Thursday 13th July 2023

At the last meeting James Creswell of Geo World Travel https://www.geoworldtravel.com/ ) spoke about the geology of Namibia. His company leads many geological field trips every year with Namibia being one of them. The talk is based on a tour of Namibia commencing and finishing at Windhoek.

Namibia has a very varied geology and covers a vast time span. In fact the geology encompasses rocks of Archaean to Phanerozoic age and as such covers more than 2.6 billion years. About half of the country's surface area is bedrock exposure, while the remainder is covered by Cenozoic deposits of the Kalahari and Namib Deserts.

The north and south of the country were once two separate cratons which were once part of of the earlier super continent of Rodinia. Then during the Ediacaran and Cambrian periods ( 550-500Ma) the two collided during the Damara Orogeny. The resultant mountains have since eroded leaving central Namibia consisting of metamorphic inliers from the eroded roots of the mountains. The Damara Orogeny was actually part of a large Pan African Orogeny that created Gondwanaland. As the Cratons collided a thrust fault was created where older carbonate rocks slid as a ‘nappe’ over younger limestone rocks lubricated by a salty layer ( the Sole Dolomite). This is the Naukluft Thrust Fault and can be viewed in the Namib-Naukluft National Park.

Nicole Grünert points to the Naukluft Thrust Fault

The oldest rocks to be seen are to be found at the Etosha national park and here you can find 2.2Ga paragneiss from the Congo Craton. Also in this area are drop-stones which fell out of ice-shelves or icebergs onto seafloor during one of the earth’s Snowball Earth episodes (630Ma). The Snowball Earth deposits themselves are covered by tropical carbonates known as Cap Carbonates. Other ancient rocks can be found near Kobos where there is an outcrop of Kalahari Craton basement granite which is 1Ga in age.
At Swakopmund basalt which formed as the super continent of Rodinia rifted apart was compressed and metamorphosed to amphibolite during the Pan-African Orogeny can be found as can rocks from the Matchless Belt which are thought to be an ophiolite.This belt lies between the Kalahari and Congo Cratons. Also at Swakopmund the world’s largest displayed quartz Crystal can be seen in the Crystal Gallery (Kristall Gallerie).

Nicole Grünert points to boudins in amphibolite which were once basalt dykes which later got metamorphosed in the Damara orogeny

As well as ancient rocks Namibia also boasts the presence of the oldest shelly fossils. These are the Namacalathus (549-542Ma) and their discovery caused the Precambrian (Edicarian)/Cambrian boundary to be revised back in time. These fossils are to be found in the area of the Tsaris Mountains.

The Erongo Complex is the eroded core of a huge volcano which has peripheral and central granite intrusions. Formed about 137-124Ma when Africa and South America were splitting apart to form the south Atlantic. The Erongo at 35km diameter is one of the largest Cretaceous granitic complexes in northern Namibia. It consists of both intrusive and extrusive rocks. The igneous activity gave rise to many different minerals which are sought after around the world. Similarly the Brandberg Massif ( Namibia’s highest mountain) is is a single mass of granite that pierced its way through the Earth’s crust into the Namib Desert during this time and is composed of homogeneous medium grained biotite-hornblende granite.

During the Carboniferous the whole of southern Africa was buried under a thick ice sheet. But during the Permian temperatures rose giving rise to an increase in fauna and flora. Now the remains of trees (Dadoxylon) can be found in rocks of the Lower Karoo Supergroup and is known as the “Petrified Forest” and has the biggest accumulation of these trees in southern Africa. They were deposited in an ancient river channel during flash flooding.

Jurassic sandstone may be found at the Waterberg Escarpment. This formed by the reactivation of a precambrian fault (Damaran) when Gondwana split to form the Atlantic. This caused Jurassic age Karoo sediments to be thrust over other Jurassic age sediments. Erosion of sediment has left the escarpment as seen today. The Waterberg Plateau is a popular national park.

One of Namibia’s great natural wonders is the Sossusvlei Pan and surrounding desert sand dunes. It is part of the Namib-Sand-Sea UNESCO site. The name translates to “dead-end marsh”. It is a clay and salt pan and is the terminus of the seasonal Tsauchab ephemeral river which is preventd from flowing all the way to the sea by the sand dunes of the Namib Desert. Some of the dunes are over 400m tall and are among the highest in the world. They are derived from material that eroded from the highlands of South Africa and Lesotho and was transported to the Namibian coast by rivers. This material was then blown onto land during the ice-age. The dunes have a red colour due to the grains of sand having a coating of iron oxide. In some places the dunes have a yellow colour due to the grains having a coating of clay particles from eroded local limestones.
What a truly remarkable place Namibia appears to be.

