1911 Encyclopædia Britannica/Llandovery Group
LLANDOVERY GROUP, in geology, the lowest division of the Silurian (Upper Silurian) in Britain. C. Lapworth in 1879 proposed the name Valentian (from the ancient north British province of Valentia) for this group. It includes in the type area the Tarannon Shales 1000–1500 ft., Upper Llandovery and May Hill Sandstone 800 ft., Lower Llandovery, 600–1500 ft.
The Lower Llandovery rocks consist of conglomerates, sandstones and slaty beds. At Llandovery they rest unconformably upon Ordovician rocks (Bala), but in many other places no unconformity is traceable. These rocks occur with a narrow crop in Pembrokeshire, which curves round through Llandovery, and in the Rhyader district they attain a considerable thickness. Northwards they thin out towards Bala Lake. They occur also in Cardiganshire and Carmarthenshire in many places where they have not been clearly separated from the associated Ordovician rocks.
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, Proëtus; among the brachiopods are Orthis elegantula, O. testudinaria, 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 Pentamerus 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 Blumenbachii; the brachiopods Pentamerus oblongus, Orthis calligramma, Atrypa reticularis; the corals Favosites, 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 soft dark shales. The group is poor in fossils with the exception of graptolites; of these Cyrtograptus grayae and Monograptus exiguus are zonal forms. The Tarannon group is represented by the Rhyader Pale Shales in Radnorshire; by the Browgill beds, with Monograptus crispus and M. turriculatus, in the Lake district; in the Moffat Silurian belt in south Scotland by a thick development, including the Hawick rocks and Ardwell beds, and the Queensberry group or Gala (Grieston shales, Buckholm grits and Abbotsford flags); in the Girvan area, by the Drumyork flags, Bargany group and Penkill group; and in Ireland by the Treveshilly shales of Strangford Lough, and the shales of Salterstown, Co. Louth.
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, Ddol shales, Dyffryn flags, Cerig Gwynion grits) lie on this horizon; at Builth also there is a 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 beds, by the Uggool and Ballaghaderin beds, Co. Mayo, and by rocks of this age in Coalpit Bay and Slieve Felim Mountains.