Devonian to the Tertiary…

The Devonian Period (416 – 359 Ma)

The Devonian geological period covers the time span 416 – 359 Ma. Named after Devon where early studies of the rocks were carried out, the period has often been referred to as the ‘Age of Fishes’. The term Devonian is also often synonymous with ‘Old Red Sandstone’, although the latter also includes some late Silurian and early Carboniferous rocks. Sedgwick and Murchison established the Devonian System in 1839, building on the earlier pioneering work of Conybeare and Phillips (1822). The sedimentary rocks of the Old Red Sandstone were derived by the erosion of Laurussia (also known as the ‘Old Red Sandstone’ continent) that had been formed during the Caledonian Orogeny when Avalonia, Laurentia and Baltica were amalgamated and the Iapetus Ocean had disappeared. The Old Red Sandstone rocks of Wales were deposited in the Anglo-Welsh Basin and accumulated under warm to hot, semi-arid conditions when Wales lay at sub-tropical latitudes of about 17ºS. Despite being dominantly non-marine rocks, they locally preserve important traces of the earliest land animals and plants, although their fossils have not been found on Anglesey.

Anglesey’s Old Red Sandstone strata (which are believed to comprise rocks of both latest Silurian and early Devonian age) form only a relatively small outcrop. Nonetheless, the distinctive red strata constitute the only Devonian rocks in North Wales and they contain unique evidence in the Anglo-Welsh Basin for the existence of perennial lakes. A sedimentary structure called epsilon cross-bedding was also first described from the coastal sections here, which also show well-developed, carbonate-rich fossil soils (palaeosols) or ‘calcretes’. These rocks also contain important evidence for continuing plate tectonic processes in the form of folds and faults formed during the Acadian Orogeny. 

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. 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”.

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.  Photos : John Conway

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.  Photos : John Conway


The Carboniferous Period (359 – 299 Ma)

The Carboniferous period covers the time span 359 – 299 Ma. Literally meaning coal-bearing, the term ‘Carboniferous’ was first coined by Conybeare & Phillips (1822) to reflect substantial coal deposits that occur in the younger rocks of the Period; the older rocks being dominated by thick limestones. By the end of Carboniferous times, the supercontinent Pangaea had been amalgamated, as the southern continents that were tied together in Gondwanaland fused with the Laurussian (North America and Europe) continent – during a mountain-building period known as the Variscan (Hercynian or Armorican) Orogeny.

At the start of the Period, Anglesey lay in the southern tropics where thick limestones formed in warm shallow seas. Marine life, including abundant corals and brachiopods, flourished and their remains are often well preserved in the limestones. As Anglesey continued its journey northwards towards the equator, sea levels fell and the warm tropical seas were replaced by vast sandy deltas where sandstones and conglomerates of the famous Millstone Grit formed. These sandy deltas were eventually choked by huge coastal swamps and dense forests, which over time were turned into coal.

Anglesey has a substantial area of Carboniferous rocks; highly fossiliferous limestones outcrop all long the east coast southwards from Lligwy to Penmon [and along the coast of North Wales from the Great Orme to Prestatyn]. These rocks 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  limestone followed by sandy beds and then mudstone were deposited. The rocks are rich in brachiopods and corals and often exhibit palaeokarstic surfaces and rarer sandstone pipes.

Colin Jago showing a group of people the geology at Red Wharf Bay

various outcrops from Lligwy round to Pennon; quarries at Penmon showing  the extent of use of limestone as a building stone.  Two views of the typical brachiopod, gigantoproductus.  Photos : John Conway

Amazonian Anglesey

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.

Delta sandstones and conglomerates of the Millstone Grit (Namurian) are found on the north coast of the Malltraeth estuary. Fossil remains of the giant horsetail, Calamites and tree-roots (Stigmaria).

Less well known, perhaps, is that a thriving coal industry they was once based on the coal measures (Westphalian) that underlie Malltraeth Marsh, although they are of no economic interest today, the seams being very narrow and the ground waterlogged.

map of coal mines

Map of coal mines, Malltraeth Marsh

Plant fossils, insect parts and fish scales have been found in these rocks 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.

Outcrops of carboniferous sandstone and gritstone occur between Moefre and Benllech on the east coast, and at Malltraeth on the west coast.  An example of the stigmaria casts that can be found.  Photos : John Conway.

The Carboniferous period saw one of the planets 5 great glaciations which affected sea levels world wide. Watch GeoMon Climate Change presentation [PPT file 14Mb] written by our chairman, Professor Colin Jago, covering both Carboniferous and Quaternary glaciations. ( Climate Change Presentation in Welsh)

Mesozoic rocks….

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 asteroid 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 [giving the dominant reddish colour to the Quaternary glacial deposits], and also along the Anglesey 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].

various views of the Triassic strata to be seen along the Strait.

Missing Strata….

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 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.


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: 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.


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