Monday 2nd May 2022
Landscape Evolution of south-west Wales
At the last meeting (20th April 2022) Prof. Peter Kokelaar gave a very interesting and informative talk on how the landscape of south-west Wales changed and evolved over time from the Carboniferous to the present day.
Although based on his work on the Gower peninsula the area covered by the talk comprised a larger area covering the south Wales coalfield, the Brecon Beacons and over to Carmarthenshire across the Southern Caledonian fold belt.
The stratigraphy of the area is dominated by Carboniferous rocks ranging from Carboniferous limestones through the Coal Measures to Pennant sandstones at the top of the sequence. It is underlain by Devonian quartz pebble conglomerates. These rocks were folded in early Permian with deformation coming from compression arising from the south. (Variscan orogeny.) Therefore the area was folded and uplifted from a southerly direction producing folding that propagated to the north. The uplifted material eventually being eroded.
There are two sets of faults in the area, one associated with the folding, and the other occurring later during the Triassic period. The latter are N-S faults related to E-W crustal extension. Hydrothermal fluids flowed through these extending faults leading to mineralisation. With good examples of calcite-haematite mineralisation to be found. At Port- Eynon there is a vestige of Triassic outcrop left. Often fault fissures contain Triassic infill and the lack of development of a karstic landscape suggests that there was no dissolution of the limestone suggesting the landscape must have been arid.The one exception being at Worm’s Head where there is a pothole with infill, suggesting it must have formed below the water table which in turn suggests the onset of Jurassic sea-level rise. Although there is are no Jurassic rocks preserved on Gower there are Jurassic sediments found in other locations eg. in the Vale of Glamorgan. Cretaceous sediments are also absent.
Mantle Plume
During Paleogene and Neogene (66Ma - 2.58Ma) times the area underwent cooling from a tropical climate to a temperate one. Global sea-level was falling and sub-arial weathering was occurring. At about 62Ma a mantle plume occurred under western Britain and Greenland causing uplift. But this uplift occurred in pulses. Therefore uplift and sedimentation associated with it must have also have occurred in pulses. It follows therefore that the peneplanation in steps that is seen in the landscape is reflecting the pulsed uplift from the plume. Observing various peneplains eg. Cefn Rhondda above Treorchy (420m OD) and Mynydd Epynt (420m OD) it becomes obvious that large volumes of rock must have been eroded over time. Similarly looking across what was the Caledonian Mountain belt it is obvious that huge areas have been eroded.
The uplift partly accounts for the tilt of Britain as the dynamic support was uneven being greatest in the west and lowest in the east. Further, at 54Ma the Atlantic opened which caused Britain to move away from the uplift of the plume due to sea floor spreading. Thus dynamic support diminished from 54-20 Ma. Proof of the loss of about 1km of rocks can be found when one observes Lundy Island ( Lundy volcano) as here the granite ( dated 60-57Ma) is now exposed.
During the Pliocene (5.33-2.58Ma) the crust is cooling and becoming denser, land subsidence was greater than sea level fall so that a marine transgression occurs. As such the transgression led to the development of marine platforms formed by marine planation. During this time those areas not under water formed small islands with the areas in between undergoing planation. The planation is caused both by mechanical abrasion and chemical dissolution of the limestone.
During the Pleistocene ( 2.58-0Ma) the opposite is true in that sea level fall is greater than land subsidence which led to marine regression. This was related to glaciations which were interspersed with interglacial periods in which sea level would rise thus giving rise to cliff formation.
Using erratics Prof, Kokelaar also mapped the extent of the last glaciation on Gower (23,000 years ago) which was the Last Glacial Maximum when global sea level was 120m lower than it is today. Looking at “Arthurs Stone” which is on the limit of the Ilston lobe, the area has a high number of erratics which is due to the ice being forced to this level due to topographic barriers. Therefore the landscape has been moulded by the ice which has also left a mantling of till. Also during this time even if the ice was not present the temperature would be low causing freeze thaw action on the rocks producing screes which flanked cliffs and buried caves.
