At our March indoor meeting on Wednesday the 20th, guest speaker, Prof. Michael Rosenbaum, gave us a talk on "Elan Valley dam construction and the Birth of Engineering Geology.”
As a result of rapid industrialisation and urbanisation of the area in the 1860s and 70s, the populace in Birmingham was largely housed in crowded tenements in insanitary conditions. Shared privies were the rule with seepage into the shallow wells owned by the water companies. The only surface water resource was the diminutive and polluted river Rea which now flows through a ditch underneath Digbeth.
Water was very short and rationed, with drinking water sold from a pony and trap. Water-borne diseases such as cholera and typhoid were rife and something had to be done to improve the health of the workforce.
The then mayor, Joseph Chamberlain, a self made man who had made his fortune as a screw manufacturer, took action. He forcibly bought up the water companies and appointed Robert Rawlinson as consultant.
Providing an adequate clean water supply was fraught with difficulty. With no adequate local river or aquifer, water would have to be found at a distance and transported. Looking at the significant rivers surrounding Birmingham, going north, the Trent was of poor quality, the Derwent required an aqueduct and pumping, in the west the Severn was too low and the Teme had too many people and high quality farms requiring compensation. The Wye was low and people in Hereford would object, so although distant, the The Elan, with the Claerwen, were selected as most suitable. They had a large catchment area with a high rainfall and enough elevation to avoid pumping.
Caban Coch Reservoir Elan Valley ( Colin Price Creative Commons)
In 1890 an act of parliament was obtained and Mansergh was engaged as engineer who designed the Elan scheme with a dam at Caban Coch where the valley was constricted. Much of the local stone was called "Cyclopian" as it contained large hard cobbles and could not be accurately shaped. Accordingly, the construction of the dam was with a core of rough irregular local stone blocks, set in concrete with an Ashlar facing of accurately cut stone ferried in from a distance. This was one factor which caused a 50% cost overrun.
We were shown a remarkable 1892 photograph of Mansergh heading a group of the "great and the good" geologists and engineers on a viewing field trip. It included Prof. Topley (Surveyed the Weald), Prof. Boyd Dawkins, Thomas Hawksley (engineer for the Vyrnwy dam), perhaps showing an early merging of geology and engineering disciplines into engineering geology.
Members were treated to an interesting evening, being given an insight into the basic structure of an iconic local heritage site.
The next meeting will be on Wednesday 17th April when Daniel Lockett will give a talk on the FISH (Fossils in Shropshire) project.
Thursday 28th February 2019
At our indoor meeting on Wed 20th February, Bob Loveridge, Research Fellow in the School of Earth and Environmental Sciences at the University of Portsmouth, gave a talk on 'Flying reptiles at Buckingham Palace', including how a team of researchers did their giant pterosaur spectacular for The Royal Society's 350th Anniversary and how they took pterosaurs to meet the Queen at Buckingham Palace.
Pterosaurs were flying reptiles, best imagined as "flying crocodiles". Finds in the early 19th century were generally described as avian, including "bird bones" described by Gideon Mantell and finds by Dean William Buckland of the first UK specimen. In 1828 Mary Anning uncovered a complete specimen, albeit as a jumble of bones. It was Cuvier who first reconstructed the skeleton and recognised them as flying reptiles.
Pterosaur reconstruction Sommerring 1812 - Wiki Commons Image
They were famously popularised by Conan Doyle when he published "The Lost World" in 1912 and have remained popular ever since.
Pterosaurs evolved in the Triassic and developed into many forms from the size of small birds to the giants with enormous wings and long necks and heads in the Cretaceous. They sported bizarre head crests the purpose of which is obscure. They could have been for maintaining heat balance or for sexual display. (It is established that they had sexual dimorphism.)
The largest, Quetzalcoatlus, was some 40ft long, with a three metre neck and head, the largest ever flying animal, comparable to a Spitfire! It probably fed on small fish and there is some controversy over how they flew, but it does look as though they were capable of powered flight.
The skeletal structure was somewhat bat-like, with the skinny wings supported by an extended fourth finger. Unlike birds, on landing, they took up a quadruped position.
One recently discovered, now in the Kimmeridge Museum, is named Cuspicephalus scarfi after Gerald Scarfe due to its resemblance to Scarfe's iconic charicature of Margaret Thatcher!
As part of the celebration of the 350th anniversary of the founding of the Royal Society, the team from the University of Portsmouth built a flight of life sized flying pterosaurs which 'flew' (suspended on wires) above the Southbank Centre in London.
In addition to celebrating the founding of the oldest scientific society, it was an opportunity to learn more about how these animals flew and walked. The models were built on a lightweight frame with assistance from Griffon Hoverwork and were fabricated from foam with a hairy textile covering. The eyes comprised finials from B & Q, suitably painted.
