Friday 17th June 2022
Introduction to Outer Hebrides Geology by Chris Simpson secretary of MWGC.
This talk is based on a guided trip round the Hebrides led by Chris Darmon and Colin Scofield from geosupplies (http://www.geosupplies.co.uk)
We took the ferry from Oban to the island of Barra at the southern end of the Hebrides. Then we worked our way north along the whole chain of islands, finally getting the ferry back from Stornoway to Skye. We had eight nights, staying at four different hotels. There was a lot of geology to see; and also, a lot of interesting non-geological sites such as the Calanais Stones – a Neolithic site erected around 2,900BC.
The islands are mainly composed of very old rocks including the oldest rocks seen in the UK. The following is a brief summary of the main aspects of Hebridean geology.
3Ba: first terrains formed, divided by faults between the islands.
2.8Ba: first metamorphosis to give predominantly meta-igneous rocks – the Badcallion event.
2.4Ba: widespread invasion by basaltic Scourie dykes.
? Age: second metamorphosis – re-worked the gneiss and converted the basaltic dykes into meta-basaltic dykes.
1.8 – 1.7Ba: pegmatitic dyke invasion.
~400Ma: Caledonian Orogeny which created the Outer Isles Thrust Fault.
The typical appearance of much of the island chain. Gneiss weathers to produce poor soil – so bare rock is very common.
The Outer Hebrides Thrust Fault is a major factor behind the geomorphology of nearly the whole island chain, being around 100 miles long. Most of the island rocks are gneiss in one form or another. Recent research is managing to differentiate the gneiss into different terrains; and also providing evidence of the precursor rock before it was metamorphosed into gneiss.
Sandstone, granite and basalt will all end up as gneiss if subjected to high-grade metamorphism – but they will retain the geochemistry of the original rock, especially the trace element geochemistry. So even after three billion years, we can still say that one area of gneiss came from Mid-Ocean-Ridge basalt, while another area of gneiss came from sandstone.
Banded gneiss. The appearance varies greatly from one area of the Hebrides to another, both in the shape and colour of the banding. There is no such thing as “typical” gneiss. (The scale is the green spectacles case.)
A closer view of the upper central part of the above photo
Increasingly complex gneiss banding.
Absolutely chaotic gneiss
Larger-scale banding and chaos.(Fence posts at the top of the picture give a scale.)
A recent dyke related to the Skye super-event approx 65Ma.The scale is an A6 field notebook
A pegmatitic dyke with huge quartz crystals.
On the geology map, this area is described as crushed gneiss and pegmatitic dykes near where thrust planes split.
A road cutting where the Outer Hebrides Thrust fault runs. The fault line runs diagonally from the top near the left down to the bottom at the centre. All the rock to the right of that line is disorganised
A closer view view of the fault boundary.
The thrust fault on another island. Here it is nearly horizontal with a more obvious contrast between the top and bottom layers.
The Calanais Stones on the island of Lewis, 12miles west of Stornoway.
Many of the stones are composed of banded gneiss. (The scale is a camera lens cap.)
Text and photos Chris Simpson
Tuesday 7th June 2022
Anglesey GeoMôn Geopark
Mid Wales Geology Club members heard an excellent talk from Professor Cynthia Burek about the Geopark on the Island of Anglesey (Môn in Welsh) on 18th May. She is a director of the Geopark and represents it at Unesco Conferences.
Geoparks became international in 2000 with several European geoparks cooperating to ensure they could protect their geological heritage and encourage sustainable development and geotourism. In 2004, the 17 European Geoparks were joined by 8 Chinese parks.
In 2015 UNESCO recognised Geoparks as one of their protected entities, joining World Heritage sites etc. A Geopark is a defined area of territory with a particular geological heritage, noted internationally for scientific quality, rarity or educational value. It also has to involve the resident population in active participation and, hopefully, economic development. By 2022, there were 177 designated Geoparks in 46 countries around the world. Europe has 88 spread around 26 countries. The UK has 8. In England: the English Riviera, North Pennines and Black Country Geoparks. In Wales: Fforest Fawr & Môn. In Scotland: Shetland and the North-West Highlands. In Northern Ireland there is the Marble Arch Caves/Cuilcagh Geopark.
GeoMôn was recognised as a Global Geopark in 2015; and recognised as the first tectonic island Geopark because it had every type of plate boundary with the associated rocks and structures. GeoMôn is the most diverse of the 8 UK Geoparks with over 100 different rock types formed over 1,800 Ma of time. Many of these rocks can be seen from the coastal path going all round the island – see the book Rocks and Landscapes of the Anglesey Coastal Footpath for details. All the main geological time periods are represented on the island apart from the Jurassic and the Cretaceous. The Geopark has produced a series of illustrated guides for walking trips to see aspects of the geology at specific locations around Môn which are suitable for both the general public and geologists.
We were shown photos of the oldest fossils in England & Wales – cyanobacteria at Llanbadrig at the Northern tip of the island – 860Ma on the latest research dating.
Llanddwyn Island, reachable at low tide from Newborough Sands, is the world type site for melange, where it was first described by Greenly. Amazingly, just a short distance away are pillow lavas formed at a constructive plate margin, rather than the destructive margin involved in the formation of a melange.
The Geopark has its headquarters in the Watch Tower Building at Amlwch, next to mica schists formed during the Caledonian Orogeny. It is open over the summer between the hours of 12noon and 4pm – but not Saturdays or Mondays. The exhibition in the building has been updated during lockdown, and the posters now cover Climate Change from Snowball Earth to modern times. Specimens of all the various rock types in the Geopark are also shown.
The South Stack formation rocks show the effects of the Caledonian Orogeny – Anglesey collided with Scotland and Eurasia as the Iapetus Ocean closed. This is an example of geodiversity supporting biodiversity, as the multiplicity of sedimentary rocks provides plenty of ledges for a wide variety of sea birds to nest safely.
The Geopark also features the best example of a thrust fault in Wales at Carmel Head, where Pre-Cambrian & Cambrian rocks have been thrust over the top of 450Ma Caradoc rocks.
Parys Mountain is well-known as having been the world’s largest copper mine in the 19th century. This is an area where the local geology led to economic activity significant in the UK and internationally.
All in all, there is an amazing variety and age of geological sites to see within the Geopark. Anyone interested in a good book covering the island’s rocks was recommended to buy Footsteps through Time – the rocks and landscape of Anglesey explained by Stewart Campbell, Margaret Wood and Brian Windley. Also worth visiting is the Geopark’s website: [https://www.geomon.co.uk]
Photos with kind permission of speaker
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.
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
Photos with kind permission of speaker
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
New data on the glaciations in Wales
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.
The research area targeted seven major massifs in Wales with the aim of understanding the timing of glacier retreat from summits to cirques:
Snowdon and the Glyders
"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 )
Photos with kind permission of speaker