The final effect on the landscape of course are humans who change the landscape in various ways.
If you are interested in learning more and in much greater detail then I encourage you to Buy Prof. Kokelaar book “All Our Own Water: Landscape Evolution, Caves and Hdrogeology of Gower”. ISBN 978-1-3999-0335-6
Tuesday 22nd March 2022
On Wednesday 16th March Tim-Holt-Wilson gave a very interesting talk entitled: " Boxstones: the Search for Miocene Suffolk. Rather than the usual summary of the talk Tim has kindly agreed that we can use the information on this subject from his blog. "Our Vital Earth"
East Beach at Bawdsey, Suffolk - 7th November 2013
A brown lump of sandstone, easily overlooked on the beach. The ghost of a shell impression draws my eye.
A boxstone. This is the first one I've ever found with a fossil in it. Looking closely, I see that the sea has abraded the shell's outlines, though the margins have survived better than the rest. It should be possible to identify it.
Boxstones are witnesses of a vanished world. They are all that remains of a lost geological stratum in Suffolk called the Trimley Sands, although deposits of similar age are still present across the sea in Belgium and other parts of Europe. Boxstones are nodules of phosphate-rich sandstone which may contain shell fossils and - if you are lucky - bones and teeth. Most are thought to date from the late Miocene period, perhaps 5.5 million years ago. Later, when the Pliocene sea swept over Suffolk and deposited the Coralline Crag and Red Crag (4.4 to 2.5 million years ago) it eroded some pre-existing marine beds. So earlier material got reworked into later deposits, which are - in their turn - being eroded today. The boxstone material at Bawdsey is coming from the base of the Red Crag strata in local cliffs and perhaps offshore.
Red Crag sands at Bawdsey Cliff, September 2013.Introduced holm oak, tamarisk and silver ragwort give the cliffs an exotic, Mediterranean aspect.
The Red Crag Basement Bed at East Lane Beach, Bawdsey. There is a boxstone in the centre of the photo. The dark brown pebbles are phosphatic mudstone.
The boxstone in my hand is a key for researching and imagining what England was like in late Miocene times. "Strata of Miocene age are very rare in Britain and none are preserved in the district".[2] Resurrecting this period will be like piecing together a jigsaw picture where most of the pieces are missing.
Also, this battered lump of rock definitely has a numinous aura for me. Hopefully I may come to understand why I think it is worth writing about.
There are a few Miocene deposits surviving in Britain, but only preserved in scattered pockets and cavities. Sands and clays have been found at the bottom of karstic sink holes at Brassington in the Peak District of Derbyshire.Blocks of fossiliferous sandstone have been found in solution pipes in Chalk bedrock at Lenham in Kent. Further afield, fossil-rich deposits in hollows in limestone at Hollymount in Ireland have been tentatively dated to the Miocene or early Pliocene. So little has survived because this period was one of crustal uplift in Britain, and erosion was active, while the North Sea was a subsiding basin area.[2] Much of Suffolk then lay beneath the waves, on the western margins of the North Sea.