Deadvlei salt pan in the Namib Sand Sea UNESCO World Heritage Site

Wednesday 7th June 2023

Coming out of the Ice Age - by Tony Thorp

On Wednesday 17th May, At Plas Dolerw, Tony Thorp gave a talk on "Coming out of the Ice Age".
The talk focussed, not on the Ice Age as such, but on what is most visible now and on what we see still happening round us in our local area.

Rather than the image we were all taught at school of Alpine glaciers, "U" shaped valleys, hanging valleys and terminal moraines, we see more the residue from a waning, melting ice sheet depositing its load of dirty mud and rocky bits (very aptly described by the outdated term "boulder clay") on the existing rock terrain. Other than on hilltops, marshy bog must have been universal in the periglacial landscape. Its extent is well shown in the BGS "Solid and Drift" or "Bedrock and Superficial" series of geological maps by the pale blue "till" colouration. Tony took us to a few of his favourite neighbouring locations.

Crychell Moor
The first location was Crychell Moor, a broad flattish valley 2km west of Llananno, just off the A483, best viewed from the trig point on Ysgwd-ffordd, on a section of Glyndwr's Way. The floor of the moor is carpeted by a number of "ramparted depressions" outlined by rushes (Figs Palsa 1, 2). There has been some debate about whether these are relics of pingos or palsas. Pingos are ice-cored conical hills found in permafrost areas, whereas palsas are rather smaller scale freeze-thaw periglacial structures comprising peat and ice mounds which grow from a mire and eventually collapse, to be succeded by new ones and often developing into complex curvilinear patterns, very evident in Palsa 1. In Palsa 2 the grass and rushes had been cut, leaving the peaty boggy depressions proud. In some conditions the periglacial feature 'patterned ground' is visible through the turf near the trig point.

Palsa 1 by Tony Thorp

Palsa 2 by Tony Thorp

His own personal landslip!
As ice wanes and massive amounts of wet "Boulder clay" are deposited on the solid topography, which will often have been oversteepened by ice action, landslipping is endemic and very often it is still mobile today. Tony showed pictures of his own landslip which is still moving downhill towards the brook at the bottom. The narrow ridge at the top is all boulder clay (As evidenced by a series of bores made for recently planted telegraph poles.) and the top scar is the steep drop below the hedge. Below is a series of terracettes. At its toe the clay is quite plastic and discharges into the brook just below the trees he planted to try to stabilise it. Fenceposts keep moving downhill!

Scar of landslip by Tony Thorp

Toe of landslip by Tony Thorp

Elsewhere, the valley is a steep "V" shaped gorge, cut into the bedrock by rock-bearing meltwaters as the ice melted. Most times it is a quietly babbling brook, but every few years, after snowmelt, it can be as noisy as a jet engine and will move large boulders!

V-shaped valley by Tony Thorp

Severn Valley
The Severn Valley was the course of a significant ice stream moving faster than the rest of the cap in a northwesterly direction. It left a train of drumlins and glacially streamlined bedrock features along its length, very evident in the BRITICE Glacial Mapping Project V.2 (2017) (Ref 1). A significant drumlin, Trehafren Hill, is in Newtown and has a large landslip on its north side and a smaller one on the south. Such slips are a common feature and when Newtown was being developed in the 60s and 70s were the reason for the Development Board commissioning the BGS to produce a special six inch to the mile geological map. Sue Cain's 3D model based on that map shows them clearly and was examined by attendees in person at Plas Dolerw.

Black Gate
Black Gate is an interesting point along the Knighton road some two km out of Dolfor. It is within a stone's throw of the source of three rivers, the Mule which joins the Severn at Abermule, the Ithon which flows south to Llandrindod alongside the A483 and also the Teme, sourced less than a km southeast and flowing to Knighton and Ludlow. The amount of boulder clay in the Mule valley shows that the col between it and the Ithon was well covered by ice during peak glacial times and must have had a complex history in the multiple advances and retreats as the ice waned. As levels one side or the other may have been higher, meltwater would have flowed in either direction. This has left a complicated pattern of meltwater channels just north of the road.

Black Gate the view from the road looking northeast to the Mule valley with meltwater channels in the foreground and hints of lateral moraine in distance on the far side of the valley.

Sarn
Sarn lies in a fertile flat bottomed valley running east through Kerry from Newtown. It is floored by shaley rocks with more resistant Bailey Hill Fm rocks forming the high ground north and south. During the last glacial it was home to a significant ice stream which, as it retreated, dumped a lot of morainic clays and gravels near Sarn. This obstructed the original river Mule forcing it to find a way north through the gorge near Abermule before joining the Severn. Its lower stretches remain as the now smaller Caebitra continuing on its former course to Churchstoke where another river diversion took place forcing it to go north, joining the Camlad and joining the Severn.
The valley is also home to a drumlin swarm just south of Bacheldre. This is more subdued than those in the Severn valley where they have been augmented by the grain of the rocks and the speed of the ice stream.