To demonstrate the size of these animals, with help, Bob unfurled a scale drawing of one wing which went two thirds the length of the auditorium. (The whole animal would not have fitted in the room.)
The models were also exhibited at Buckingham Palace as part of the Queen's 80th birthday celebrations when they were enjoyed by her, other members of the Royal Family and by many schoolchildren invited for the occasion.
Bob was thanked for showing us models, books, drawings and fossils and giving a memorable talk with a difference.
The next meeting will be on Wednesday 20th March when Professor Michael Roenbaum wiii give a talk entitled: "The Elan Valley Aqueduct and the Birth of Engineering Geology."
Monday 28th January 2019
Twenty six members attended the AGM which was followed by a talk on Anglesey's geology based around the geology observed during the club's annual summer weekend. The weekend was led by Dr. Charlie Bendall.Below is an outline of David's talk.
The standard view of Anglesey geology, based on the detailed work of Edward Greenly in 1919, dates from long before plate tectonics was understood. There has been much controversy about the exact chronological order of the main units, and whether they might perhaps be “upside down”.
Since 2007, some Japanese scientists, working with Brian Windley of Leicester University, have put forward a new view, where the Anglesey rocks are seen as a classic example of what happens when an ocean plate is subducted under a continental plate. The main components, including an accretionary complex, extensive mélange, an exhumed blueschist, an ophiolite sequence, and continental margin sediments, are exactly what one sees in other parts of the world where an ocean plate has been subducted. An example is the San Francisco region - as was well illustrated in Chris Simpson's recent talk to our group. As Chris pointed out, one of the key features of such a scenario is that the older units are on top, with the younger units thrust underneath - the opposite of the usual stratigraphic order!
The “Japanese” analysis is summarised in the following diagram by Professor Shigenori Maruyama, taken from my photo of the GeoMôn geopark information board at Llanddwyn Island:-
GeoMôn geopark information board at Llanddwyn Island
To summarise the story, around 680Ma, an oceanic plate was being subducted under Avalonia, leading amongst other things to the magmatism of the Malvern Hills. Subduction was building up an accretionary complex corresponding to the Gwna Group. The accretionary process continued over a long period of time until at least the early Cambrian.
The accretionary complex mainly consists of ocean plate sediments scraped off the descending plate by the overriding continental plate. It typically comprises a succession of small units thrust one under another in succession. Each unit, or “horse”, represents a slice of seafloor. Each typically encapsulates, at the base some pillow basalt from a mid-ocean ridge, capped by bedded chert and sometimes limestone, with possibly mudstone and turbidites on top. The units are particularly well exposed over the entire Llanddwyn island, where the Japanese scientists have mapped out 23 “horses”, which they believe represent over 7,800m of seafloor shunted into an accretionary deposit only about 300m thick.
At the core of the subduction's accretionary process is a deep oceanic trench, with a particularly steep slope on the continental side. In many situations, material tumbles down this slope, creating a chaotic mixture of various rock types, mostly derived from previously accreted seafloor, with components ranging from kilometre-scale chunks down to small pebbles. This is the famous Gwna Mélange. As I understand it, it's more likely to be the result of an ongoing process, rather than being produced by a single, huge, catastrophic event. The mélange can be seen on the north coast of Anglesey, and less dramatically at the southern tip of Llanddwyn island as well as the famous example on the mainland at the southwestern tip of the Lleyn peninsula.
Returning to the detailed story of the subduction, it is thought that at around 620-600Ma or perhaps earlier, Avalonia advances to the point where a mid-ocean ridge gets subducted. Although the exact effect of such an event is unclear, it is suggested this led to the creation of the Coedana granite within the continental crust at the edge of Avalonia.
Around 575-550Ma, a ductile wedge of the accretionary complex, a few kilometres thick, gets subducted, recrystallised under blueschist conditions (high pressure but relatively low temperature), and then is squeezed back to a higher level, possibly assisted by a shallowing subduction angle. In this way, the Blueschist Unit is formed.
After the emplacement of the Blueschist Unit, the accretionary process continues, with conditions producing mélange becoming more prevalent.
During the final stages of the subduction, in the Cambrian, continental margin sediments corresponding to the New Harbour Group, and subsequently the South Stack Group, are underthrust more-or-less intact into the base of the accretionary complex. This underthrusting leads to doming of the already accreted complex, causing a fragment of the western margin of Avalonia containing the Coedana granite to become separated from the rest.
At some point after the New Harbour Group turbidites were emplaced, a deep slice of ocean crust was thrust within it - as a so-called “dismembered ophiolite”.
After subduction had largely ceased, magmatism continued into the Ordovician, resulting in the igneous rocks of North Wales. And there the subduction story ends.