My boxstone shell's habitat was evidently a sandy one, given its sandstone matrix. Some of this sand must have been washed into the sea from rivers streaming off the English land-mass, although mineral studies suggest that sediment may also have come from metamorphic rocks in the Ardennes region of Belgium.[4]The sandstone then became solidified on the sandy sea bed, along with its enclosed fossil. Deposits of a similar age have survived at Deurne in Belgium, and contain fossil shells, shark and whale bones.[5]
Two parts of a Miocene Plesiocetus whale skeleton from the Deurne Sand Member at Antwerp, Belgium (Bosselaers et al 2004)
What was life like onshore in late Miocene times? The Brassington deposits contain an assemblage of plant fossils which can be used to reconstruct the climate. They suggest that the mean annual temperature was about 16ºC; by way of comparison, Suffolk today has a mean of 10ºC while Madrid has 15ºC. So we are talking about a period of warmer climate, before the general cooling trend which took place after 2 million years ago which ushered in the successive ice ages of the Pleistocene. The list of species seems to have more in common with an Asian forest garden than an English woodland. My imagination goes travelling Exotics include Cedrus (cedar), Tsuga ( hemlock), Liquidamber (sweetgum) Sciadopitys ( Japanese Umbrella pine), Symplocus (sweetleaf), Cryptomeria ( Japanese cedar). The forests of Derbyshire must have been richly aromatic places. There are also more familiar British trees such as alder, spruce and hazel.The ground flora includes mosses, ferns, herbs and grasses. The salt-tolerant herbs Armeria ( Sea Thrift), and Limonium ( Sea Lavender) suggest the sea may not have been far away. Given that the Brassington site is now some 330 m (1082 ft) above sea level, this indicates how much crustal uplift may have taken place here over the last 5 million years.[6] Brassington shows just how much difference five million years has made to the geography of Britain.
Leaves of Liquidamber from the Pliocene Project Sutton Suffolk
A few boxstones contain mammal fossils. As one would expect, marine mammals are most often represented, for example the sea cow Halitherium, the beaked whale Mesoplodon and the sperm whales Hoplocetus and Scaldicetus. A handful of land mammal specimens have been found, their bones and teeth presumably washed out to the sea. These include the elephant-like Mastodon, the ancestral pig Sus palaeochoerus and the mustelid Pannonictis.
If we want to expand this meagre evidence for land life we can turn to the fascinating Dorn-Dürkheim site in Germany, dated about 8 million years ago. Fossils have been recovered from mud in an abandoned meander of the early Rhine. A wealth of over 80 mammal species have been identified, and looking through the list of them I have a sense of a modern zoo fauna seen through the distorting mirror of time and change. The mastodon Anancus roamed the forest, along with the hornless rhinoceros Aceratherium, the horse Hipparion, the dirk-toothed cat Machairodus, the bear Ursavus and the deer Procapreolus[8] These are all extinct genera, but some Miocene mammal types have survived pretty much unchanged to the present day. I even have one of them living in my garden, the Muntjac deer, which has same the long canine teeth and prong-like horns of its ancestors from Dorn-Dürkheim. It is a native of Asia which has been introduced here and has unwittingly recovered old ground in Europe. Other Miocene descendants are living today in warmer parts of the world, where they found refuge during the cold phases of the Pleistocene or remained living in secure habitats. An example is the White rhinocerus Cerathotherium simum from Africa, closely related to the Miocene species C.neumayri from subtropical Greece.[9] Other examples are the raccoon dog Nyctereutes (a native of the Far East) and the tapir Tapirus (south-east Asia and south America). These animals remind me of the currents of evolution that flow through our present day wildlife, and which have their sources in the exotic, warmer world of the late Neogene period of Earth history. As for us, are we not the descendants of Miocene apes?
A late Miocene scene at Dorn-Dürkheim
This is an artist’s impression of how life and palaeoenvironment may have been during the late Miocene (specifically the Turolian mammal stage). A small tributary of the early Rhine meanders through a rather flat landscape covered with patches of woods and savannah. In the foreground, beavers have dammed the creek creating a small pond that stimulated a group of deinotheres to take a refreshing bath. While two chalicotheres are feeding on leaves and fruits, a group of deer is searching for shelter in the shade of trees. In the middle on the right side, a small herd of hipparions is fleeing, whereas on the far left some mastodonts are crossing a meadow. Above the deinotheres a dwarf tapir is approaching the creek, while on the right side above the hipparions a gathering of carnivores is feeding on a carcass. Painting by Wolfgang Weber. Image courtesy Franzen et al. 2013.
A Miocene scene by Mauricio Anton, showing the mastodont Gomphotherium and the rhino Aceratherium in a Spanish landscape. From the book Madrid antes del Hombre by Jorge Morales and illustrated by Mauricio Anton (Comunidad De Madrid, 2010).
