The Geology of the Prescelly Hills, North Pembrokeshire

Summary The Ordovician rocks of this district range from the Arenig to the Bala, with the succession generally ascending northwards. The rocks usually possess a Caledonoid strike and are disposed about two eastwardly pitching anticlines and a complementary syncline, which are dislocated by faults and overthrusts to the south-east. The Sealyham Volcanic Series is composed of rhyolites and tuffs in the western part of the area, but in the east the formation is almost entirely represented by trachytes. The Didymograptus bifidus beds occupy nearly the whole of the Prescelly Hills and they are mainly composed of shales and mudstones. The Fishguard Volcanic Series consists of fine-grained spherulitic and flow-brecciated rhyolites with thick beds of tuffs and local developments of ashy shales and mudstones. Sills of quartz-dolerites (among them the “spotted dolerite” which has been described as the analogue of the blue-stones of Stonehenge) and a single intrusion of a keratophyria rock occur in the Arenig and Llanvirn rocks but do not penetrate the shales, mudstones and sandstones of the Llandeilo and Bala formations. Petrographic descriptions of the igneous rocks are given, as well as a figure illustrating the stratigraphical correlation of the beds with those of other areas.