Drumlin by Tony Thorp

Tony ended with a small sad sequence showing the (probable) demise of a kettle hole on the Severn outwash plain near Shrewsbury as it was demolished by a 5 furrow plough! Never mind, perhaps the peat, the wet and the roots of rushes will re-establish!

Ref1 The BRITICE Glacial Mapping Project: version two (2017) accessible at:
https://shefuni.maps.arcgis.com/apps/webappviewer/index.html?id=fd78b03a74bb477c906c5d4e0ba9abaf

Thursday 11th May 2023

The River and the Rock: River Potholes of Wales By Dewi Roberts

At the last meeting Dewi Roberts gave a very interesting talk on the formation of fluvial potholes. His talk was accompanied by his superb photography taken both above and under the water and which had been taken mainly in welsh rivers.

General View of a pothole by Dewi Roberts

Potholes are an erosional and very visible feature of a fluvial landscape and occur mainly in the upper reaches of a river were the flow of water has sufficient energy to carry the boulders, cobbles , pebbles and sand that are required to cause abrasion of the bedrock. Therefore one of the main factors in pothole formation is hydraulic. They are one of the ways in which rivers erode the landscape but they create superb river scenery.

Underwater image of turbulence moving cobble by Dewi Roberts

Formation begins with a minor depression in the bedrock. The turbulent flow of the river will then develop eddy currents which enter the depression. This will then cause the sediment to enter the depression and begin to abrade the sides and the bottom causing it to enlarge. A type of positive feedback mechanism which causes an increase in turbulence and eddy currents. The pothole will continue to deepen and widen over time unless the erosive effects are halted for some reason. The sediment particles involved in the erosion are known as tools.

Potholes on the Clywedog River by Dewi Roberts

If the pothole continues to erode overtime then eventually neighbouring potholes may coalesce which can lead to the development of an inner channel through which water can flow and produce areas of sculpted rock.

Abergwesyn Common by Dewi Roberts

Other factors that play a role in the formation of potholes will be the type of rock over which the river flows and the presence of joints, veins or dykes. The bedrock topography will have an influence on flow and turbidity and joints can lead to block quarrying which may limit the length of time available for pothole formation.

A thoroughly enjoyable talk especially as we were taken under the water to see the process at work.

Dewi along with Stephen Tooth and Hywel Griffiths have produced a book entitled:

River and the Rock, The - River Potholes of Wales

ISBN: 9781845278878

This should prove a very interesting read and the photography will be superb.

Dewi also has an exhibition at The Aberystwyth Arts centre:

Hydro Psyche

Wed 17 May 2023 to Sun 23 July 2023

“Inspired by the Rheidol, Ystwyth, Dulas and other Welsh rivers, Hydro Psyche explores the benefits of re-connecting with nature. A collaborative project by painter Karen Pearce, photographer and videographer Dewi Roberts and writer and performer Liz Pearce that fuses paintings, film, photography and sound to celebrate the beauty and power of rivers and the life within them.”

Wednesday 19th April 2023

The last talk which was given by Chris Darmon was on the geology of the Canary Islands. Chris has led and will be leading further field trips to the Islands. More can be found about his field excursions at https://www.geosupplies.co.uk/geological-supplies-company.php

The Canary Islands are a group of seven main volcanic Islands located off the north-west coast of Africa trending in a south-west to north-east direction. They are located on ancient oceanic crust of the African plate and are a good example of intra-plate volcanism. The ages of the islands varies between the youngest El Hierro at 1.12Ma in the south west to Fuerteventura at 20.6Ma in the north east. It has been suggested that like the volcanism in Hawaii the islands have been produced by the presence of a volcanic plume over which the African plate moves in a direction from west to east although other ideas have been put forward and the topic is controversial.

Lava flow Lanzarote with columnar jointing by Chris Simpson MWGC

The canary Islands form what is known as the Canary Volcanic Province but this is not the only area on the African plate that is volcanic as the area around Madera is also volcanic and known as the Madeira Volcanic Province and volcanism is also around Cape Verde islands. ( Cape Verde Volcanic Province) But all of these are linked to a much larger area of seamounts and these cover an area that is approx 1300 km long and 350 km wide which is known as the Canary Islands Seamount Province. The sea mounts vary in age with the oldest in the region of 142Ma.

