Above: on the eastern coast of Anglesey, Old Red Sandstone crops out near to the popular beach at Traeth Lligwy. The characteristic red-brown colour is evident.
Photo: Stewart Campbell.
Part 04: Devonian to the Ice Age...
The Old Red Sandstone Continent
Part 3 ended with the collision of Avalonia with Laurentia and Baltica to form yet another new continent. Thus amalgamated, the large landmass drifted on northwards throughout the Devonian Period, with Anglesey finally crossing the Equator during the latter part of the Carboniferous Period. By Permian times, the rest of Gondwana had moved north and collided with the amalgamated Avalonia, Laurentia and Baltica; in addition, Siberia had joined Baltica in the collision that created the Ural Mountains. In this way the supercontinent of Pangaea came together. How did these events affect Anglesey?
During Devonian times, arid, terrestrial conditions prevailed. The sediments which accumulated vary from conglomerates consisting of Precambrian quartzite pebbles in a red muddy matrix, exposed at the base of Bodafon Mountain, to red mudstones containing yellow evaporite spots and patches (cornstones), typical of dried-out lake deposits, as seen on the coast at Lligwy Bay. Collectively they are referred to as the Old Red Sandstone, and the rather hostile environment they were deposited in was widely developed across the UK, stuck firmly within what has been termed the "Old Red Sandstone Continent".
The sea returns
However, the terrestrial conditions of the Devonian gave way to the sea once again in early Carboniferous times, when, in warm, tropical conditions and a shallow-sea environment, a thick sequence dominated by limestones was deposited. These beds were laid down in cycles as the sea transgressed and retreated as many as 11 times over North Wales. On Anglesey, only four of these cycles, which consist of sandy beds followed by limestone and then mudstone, were deposited on this, the northern margin of the Wales-Brabant Massif - the remains of Avalonia. The rocks are rich in fossilised brachiopods and corals and often exhibit palaeokarstic surfaces and rarer sandstone pipes.
Later Carboniferous strata are restricted in their outcrop to a linear NE-SW-trending belt which reaches the south-west coast at Malltraeth Bay, and a second smaller area around Abermenai Point. Westphalian coal-bearing strata were formerly mined for coal beneath Malltraeth Marsh, although they are of no economic interest today, the seams being very narrow and the ground waterlogged. Fossil remains of the giant horsetail, Calamites and tree-roots (Stigmaria), as well as insect parts and fish scales have been found in these deposits and their palaeoecology demonstrates that, during the time of formation, the environment was a tropical rain-forest situated close to the Equator, in a similar situation to the Amazon forests of today.
By Permian times, the formation of Pangaea was complete. It straddled the globe, extending towards both poles, with Anglesey lying just north of the equator. Large continental landmasses create climates with extreme variations of heat and cold and highly seasonal rainfall patterns, with desert conditions widespread. The closing of the Palaeozoic era, 250 million years ago at the end of the Permian Period, was characterised by inhospitable conditions, further complicated by a probable asterioud impact, which resulted in a tremendous and wide-ranging mass-extinction with over 70% of terrestrial and 90% of marine species disappearing.
Anglesey's journey northwards continued through the following Mesozoic era (consisting of the Triassic, Jurassic and Cretaceous Periods). During this time, evidence from elsewhere in the UK shows how conditions changed from the saline desert lakes of the Triassic to the warm tropical seas of the Jurassic and Cretaceous, which covered low-lying areas. Triassic strata outcrop on the sea-bed to the east of Anglesey, and also along the coast of the Strait. New Red Sandstone strata outcrop in the grounds of Plas Coch [all on private land] and this fine Tudor house is built of these rocks [hence its name].
but whether the sea ever covered the island again is uncertain, although it is now suspected that the Chalk sea of the Upper Cretaceous covered virtually the entire UK. However, there are no sedimentary rocks on Anglesey representing the time from the end of the Triassic Period through to the Miocene - a gap of over 200 million years.
The Atlantic opens
The breakup of Pangaea began about 170 million years ago, in Jurassic times, and in the following 100 million years, Laurentia separated from the amalgamated Baltica, Siberia and Africa (and the rest of the microcontinents that make up Western Europe), whilst the Gondwana split later in the Cretaceous Period and both North and South America drifted westwards. Incidentally, most of old Avalonia remained with Western Europe, excepting the area of the modern North American continent between New England and Newfoundland, which consists of Avalonian rocks. A new ocean - the Atlantic - was developing and widening, with much volcanic activity, including an intense episode of volcanism and associated igneous intrusions, focussed along the western seaboard of Scotland, early in the Cenozoic Era. By this time, Anglesey lay between 40 –50 degrees north of the equator.
The contribution of the igneous activity, 55 million years ago, to the bedrock geology of Anglesey was in the form of numerous basic dyke intrusions. The dykes are rich in iron-bearing silicate minerals, and are rather prone to deep spheroidal weathering, so that their outcrops often display the classic "onion-skin" texture. Mapping of the dykes, by aeromagnetic survey, has revealed a series of apparent offsets that have been interpreted as evidence that significant amounts of sinistral faulting have occurred in North Wales in the last 55 million years. However, because some of the dykes are exposed at outcrop, these apparent offsets can be examined in detail. The outcrops show that the intrusions ‘skipped’ from following one plane of weakness to another, and so the offsets appear not to be due to later faulting. The uncertainties require that the outcrops are preserved and subjected to future investigation.