I've done some research, and I think my boxstone fossil is an example of the extinct clam Glycimeris obovata ringlei, or perhaps the extinct cockle Laevicardium decorticatum. It lived and died on the seabed of the North Sea, in a moment of time well beyond human memory, even before the dawn of human awareness. Perhaps the Miocene is a metaphor for an Eden-like world before the mythical Fall of Man. If so the present Anthropocene epoch is a bitter confirmation of that Fall, as human beings para-consciously consume and abuse ever more of the world's resources. Also, perhaps my boxstone fossil reminds me obscurely of life before the dawn of my own awareness. Hence its numinous power.
Exploring this fossil's world has put me in touch with a continuum of genetic memory streaming through the world of plants and animals from the Miocene into the present. It reminds me of the biodiverse richness of the tropical parts of our planet, now threatened as never before. It reminds me of the tides of change operating on million-year timescales, transforming species and environments but also conserving elements of them. It reminds me of the fragile bubble of my own animal awareness and genetic identity which is floating - for a moment in time - on the surface of the Earth, yet part of an ancient continuum of being. In this I am no different from a mollusc which was alive 5.5 million years ago - or one alive today.
Glycymeris glycymeris - a living example of the genus. Photo by Philippe Le Granché, courtesy DORIS database.
References
[1] - Balson, P (1990): 'The Trimley Sands': a former marine Neogene deposit from eastern England; Tertiary Research, vol.11.
[2] - Mathers, SJ et al (2007): Geology of the Ipswich District. A brief explanation of the geological map Sheet 207 Ipswich; British Geological Survey, Keyworth.
[3] - Jones, RL and Keen, DH (1993): Pleistocene Environments of the British Isles; Chapman and Hall.
[4] - Boswell, PGH (1928): The Geology of the Country around Woodbridge, Felixstowe and Orford; HMSO.
[5] - Bosselaers, M et al (2004): Geology & Palaeontology of a temporary exposure of the late Miocene Deurne Sand Member in Antwerpen (N. Belgium); Geologica Belgica 7.
[6] - Pound, MJ et al (2012): The palynostratigraphy of the Brassington Formation (Upper Miocene) of the southern Pennines, Central England; Palynology 36.1.
[7] - Spencer, HEP (1970): The Early Pleistocene. The Crag Epochs and their Mammals; Transactions of the Suffolk Naturalists' Society, vol.15, pt.4.
[8] - Franzen, JL et al (2013): Palaeobiodiversity, palaeoecology, palaeobiogeography and biochronology of Dorn-Dürkheim 1—a summary; Palaeobiodiversity and Palaeoenvironments, Vol.93, no.2.
[9] - Agusti, J & Anton, M (2002): Mammoths, Sabertooths and Hominids - 65 million years of mammalian evolution in Europe; Columbia University Press.
Sunday 20th February 2022
The last meeting (16/2/22) Chris Darmon and Colin Sutherland took us on an exploration of the geology of Iceland.
The island of Iceland sits astride the mid-Atlantic ridge, which is an active spreading ridge between two continental plates i.e. the north American and Eurasian plates. It is this location that gives Iceland it’s unique and varied geology. It is geologically young with the oldest sub-arial rocks some 16 million years old and the varied landscapes being the result of the interplay between plate tectonics, volcanics and glaciation.
Map showing midatlantic ridge splitting Iceland. USGS Wiki Commons
Chris and Colin took us on a whistle-stop tour of some of the more interesting geological locations on the island. Below are just three of them.
Laki volcano, located in southern Iceland, is known for the very large, 27 km fissure eruption which took place from June 1783 until February 1784. The eruption led to the release of of about 14 cubic km of basaltic lava and large amounts of ash and gases. Not only is the eruption known for it’s size but the fact that about 25% of the population of Iceland died due to famine and disease brought on by the death of their livestock ( due to eating fluorine contaminated grasses) and crop failure ( due to acid rain). The eruption also caused several years of extreme weather in Europe and north America.
