The Mumbles giant erratic -- progress report

There is good progress on the analyses of the two samples (one sample originally, but cut up to eliminate weathered material and to allow analyses using two different techniques, in different labs) taken by Phil Holden from the famous erratic which he discovered in Limeslade Bay.  (Actually Rob Ixer claimed that we were lying when we said we had said we had sampled the erratic -- which was really rather silly of him.  What would be the point misleading anybody on this?  But one gets used to a certain amount of silliness in all things pertaining to Stonehenge.........)

Anyway, the work is going well, and we should soon be in a position to see the geology reports.

On a related matter, I notice that there have been 1353 reads of this post on the blog:

https://brian-mountainman.blogspot.com/2022/02/the-mumbles-giant-erratic-sound-and-fury.html

In general, I count 500 reads as a "popular post" -- so there seem to be rather a lot of people who are showing an interest in this one, in spite of Rob Ixer pretending that it was all much ado about nothing.

And what intrigues me even more is the fact that the Limeslade giant erratic, on the edge of the Gower,  is on the other side of the Bristol Channel from that famous assemblage of erratic boulders -- great and small -- around Saunton and Croyde on the North Devon Coast.  That all makes sense -- if the Irish Sea Glacier was pushing eastwards, in one glaciation or several, of course it would have dumped a trail of erratics on both flanks.  But of course most of the erratics will have been dumped in the channel that is now submerged.

The Saunton erratic cluster

The Ramson Cliff erratic, about 80m above sea level.  Since this photo was taken it has sunk more deeply into the undergrowth.......

As we dig deeper into the Quaternary history of the Bristol Channel region, the collection of (mostly igneous) erratics around Croyde and Saunton appear more and more important. As indicated by Keene and Cornford in their booklet called "The Cliffs of Saunton" and Madgett and Inglis (1987) there can be no doubt that the 50 or so erratics described in the literature represent the last remnants of an ancient glaciation. Some of the recorded erratics are quite small, but of the 37 larger erratic recorded in the tables below, there are a number (dolerite, rhyolite, rhyolitic tuff and other volcanics) that could have come from North Pembrokeshire, but the geologists who have looked at them prefer, at the moment, to suggest provenances in Scotland or other northern parts of the British Isles.

https://devonassoc.org.uk/wp-content/uploads/2018/11/A-Reappraisal-Madgett-TDA-1987.pdf

These erratics (including one at about 80m asl) must have been a part of a spread of glacial deposits; they cannot all have been dumped from grounded icebergs or ice floes since at times of cold climate seal level would have been many kilometres away from the present position of the coast.

The glacial episode responsible must also have affected (if not overrun) Lundy Island, and the other islands (including Flat Holm) further up the Bristol Channel. Even if the ice surface gradient was very shallow, ice at the same time must have penetrated eastwards past Ilfracombe, Lynmouth and Minehead, and into the Somerset Levels. It may well have impinged upon the west-facing chalk escarpment of Salisbury Plain. It may of course have coalesced with ice coming off an independent Exmoor ice cap. But in general, it may well be that the North Devon cliffline was the prominent landscape feature that resisted the flow of the Irish Sea Glacier and channelled it eastwards.

https://devonassoc.org.uk/wp-content/uploads/2018/11/A-Reappraisal-Madgett-TDA-1987.pdf

https://devongeography.wordpress.com/2021/10/27/coastal-walk-at-baggy-point-north-devon/

The Ramson Cliff erratic  above the north-facing Ramson Cliff, near Morte Bay, about 150m inland and on the edge of the coastal slope.

About halfway along the north side of Baggy, there is another (smaller) glacial erratic sat right next to the path. It is a 500 kg block of epidiorite of Scottish origin. It is a little difficult to spot, as it is surrounded by gorse, and is slowly disappearing into the surface. Part of the surface has been chipped away, exposing the obvious crystalline structure of igneous rock. It once stood upright in the middle of a nearby pasture field and was used as rubbing post by sheep and cattle. In the early 1970s, the field was ploughed and the rock dislodged and then laid prone. It was then dragged to edge of field where it has been ever since. The erratic can be located at grid reference SS 4356 4070.

Dark coloured crystalline rock (epidiorite) adjacent to coastal footpath at c 80 m asl. 

(Photo: Paul Berry)

The Baggy Point (Freshwater Gut) giant erratic

Thanks to Paul Berry for this info on a 50-tonner at Freshwater Gut, Baggy Point, near Croyde.  I have featured this erratic in a number of previous posts, but I found this new source of information:

https://www.geographysouthwest.co.uk/wp-content/uploads/2021/10/PTV-Baggy-Point-Erratic.pdf

The Baggy Point erratic is a large granulite gneiss boulder that has been carried by ice from Western Scotland. It weighs some 50 tons, but sea erosion and encrusted lichens make it a little tricky to spot.

The Baggy boulder is one of a suite of over twenty glacial erratics (according to Paul Madgett, there are 37 of them) that can be found along this stretch of north Devon coastline. Two of them are quite accessible (this one at Baggy Point, and another at Saunton are relatively easy to identify, but most of the others are much smaller and quite difficult to find.