The Cenozoic Era also brought into existence a mineral that is named after Anglesey. The lead sulphate, anglesite, was first described by Beudant in 1832, although the existence of the mineral, previously known as "vitriol de plomb", at Parys Mountain had been known for many years. It formed during the deep, prolonged subtropical weathering that the climate permitted at the time, during which the sulphide orebodies were deeply oxidised into a thick "gossan", much of which survived later erosional processes and was instead removed by the miners in their search for copper ore. The recent discovery of Miocene deposits in northern Anglesey could indicate that a substantial Miocene cover once existed, but was largely eroded following Plio-Pleistocene uplift. It seems likely that the Menaian surface (and the adjacent Snowdonian massif) were well-established landscape features before the end of the Miocene. The few areas of upstanding relief may represent inselbergs exhumed from a deep saprolitic cover.
Sudden catastrophic Climate Change
By late Cenozoic times, the UK was pretty much where it is now in relation to the Poles and the Equator. But there was one last sting in the tail. A mere 1.6 million years ago, Earth went from its stable warm climate into a series of violent climatic oscillations - the ice-ages and intervening warm periods of the Quaternary. Anglesey contains an exceptional range of glacial geomorphological features and sedimentary deposits. Although the gross morphology of Anglesey’s land surface is pre-glacial in origin, its landscape was modified considerably during the Pleistocene glaciations and the island is now dominated by an erosion surface cut across the older rocks.
The Quaternary has featured at least four intensely cold periods during which large parts of upland Britain were covered by ice sheets for long periods. Although Anglesey was probably overrun by ice on these occasions, only evidence from the last major glacial phase - the Late Devensian - is known. During that glaciation, around 20,000 years ago, Anglesey was completely submerged by ice. Two ice sheets from different sources were involved. The Snowdonian mountains were the source of ice streams that moved broadly northwards towards Anglesey, while the massive Irish Sea ice sheet, fed by glaciers from Scotland, Ireland and Cumbria, moved onto the island from the north. The Irish Sea ice stream was dominant, and travelled north-east to south-west across the island, broadly in keeping with its NE-SW-trending, structurally controlled rock ridges. The Welsh and Irish Sea ice streams met in the region of the present-day Menai Strait and produced a confluent south-westward flow.
Deposits from the Irish Sea Glacier tend to contain a wide range of rock types from its diverse source areas and from the varied geology of the seafloor traversed. A red colouration is common, being derived partly from Permian and Triassic rocks offshore such that it used to be known as the Red Northern Drift. The origin of these glacial sediments is shown by the granites from Ailsa Craig [ Firth of Forth], Shap [Lake District], sandstone & coal fragments [Lancashire] and basalt & flint [Antrim]. Deposits from the Welsh Ice sheet [very rare on Angelsey] reflect the geology of its source areas, with a high proportion of Cambrian slates and mudstones, varied Ordovician igneous materials and a blue-grey colouration. Their virtual absence shows the power of the Irish Sea Glacier which displaced them along the coast of the mainland.
And so to the present.....
Landscape development during the last 10,000 years (post-glacial or Holocene) saw the gradual re-colonisation by vegetation, the development of soils and progressive sea-level rise. Although the broad form of Anglesey’s coastline can be traced back to Permo-Triassic times, the soft-sediment features seen today are largely an inheritance of the Quaternary, particularly the present marine cycle which has cliffed glacial deposits and reworked them into estuarine mudflats, saltmarshes and sand dunes. Recent borehole studies in the Menai Strait have revealed one of the most comprehensive records of sea-level change yet known in Europe. Extensive basin mires with deep stratigraphic sequences complement the marine evidence.
Above: that part of the geological time-scale from the end of the Mesozoic (the "K-T Boundary"), 65 million years ago, up to the present day, consisting of one era - the Cenozoic - which is subdivided into two periods, and then into epochs, such as "Eocene". The current epoch, the Holocene, began only 0.01178 million years ago, so that it is too small to show up on the chart. The Quaternary consists of the Pleistocene and Holocene lumped together. Graphic: John Mason.
Above: thinly bedded greyish limestones of Lower Carboniferous age, cropping out on the coast near Penmon. Photo: Stewart Campbell.
Above: that part of the geological time-scale from the end of the Precambrian, 540 million years ago, up to the present day, consisting of one eon - the Phanerozoic, which is subdivided into three eras, such as "Mesozoic" and then into periods, such as "Jurassic". Graphic: John Mason.
Above: an early Cenozoic dyke cutting older rocks on the coast at Porth Namarch, near Holyhead. Basic in composition, with a lot of iron-bearing silicates, the dyke-rock shows the distinctive, spheroidal "onion-skin" weathering pattern.
Photo: Stewart Campbell.
Above: till - deposited by the Irish Sea ice-sheet, about 20,000 years ago - is now being reworked by the sea at Beaumaris - part of the seemingly endless cycle of sedimentary erosion and deposition.
Photo: Stewart Campbell.