In geology, the Llandovery Group refers to the lowest division of the Silurian period (Upper Silurian) in Britain. It is named after the town of Llandovery in Wales, although Charles Lapworth had proposed the name Valentian (from the Roman British province of Valentia) for this group in 1879. It includes 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 upon Ordovician rocks. These rocks occur with a narrow crop in Pembrokeshire, which curves round through Llandovery, and in the Rhayader district they reach a considerable thickness. They also occur in Ceredigion and Carmarthenshire.
The Upper Llandovery has 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 Long Mynd, which the conglomerate wraps round. As it is followed along the crop it rests upon the Lower Llandovery, Caradog, Llandeilo, Cambrian and pre-Cambrian rocks. The fossils include the trilobites Phacops caudata, Encrinurus punctatus and Calymene blumenbachis; the brachiopods Pentamerus oblongus, Orthis calligramma and Atrypa reticularis; the corals Favosites and Lindostroemia; and the zonal graptolites Rastriles maximus and Monograptus spinigerus.
The Tarannon shales, grey and blue slates, designated by Adam Sedgwick the Paste Rock, is traceable from Conwy into Carmarthenshire; in Ceredigion, there are gritty beds; 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 Rhayader Pale Shales in Powys; 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; 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 Meirionnydd and Denbighshire. In the Lake district the lower part of the Stockdale shales (Skelgill beds) is of Llandovery age. 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.
Economic deposits in Llandovery rocks include slate pencils (Teesdale), building stone, flag-stone, road metal and lime.