Acritarchs are organic microfossils, known from approximately 1800 million years ago to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.
John William Salter was an English naturalist, geologist, and palaeontologist.
First appearance datum (FAD) is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest fossil discovered, to date, of a particular species. A related term is last appearance datum (LAD), the last appearance of a species in the geologic record.
The Caledonian orogeny was a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the Laurentia and Baltica continents and the Avalonia microcontinent collided.
The Dalradian Supergroup is a stratigraphic unit in the lithostratigraphy of the Grampian Highlands of Scotland and in the north and west of Ireland. The diverse assemblage of rocks which constitute the supergroup extend across Scotland from Islay in the west to Fraserburgh in the east and are confined by the Great Glen Fault to the northwest and the Highland Boundary Fault to the southeast. Much of Shetland east of the Walls Boundary Fault is also formed from Dalradian rocks. Dalradian rocks extend across the north of Ireland from County Antrim in the north east to Clifden on the Atlantic coast, although obscured by younger Palaeogene lavas and tuffs or Carboniferous rocks in large sections.
The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~450 million years ago, and exposed in northwest England, including the Pennines and correlates along its strike, in the Isle of Man and Ireland, and down-dip in the Southern Uplands and Welsh Borderlands. It underlies much of north England's younger cover, extending south to East Anglia. It formed as a foreland basin, in a similar setting to the modern Ganges basin, fronting the continent of Avalonia as the remains of the attached Iapetus ocean subducted under Laurentia.
Ediacaria is a fossil genus dating to the Ediacaran Period of the Neoproterozoic Era. Unlike most Ediacaran biota, which disappeared almost entirely from the fossil record at the end of the Period, Ediacaria fossils have been found dating from the Baikalian age of the Upper Riphean to 501 million years ago, well into the Cambrian Period. Ediacaria consists of concentric rough circles, radial lines between the circles and a central dome, with a diameter from 1 to 70 cm.
The Ordovician Martinsburg Formation (Om) is a mapped bedrock unit in Pennsylvania, New Jersey, Maryland, Virginia, and West Virginia. It is named for the town of Martinsburg, West Virginia for which it was first described. It is the dominant rock formation of the Great Appalachian Valley in New Jersey and Pennsylvania.
The Ceibwr Bay Fault is a WSW-ENE trending fault zone that cuts Ordovician rocks of the Ashgill Nantmel Mudstones Formation and the Caradoc Dinas Island Formation. The fault is exposed on the south side of Cardigan Bay in Wales and forms part of the Fishguard-Cardigan Fault Zone. It extends from the coast at Ceibwr Bay at its western end to the coast at Aberporth at its eastern end. The fault zone is thought to have been active as a normal fault throughout the deposition of the Ordovician sequence.
The Welsh Basin was a northeast-southwest aligned back-arc depositional basin during the Cambrian, Ordovician and Silurian periods during which a considerable thickness of marine sediments was laid down in the area. To the southeast lay the Midland Platform and to the northwest, within the Iapetus Ocean, through what is now Ireland and the Lake District, was an island arc; a northeast-southwest aligned Irish Sea landmass which was associated with volcanic activity. Examination of the sediments and associated fossils allows the deeper centre of the basin to be distinguished from shallower 'platform' areas along its southeastern margins. From the middle Silurian onwards, collision of Avalonia with the more northerly continent of Laurentia occurred giving rise to the Caledonian Orogeny. The inversion of the basin occurred at that time i.e. its uplift and deformation.
Pleuroctenium Hawle & Corda (1847) is an agnostid trilobite belonging to the family Condylopygidae Raymond (1913). The genus occurs in Middle Cambrian (Drumian) strata of Canada, the Czech Republic, England and Wales, France, and Sweden.
The Hells Mouth Grits, formally defined and renamed the Hells Mouth Formation by Young et al., is a geological formation composed of Cambrian Greywackes in the south west part of St. Tudwal's Peninsula. Equivalent to the Rhinog Formation in the Harlech Dome, the grit beds exposed at St Tudwal's are very uniform in lithology and thickness when traced along the outcrops, with a gradual thinning southwards. They exhibit the characteristic textures and structures of greywackes but differ from the normal type in being relatively well sorted and commonly laminated. Intercalated mudstones are more variable both in thickness and in lithology and contain laminated mudstones rich in sponge remains. The sandstones have sharply defined bases, often bearing sole structures and occasionally loaded. Sandstone dykes cut down from the bases of some beds and extend through up to 0.6 m of underlying siltstones. The sandstones may form sheets up to 4 m thick, although a bed thickness of up to 1m is more usual, and have been interpreted as turbidites deposited by currents from the northeast.
