Fawn sites, Lindostroemia, &c.; and the zonal graptolites Rastrites See also:maximus and Monograptus spinigerus and others (Monograptus Sedgwicki, M. Clingani, M. See also:proteus, Diplograptus Hughesii). The Tarannon shales, See also:grey and See also:blue slates, designated by A. See also:Sedgwick the " See also:paste See also:rock," is traceable from See also:Conway into Carmarthenshire; in Cardiganshire, besides the slaty facies, gritty beds make their See also:appearance; and in the neighbourhood of See also:Builth soft dark shales. The group is poor in fossils with the exception of graptolites; of these Cyrtogra plus grayae and Monograptus exiguus are zonal forms. The Tarannon group is represented by the Rhyader See also:Pale Shales in See also:Radnorshire; by the Browgilt beds, with Monograptus crispus and M. turriculatus, in the Lake district; in the See also:Moffat Silurian See also:belt in See also:south See also:Scotland by a thick development, including the See also:Hawick rocks and Ardwell beds, and the See also:Queensberry group or Gala (Grieston shales, Buckholm grits and See also:Abbotsford flags); in the See also:Girvan area, by the Drumyork flags, Bargany group and Penkill group; and in See also:Ireland by the Treveshilly shales of See also:Strangford Lough, and the shales of Salterstown, Co. See also:Louth. The Upper and Lower Llandovery rocks are represented in descending order by the Pale shales, Graptolite shales, Grey slates and See also:Corwen grit of Merionethshire and Denbighshire.

In the Rhyader district the Caban group (Gafalt beds, shales and grits and Caban conglomerate), and the Gwastaden group (Gigrin mudstones, Ddolshales, Dyffryn flags, Cerig Gwynion grits) See also:

lie on this See also:horizon; at Builth also there is a series of grits and shales. In the Lake district the lower See also:part of the Stockdale shales (Skelgill beds) is of Llandovery See also:age. In south Scotland in the central and See also:southern belt of Silurian rocks, which extends across the See also:country from Luce See also:Bay to St Abb's See also:Head, the Birkhill shales, a highly crumpled series of graptolitic beds, represent the Llandovery horizon. In the Girvan area to the north their See also:place is taken by the Camregan, Shaugh Hill and Mullock Hill See also:groups. In Ireland the Llandovery rocks are represented by the Anascaul slates of the See also:Dingle promontory, by the Owenduff and Gowlaun grits, Co. See also:Galway, by the Upper See also:Pomeroy beds, by the Uggool and Ballaghaderin beds, Co. See also:Mayo, and by rocks of this age in Coalpit Bay and Slieve Felim Mountains. Economic deposits in Llandovery rocks include See also:slate pencils (Teesdale), See also:building See also:stone, See also:flag-stone, road See also:metal and See also:lime. See also:Lead ore occurs in See also:Wales. (See SILURIAN.) (J. A.

End of Article: LLANDOVERY GROUP

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