Laki from Lambavatnsgigar by Remih Wiki Commons
Vatnajökull glacier is the largest ice-cap in Iceland and the second largest in Europe. It is the second largest glacier in area after Austfonna on Svalbard in Norway but larger by volume. It has 30 outlet glaciers which are channels of ice that flow out of ice-caps but remain constrained on the sides of the valley.
Vatnajökull glacier by Adam Jang Wiki Commons
Gulfoss waterfall is Iceland’s most famous waterfall. Gulfoss translates to “Golden waterfall” as on a sunny day Gulfoss may reflect sunlight so giving it a golden hue. The canyon into which the water surges grows at an incredible speed of about 25cm per year due to erosion, such is it’s power.
Gulfoss waterfall by Ragnhild and Neil Crawford Wiki Commons
Chris went on to discuss new ideas about the formation of Iceland. The established theory is that geological features such as Iceland, are generated by the interaction of ocean-ridge sea-floor spreading with a hot mantle upwelling. This idea is now being challenged. Chris outlined several reasons why the original theory is not correct.
1.the spreading rate does not add up as Iceland is wider than the 2cm/year spreading rate can account for.
2.There is too much rhyolite. In other words how can an island on a divergent plate boundary have both mafic and felsic magma sources?
3.Mesozoic age zircons have been isolated from rocks in south-east of the island
4.oceanic crust under Iceland is too thick. This crust is up to 40km thick, far greater than normal oceanic crust.
A new theory has been put forward by Prof Foulger (Durham University) and her team. They argue instead that Iceland is made of continental crust and so are large areas of the surrounding seabed. This hidden continent of Icelandia, if it exists, has a surface area of 600,000 square kilometres. Prof. Foulger thinks Icelandia is one chunk of Pangaean continental crust that survived and now sits underneath Iceland.
The research into this goes on.
Thursday 4th November 2021
At the last meeting Prof. Neil Glasser gave a highly informative talk regarding new data on the glaciations in Wales based on his recent research, in collaboration with Prof. Philip Hughes of Manchester University and Dr David Fink of the Australian Nuclear Science and Technology Organisation. (ANSTO).
Their research has been ongoing for a number of years and commenced by formulating a question regarding the thickness of ice over land in Wales during the ice-ages. Were the high elevations of Wales above the ice in the form of nunataks during this time or had they been covered by the ice? This led to the research project to determine exposure ages for parts of Wales which would determine if the high elevations had been covered by ice or not and would lead to a greater understanding of glacier retreat.
Research Area
The research area targeted seven major massifs in Wales with the aim of understanding the timing of glacier retreat from summits to cirques:
Moelwyns
Arenigs
Rhinogs
Arans
Snowdon and the Glyders
Cadair Idris
Taking Samples
"In order to determine the age at which the rocks were exposed they used the method of Cosmogenic Nucleotide Dating (CND). CND uses interactions between cosmic rays and nuclides in glacially transported boulders or glacially eroded bedrock to provide age estimates for rock exposure at the earth’s surface. That is how long the rocks have been located at the surface. Cosmogenic nuclide dating is effective over short to long timescales (1,000-10,000,000 years), depending on which isotope you are dating. Different isotopes are used for different lengths of times. This long period of applicability is an added advantage of cosmogenic nuclide dating."(http://antarticglaciers.org)
Sampling strategy is important as is choosing the correct rock type. Granite and sandstone boulders are frequently used as they contain a large amount of quartz which yields 10Be and 26Al via spallation of oxygen and silicon respectively. Using these paired cosmogenic isotopes produces more reliable results than when used in isolation.
Although Prof. Glasser touched on results from previous work on some of the targeted areas his talk presented us with the latest ( still unpublished) results from work carried out on Snowden and the Glyders and Cadair idris.