In a later paper from 2013, Windley and others show that the Anglesey story is far from unique. For at least 3.8 billion years, the Earth has experienced a remarkably consistent process of sea floor spreading, subduction and accretion at continental margins. Other examples are to be found in Greenland, Central Asia, Japan, and the previously mentioned California coastal range. A further example is nearer at home in the southern uplands of Scotland, where an accretionary complex was formed on the margin of Laurentia as the Iapetus Ocean was being subducted. All of these examples show very similar rock types and structures. It appears that the overall process is a major factor in the creation of continental crust. It is even suggested it has played a part in plate tectonics being a major heat loss mechanism on Earth since the early Precambrian.
Friday 7th December 2018
Twenty members attended the November evening during which Chris Simpson presented a short talk related to two geological sites of interest he had recently visited in Catalonia.
The talk was followed by a presentation to Colin Humphrey and his wife Mary as they are moving down to Hampshire. Colin has been a driving force behind the club for the last sixteen years and we wish them both all the best for the future.
Summary of talk:
Castellfollit de la Roca This is situated in Northern Catalonia about 50 – 60 miles South of the Pyrenees and is one of the eleven townships which compose the Garrotxa Volcanic Area Natural Park. Two rivers ( rivers Fluviá and Turonell) merge into one, and there is a narrow triangle of land between the two where they merge. Over a long period of time, the two rivers have eroded down, leaving the triangle of land standing out as a 50m high feature with sheer sides facing the two rivers. A medieval town was built on this land, presumably because it was so easy to defend from any attackers. The 50m high cliffs show beautifully the two lava flows with obvious columnar jointing within each flow. The lava flows have been dated, the first with a date of 217000 ± 35000 and the second at 192000 ± 25000
Montserrat is the highest mountain in a chain situated about 50miles West of Barcelona. It is 1,236m high and has a distinctive saw-toothed appearance which gives it its name. It is largely composed of conglomerates which were deposited in the Eocene era as a large fan delta. The clasts are mostly limestone and cemented with calcite. Montserrat began its formation due to the upthrusting of the Catalan Coastal Range. Denudation of this new mountain range front produced the fan delta conglomerate at its base. This sequence was about 1300m thick. During the Upper Oligocine to Lower Miocine there was an extensional phase of the Iberian peninsula causing the massif to rise, which led to the development of joints. Now the massif was also exposed to the elements leading to differential erosion and the development of a karst landscape seen today.
The Next meeting will be the AGM on Wednesday 16th January 2019. This will be followed by a short talk by member David Warren entitled: " The Geology of Anglesey."
Saturday 22nd September 2018
Indoor meeting on Wednesday 19th September
"Fossil Plants of the Carboniferous Period: a personnel collector’s perspective." Paul Lane.
Paul gave an outline of his history and how he became interested in fossils. His grandfather came to South Wales in 1910 from a rural background to work in the pits and his father followed him into the mines. Paul started collecting fossils when he was about 9-10 years old most being collected from the tips around where he lived in the Rhondda Valley. His father made him a “pick” and gave him an old gas mask bag to hold the specimens.
When at school a new teacher arrived from Aberystwyth who offered geology as a subject and therefore he could study geology at ‘O’ and ‘A’ level after which he undertook a teaching degree with geology as the main subject and the rest (as he states) is history.
Paul then proceeded to describe the Carboniferous Period and the plants that occurred:
Around 359 Ma, at the start of the Period, Great Britain was lying at the equator, covered by the warm shallow waters of the Rheic Ocean, during which time the Carboniferous Limestone was deposited, as found in the Mendip Hills, North and South Wales, in the Peak District of North Lancashire, the Northern Pennines and Southeast Scotland.
These were followed by dark marine shales, siltstones and coarse sandstones of the Millstone Grit. Later, river deltas formed and the sediments deposited were colonised by swamps and rain forest. It was in this environment that the cyclic Coal Measures were formed, the source of the majority of Great Britain's extensive coal reserves that powered the Industrial Revolution.
Lepidodendron by Paul
Average global temperatures in the Early Carboniferous Period were high: approximately 20°C (68°F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12°C (54°F). Lack of growth rings of fossilised trees suggest a lack of seasons of a tropical climate.
Glaciations in Gondwana triggered by Gondwana's southward movement, continued into the Permian.
3 pieces of coal by Paul
The thicker atmosphere and stronger Coriolis Effect due to earth's faster rotation (a day lasted for 22.4 hours in early Carboniferous) created significantly stronger winds than today.
The cooling and drying of the climate led to the Carboniferous Rainforest Collapse (CRC) during the late Carboniferous. Vast tropical rainforests collapsed suddenly as the climate changed from hot and humid to cool and arid. This was likely caused by intense glaciation and a drop in sea levels.
At the next meeting Kit Moorhouse will give a talk entitled: “North Sea Oil Fields and Debris Flows”