Aa Lava flow Lanzarote by Chris Simpson MWGC

The Canary Islands similar to the Hawaiian Islands, show different stages in their development over time. Firstly there is the submarine stage, followed by shield-building, declining, erosive and rejuvenation stages. The stages commence with the submarine seamount stage which contain submarine sediments and for example, alkali basalt pillow lavas and dyke swarms. This is followed by the emergent stage i.e. the stage at which the volcano is at the surface. It then begins to develop into a large shield volcano characterised by subaerial alkaline basalt and trachybasalt lava flows. As the volcano begins to decline then rocks like phonolite begin to appear. This stage is followed by erosion. Here there is little or no volcanic activity and there is vast erosional removal of previous lavas and ashes. Finally there is rejuvenation with minor volcanic activity and the formation of rocks such as nephelinites. Thus these structures need at least 10-15 million years to go through the various stages.

Multiple balsaltic dykes Fuerteventura by Chris Simpson MWGC

So what are the arguments for the formation of these islands. Firstly the idea that there is a large mantle plume over which the African plate moves but there are many arguments against this idea. Another suggestion is that they are related to a propagating fracture associated with the Atlas tectonic chain. Objection to this hypothesis is that there is no evidence of continuous faults connecting the two areas.
A third idea put forward by Paul van de Bogaard in 2013 is as follows: "shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean and most long-lived preserved on earth.” (van den Bogaard, P. The origin of the Canary Island Seamount Province - New ages of old seamounts. Sci Rep 3, 2107 (2013). https://doi.org/10.1038/srep02107)

The geology of these islands appears to be complex with much more research necessary to understand their geology.

Saturday 11th March 2023

What triggered the Cambrian Explosion by Prof Rachel Wood University of Edinburgh

At the beginning of the Cambrian period there was a huge evolutionary burst of life markedly increasing the diversity of animals which includes many of the major groups of life we see today. The question has always been: “what triggered this event?”. The talk by Prof Rachel Wood” ( University of Edinburgh) on February 13th shows just how much work has gone into that question and that answers are finally being found. What is proposed is that the roots of the Cambrian explosion are not in the Cambrian but in the Ediacaran.

Prof. Wood explained that for the vast majority of time the planet was dominated by unicellular life. Then between 575-540Ma organisms on the sea floor changed dramatically. These organisms - the Ediacara biota, were much more complex but at what point the first animal appears is actually still disputed. Although if one looks at various characteristics that suggest complexity eg. burrowing, surface locomotion or biomineralisation, it can be seen that they do start to appear in the Ediacaran. Then at about 540Ma a huge abundance and diversity of life appeared. This being the “Cambrian Explosion”.

Prof. Wood and her multidisiplinary team along with international collaborators have been working on this problem for many years. The main location for their field work is in Namibia. The reason this area is so important is due to the ability to a.) date the ash beds to a narrow time sequence of between 547 - 538 Ma a critical 10 million year period where they find these first fossils that are known to be bilaterians b)the area is divided into two basins, c) the necessary outcrops are present and vegetation free.

Research site in Namibia

One fossil find they have named Nomacalathus shows bilateral symmetry which suggests it is an animal and although from the Ediacaran biota is very similar to the Entoprocta found in China from about 520Ma. Therefore she suggests that they are now pulling the Cambrian Explosion down into older rocks.

One suggestion as to the cause of this increase in diversity is related to changes in oxygen levels. When oxygen levels increase it leads to an increase in cellular energy production. This could lead to a development of greater complexity with an increase in size, more complex body structures and enable more energy-intensive movements.

Part of the work in this area is in tracking anoxia. For this they use proxies one being the iron speciation paleoredox proxy which can distinguish three major oceanic redox states - oxygenated, ferruginous and euxinic. Anther proxy is the Cerium anomaly which can track manganous conditions. These two provide detail necessary to understand oxygen distribution in the basins under study.

From this work they discovered that the organisms only lived in well oxygenated areas of the basins. To begin these areas were very restricted but as oxygen levels increased so did habitable area which itself allowed spread into new areas and also an increase in diversity. Not only is change in habitat important but most probably nutrient levels will play a part but is difficult study. Individual animal physiology and adaptability will also play a part.

Prof. Wood then summed-up the talk with the following conclusion:

Late Ediacaran metazoans restricted to well-oxygenated, but dynamic, oases

There were differential turnovers and body size changes within the Cambrian

• may have been structured by intervals of dynamic shallow marine
  redox/nutrient input.

Physiology may cause differing responses to these drivers
-created distinct record of early metazoans during the Cambrian Explosion, and also the form of its demise.

Oxygenation expanded habitats - dynamic chemocline - and governed ecological complexity

Cambrian Radiation was not linear, but cyclic, driven by oxic pulses