Bailiaspis Resser, 1936, is a Middle Cambrian (Miaolingian) trilobite genus belonging to the Family Conocoryphidae Angelin, 1854. Within the Acado-Baltic region, the genus ranges from Wuliuan into Guzhangian age strata.
The geology of the Isle of Man consists primarily of a thick pile of sedimentary rocks dating from the Ordovician period, together with smaller areas of later sedimentary and extrusive igneous strata. The older strata was folded and faulted during the Caledonian and Acadian orogenies The bedrock is overlain by a range of glacial and post-glacial deposits. Igneous intrusions in the form of dykes and plutons are common, some associated with mineralisation which spawned a minor metal mining industry.
Ganderia or Gander Terrane is a terrane in the northern Appalachians which broke off the supercontinent Gondwana c.570 million years ago (Ma) together with Avalonia, Megumia, and Carolinia.
The Durness Group is a geological group, a carbonate-dominated stratigraphic unit that forms a c. 170 km long narrow and discontinuous outcrop belt along the north-western coast of Scotland from the Isle of Skye and Loch Kishorn in the south to Durness and Loch Eriboll in the north. It forms the youngest part of the foreland basin of the Moine Thrust Belt in the Scottish Northwest Highlands and is incorporated into this belt's lowermost thrust sheets, where it is often affected by thrust faulting. It overlies the Ardvreck Group.
The Ross orogeny was a mountain building event in Antarctica in the early Paleozoic. The ancestral Trans-Antarctic Mountains were uplifted earlier by the Beardmore orogeny but had eroded as a broad epicratonic sea flooded much of Antarctica in the Cambrian. Shallow water sedimentary rocks, platform carbonates and deepwater turbidites from this period are found in the mountain range. The Ross orogeny was one of the most extensive orogenic events in Antarctica, causing widespread plutonism and metamorphism. Bimodal magmatism and extension mark the beginnings of the orogeny, while during the later phase sedimentary rocks at the continental margin were deformed, metamorphosed and intruded with granite batholiths. Interpretations of rock forms in Antarctica during the 1980s suggested a westward-dipping subduction zone may have formed along the paleo-Pacific Ocean shoreline of East Antarctica. This is inferred from a large number of I-type and S-type granitoids which are similar to large circum-Pacific batholiths.
The geology of Snowdonia National Park in North Wales is dominated by sedimentary and volcanic rocks from the Cambrian and Ordovician periods with intrusions of Ordovician and Silurian age. There are Silurian and Cenozoic sedimentary rocks on the park's margins. The succession was intensely faulted and folded during the Caledonian Orogeny. The region was uplifted as the North Atlantic Ocean opened during the Cenozoic. The current mountainous landscape arises from repeated glaciations during the Quaternary period.
The geology of Anglesey, the largest (714 km2) island in Wales is some of the most complex in the country. Anglesey has relatively low relief, the 'grain' of which runs northeast–southwest, i.e. ridge and valley features extend in that direction reflecting not only the trend of the late Precambrian and Palaeozoic age bedrock geology but also the direction in which glacial ice traversed and scoured the island during the last ice age. It was realised in the 1980s that the island is composed of multiple terranes, recognition of which is key to understanding its Precambrian and lower Palaeozoic evolution. The interpretation of the island's geological complexity has been debated amongst geologists for decades and recent research continues in that vein.
The geology of Pembrokeshire in Wales inevitably includes the geology of the Pembrokeshire Coast National Park which extends around the larger part of the county's coastline and where the majority of rock outcrops are to be seen. Pembrokeshire's bedrock geology is largely formed from a sequence of sedimentary and igneous rocks originating during the late Precambrian and the Palaeozoic era, namely the Ediacaran, Cambrian, Ordovician, Silurian, Devonian and Carboniferous periods, i.e. between 635 and 299 Ma. The older rocks in the north of the county display patterns of faulting and folding associated with the Caledonian Orogeny. On the other hand, the late Palaeozoic rocks to the south owe their fold patterns and deformation to the later Variscan Orogeny.
Ireland 