His previous work has shown that the summits of the Rhinogs, Moelwyns and Arenigs in north Wales were covered by ice when the last Welsh Ice Cap was at its maximum and evidence of ice scouring and transport of glacial boulders was found. The evidence was particularly pronounced in the Rhinogs and Moelwyns. This work also provided very strong evidence that these summits became exposed as nunataks at 20–19 ka.(Hughes et al.,2016)
Other work on the Aran ridge showed that the welsh Ice Cap was thick enough to completely cover the Aran ridge and achieve glacial erosion at the Last Glacial Maximum. The work also showed that between 20 and 17,000 years ago the ridge summits were exposed as nunataks at a time when glacial erosion at lower elevations was achieved by large outlet glaciers in the valleys surrounding the mountains.( Glasser et al.,2012)
From the early works and the latest research on Snowdon and the Glyders and Cadair Idris the following conclusions have been derived:
Wales was glaciated by a large Ice cap that buried the highest mountains and transported erratics over the highest peaks in Marine Isotope Stage 4( MIS 4) 60-70,000 years ago. This latest research producing the first actual dates to show that this took place.
The highest summits of Wales then stood above the ice at LGM c 25,000 years ago.
Ice thinned quickly revealing most other summits 19-20,000 years ago after which there was only ice in the main valleys in Wales.
There was a pulse of glaciation at the Younger Dryas 12-10,000 years ago and ages for this have been obtained from both sides of Cadair Idris and also from Cwm Cau.
(Hughes PD, Glasser NF, Fink D, 2016. Rapid thinning of the Welsh Ice Cap at 20–19 ka based on 10Be ages. Quaternary Research 85: 107-117)
(Glasser NF, Hughes PD, Fenton C, Schnabel C, 2012.10Be and 26Al exposure-age dating of bedrock surfaces on the Aran ridge, Wales: evidence for a thick Welsh Ice Cap at the Last Glacial Maximum. Journal of Quaternary Science 27 (1): 97-104 )
Friday 24th September 2021
At the last indoor (Zoom) meeting Prof. Sarah Davies ( Head of Geography and Environmental Sciences Aberystwyth University) gave a highly interesting talk entitled “Climate change and coastal heritage in Wales - insights from the palaeo-environmental record”
Prof. Davies is involved in a collaborative project known as CHERISH, i.e. Climate, Heritage and Environments of Reefs, Islands and Headlands. The works covers both Ireland and Wales and there are four partners:
Royal Commission on Historical Monuments of Wales, lead on the project.
Aberystwyth University
The Discovery programme - Centre for Archaeology and Innovation Ireland.
Geological Survey, Ireland.
Using an array of techniques and a land, sea and air approach the project has the following aims:
Reconstructing past environments and weather history.
Discovering, assessing, mapping and monitoring heritage on land and beneath the sea.
Targeting data and knowledge gaps to raise awareness of heritage in these remote coastal locations.
Establishing new metrically accurate baseline data and recording standards.
Although the project covers both Ireland and Wales Prof. Davies focused her talk on Wales and the Holocene Period. Her team are focusing on trying to put some contemporary climate change problems into a longer term context i.e. the last few millennia. Using palaeoenvironmental records and a range of physical techniques they are examining coastal wetlands, lakes, bogs and sand dune systems, with the aim to date past storm activity and how periods of storminess have changed over time in frequency and intensity. The link to sea level change is also important.
The palaeoenvironmental evidence can also be used to link to the archaeology to provide environment and landscape context and how changes affected the societies at the time.
One study area covers parts of north Wales( Anglesey and Gwynedd) with different sites of different types and putting the data together from these sites it is hoped to develop a picture of change. These areas can be seen on the map below.
A key tool in the research is luminescence dating which allows dating of when minerals, especially quartz, was last exposed to light. In other words it is a burial signal for the minerals.
Prof. Davies then went on to describe the work undertaken at the Welsh sites. From the work undertaken a whole range of evidence has been obtained which is allowing them to build a picture of landscape and environmental change which can be integrated with the archaeology. This is aiding in the longer term understanding of environmental and climatic change and how past societies coped with those changes. This knowledge may help with the changes which we are undergoing now and in the future.