Friday 3rd November 2023

Quartz and other forms of silica by Bill Bagley (chair MWGC) 18th October 2023

All photos by Bill Bagley

Bill commenced the talk by showing that the vast majority of minerals that make up the rocks of the earth’s crust are silicate minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and many clay minerals. These silicate minerals account for 92% of the crust. Quartz forms 12% and it was on quartz that Bill concentrated the rest of the talk.

The building blocks of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. These silica tetrahedral units can share oxygen atoms and be linked in a variety of ways, which results in different structures

Quartz is composed of pure silica, SiO2 with the tetrahedra arranged in a three-dimensional framework. It is one of the most commonest minerals on earth. It can be found in many geological environments and is a constituent of many rocks. Quartz occurs in either a crystalline or cryptocrystalline form. The crystalline forms of quartz occur as distinct crystals for example amethyst, citrine and smokey quartz, whereas cryptocrystalline quartz is composed of inter-grown aggregates of microscopic or submicroscopic quartz crystals for example agate and onyx. The cryptocrystalline form of quartz may be divided into two general classes i.e. fibrous (eg. agate) and granular (eg.jasper)

Pure quartz is usually clear or milky in colour but the presence of other elements in the lattice structure or structural change produces a myriad of colours. For example amethyst (violet/purple) is due to the presence of iron within the lattice; rose quartz due to the presence of titanium; smokey quartz due to radiation damage.

Jasper, Mookaite var.

Cryptocrystalline quartz is coloured similarly. For example agate which is characterised by concentric banding of colours is formed by silica-rich groundwater entering a void in a rock eventually forming a thin layer of chalcedony. This is then repeated over time. The differing colours are due to the presence of different impurities in each successive inflow of silica-rich groundwater. The most frequent impurities being iron oxide (red, brown or yellow); manganese oxide (purple or pink); chlorite (green); hematite (black).

Australian agates.

Bill also explained some of the uses that silica can be put to both from the past and newer technologies.
The use of quartz dates back to the Palaeolithic where flint was used to make hand axes and scrapers.
In more modern times quartz is used in large quantities as it is an essential ingredient in concrete and mortar. Sandstone and quartzite are used in buildings and crushed in road and rail construction. Further, the properties of quartz means it has a wide variety of uses. Quartz has a hardness of 7 on the Mohs scale and may be used as an abrasive (eg. Glass grinding). It has a high melting point and can be used in the manufacture of refractory products like firebricks. Quartz crystals possess a property known as the piezoelectric effect. In other words it is the appearance of an electrical potential (a voltage) across the sides of a crystal when you subject it to mechanical stress. This for example is used in a quartz watch but there are many other uses in electronics.

A very interesting talk.

Monday 21st August 2023

Chris Simpson gave a talk summarising the main features of the geology of the Northwest Highlands of Scotland based on a 2022 guided tour with Chris Darmon and Colin Schofield.
The area is famous in geological circles because of the "Highlands Controversy" which divided 19th Century geologists – how could it be that old rocks should be found lying on top of younger rocks? The British Geological Survey sent two geologists to conduct an in-depth survey of the whole area and settle the controversy once and for all. Messrs Peach and Horne did just that. They found that older rocks were indeed emplaced overlying younger rocks as the result of large-scale thrust faults. Crustal compression during the Caledonian Orogeny had pushed millions of tons of rock over many miles to produce the Moine Thrust in the Northwest Highlands (and the Outer Islands Thrust Fault in the Outer Hebrides).
Chris also showed pictures of many of the other geological delights to be found in the Northwest Highlands.
(Summary and photos by Chris Simpson)

The Moine Thrust as seen at Knockan Crag. The dark layer at the top of the hillside is Moine schists which are thrust over pulverised Durness Limestones.

Close-up photograph showing contact between the Moine schists and Durness Limestone

An information board about the Glencoul Thrust.

The Glencoul Thrust as seen on a cloudy day.

The chance sighting of a vogesite sill alongside the A837 a few miles South of Inchnadamph.

The vogesite sill intrudes the Ghrudaidh Formation – dolostones forming the base of the Durness limestones.
Vogesite is a plagioclase-rich lampropyre

The “multi-coloured rock stop”. Three ages of rock intermingled. Gneiss around 2.8Ba, basaltic dykes around 2.3Ba and pegmatitic granites around 1.75Ba

A close-up photograph of the pegmatitic granite -- Ignore the rock drill marks made when this road cutting was made. Nobody knew about this conjunction of rocks until the road was dug!

Achmelvich. Banded gneiss with olivine nodules.
The presence of these nodules suggests that the gneiss originated from a piece of oceanic crust

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.”