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Encyclopedia Britannica - Main :: LEO-LOB |
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LLANDOVERY GROUP , in geology, the lowest division of the Silurian
Silurian
Lapworth
The Lower Llandovery rocks consist of conglomerates, sandstones and slaty beds. Ai Llandovery they rest unconformably upon Ordovician rocks ( Bala
district
Bala
There is a change in the fauna on leaving the Ordovician and entering the Llandovery. Among the graptolites the Diplograptidae begin to be replaced by the Monograptidae. Characteristic graptolite zones, in descending order, are:Monograptus gregarius, Diplograptusvesiculosus, D. acuminatus. Common trilobites are:Acidaspis, Encrinurus, Phacops, Proetus; among the brachiopods are Orthis elegantula, O. tesludinaria, Meristella crassa and Pentamerus (Stricklandinia) 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, Gloucestershire, is an arenaceous series generally conglomeratic at the base, with local lenticular developments of shelly 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 conglomerate wraps round. As it is followed along the crop it is found to rest unconformably upon the Lower Llandovery, Caradoc, Llandeilo, Cambrian and pre-Cambrian rocks. The fossils include the trilobites Phacops caudata, Encrinurus punctatus, Calymene Blumenbachis; the brachiopods Pentamerus oblongus, Orthis calligramma, Airy pa reticularis; the corals Fawn sites, Lindostroemia, &c.; and the zonal graptolites Rastrites maximus and Monograptus spinigerus and others (Monograptus Sedgwicki, M. Clingani, M. proteus, Diplograptus Hughesii).The Tarannon shales, grey and blue slates, designated by A. Sedgwick the " paste rock," is traceable from Conway into Carmarthenshire; in Cardiganshire, besides the slaty facies, gritty beds make their appearance; and in the neighbourhood of Builth
Pale Shales in Radnorshire; by the Browgilt beds, with Monograptus crispus and M. turriculatus, in the Lake district
Strangford
The Upper and Lower Llandovery rocks are represented in descending order by the Pale shales, Graptolite shales, Grey slates and 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) lie on this horizon; at Builth
series of grits and shales. In the Lake district the lower part of the Stockdale shales (Skelgill beds) is of Llandovery age. In south Scotland in the central and southern belt of Silurian rocks, which extends across the country from Luce Bay to St Abb's Head, the Birkhill shales, a highly crumpled series of graptolitic beds, represent the Llandovery horizon. In the Girvan area to the north their place is taken by the Camregan, Shaugh Hill and Mullock Hill groups. In Ireland the Llandovery rocks are represented by the Anascaul slates of the Dingle promontory, by the Owenduff and Gowlaun grits, Co. Galway, by the Upper Pomeroy
Economic deposits in Llandovery rocks include slate pencils (Teesdale), building stone, flag-stone, road metal and lime. Lead ore occurs in Wales. (See SILURIAN.) (J. A. H.) End of Article: LLANDOVERY GROUP If you wish, you can link directly to this article.
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