As at Saunton, the Baggy erratic sits on a wave cut platform created from the local rocks. At this location, the foreshore consists of Baggy sandstones that overlie the Upcott slates that form Baggy Point itself. These sedimentary beds of mudstones, siltstones and sandstones were formed in the Devonian Period between 359 to 372 million years ago. They were deposited as horizontal layers on the sea bed, but have since been uplifted and contorted, and hardened into vertically aligned layers. The sea has since carved this rock into sharp ridges and ancient fault lines are marked by long, straight and deep gullies, created where less-resistant beds have been eroded at a faster rate.

Behind the erratic boulder is a clear exposure of an old cliff line formed from Pleistocene raised beach material, or sand rock, that was created in the Ipswichian interglacial. This provides clear evidence of how sea levels at this location have been at different heights in the past.

----------------------------

This massive erratic must indeed have been emplaced by ice, but I disagree with Paul on a couple  of points.  First, the shore platform on which the erratic rests has to be older than Ipswichian -- and indeed, evidence from all around the Bristol Channel suggests that although the raised beach deposits may well be of Ipswichian age, the rock platform is certainly older -- and indeed may be a composite feature, cut and refreshed during a number of older interglacial episodes.

Secondly, the idea of emplacement by floating ice is no longer tenable.  An erratic of this size cannot have been transported on slabs of floating sea ice.  It must have been carried either in an iceberg or in a large and deep chunk of brash ice -- and that means deep water.  Deep water cannot have existed here at a time of iceberg transport, because when the Celtic Sea was awash with the debris of glacier disintegration, the sea was not here at all -- it was probably at least 100m lower down, and most of the Bristol Channel was dry land.  

So the erratic must have been dumped from a melting glacier -- but which glacial episode are we talking about?  Anglian?  Wolstonian?  Happisburgh?  The jury is still out.

See also:

https://brian-mountainman.blogspot.com/2015/01/the-erratics-at-baggy-point-croyde-and_18.html

https://brian-mountainman.blogspot.com/2015/01/the-erratics-at-baggy-point-croyde-and.html

https://brian-mountainman.blogspot.com/2015/01/croyde-saunton-baggy-point-erratics-list.html

https://brian-mountainman.blogspot.com/2015/01/baggy-point-erratic-at-80m-altitude.html

The Devils Stone, Shebbear

 

Thanks to Tony for reminding up of this strange stone in Shebbear, deep in the Devon countryside, 20 kms from the sea.  It's referred to in more articles as a glacial erratic, and one had better believe that, unless one insists on some other fantastical explanation.  it is reputed to weigh about 1 tonne.  It has to be turned over every year, on Nov 5th, by the church bellringers, in order to stop the Devil from escaping.  So there we are then.

There are a few photos of it on the web, and it clearly has one side that is has relatively unweathered, with quite angular edges, and another side that is very crumbly and heavily weathered.  That in itself suggests great age.

Some people refer to it as a pinkish granite, or as a "quartz boulder" -- but on close examination of some of the photos it seems to have a dark bluish colour, and looks as it it is very coarse-grained. It appears to have a red-brown weathering crust, just like many of the erratic boulders on Preseli.   Is it a dolerite or a gabbro?  Interesting........ geological opinion needed.

https://en.wikipedia.org/wiki/Shebbear

 

Rhosyfelin RIGS -- official notification

Here is the 2016 official notification / citation for Craig Rhosyfelin. It has taken a long time to get onto the official record!   It is based on its geological / geomorphological interest, but there is also a mention of the disputed quarrying hypothesis.  What is very intriguing, to all who know this site, is that Parker Pearson, Ixer and Bevins have never acknowledged, in any of the papers published between 2015 and the present day, that their ideas are disputed by anybody. That is not the sort of thing that is supposed to happen in the world of serious scientific research.......  

Say what you like about this blog, but we do at least admit to all and sundry that there are normally at least two different interpretations of anything worth thinking about...... and we have never claimed to be uniquely in possession of the truth.

https://www.pembrokeshirecoast.wales/wp-content/uploads/archive/npa_2016_april_27_20_16_rigs_spg.pdf 

Geology of Brynberian Moor

The surface of a possible moraine 200m from Brynberian Bridge

In my hunt for spotted dolerite sources, I wandered about a bit on the moor today, starting near Brynberian bridge and wandering eastwards as far as Glan yr Afon.  I confirmed that the geology map is not all that accurate.  

In the headwaters of the Afon Brynberian, where several streams flow down to Penllyn, therse is one exposure on the river bank which confirms that the bedrock there is Abermawr Shale, but mostly the bedrock is buried beneath a thick layer of stony till.  Around SN1046348 there is undulating terrain with many dolerite boulders; I think this is a morainic accumulation.  The boulders are heavily abraded and weathered, and most of them are dolerites -- some fine-grained and others coarse-grained and maybe best described as gabbro.  Some appear to be similar to the dark-coloured porphyritic rock with large phenocrysts which outcrops at SN103352 in the old Brynberian Quarry:

Because of the distinct whitish spots which are obvious on the outer exposed surfaces of this rock, I think we could refer to it as spotted dolerite, but the matrix is very fine-grained and almost flinty -- and on balance I think it's probably a lava..........but just a few metres away there is another outcrop of dark blue unspotted dolerite.  As at Cilgwyn, there seems to be a close juxtaposition of rhyolites / lavas and dolerites.  So there must be a sill of dolerite in the Brynberian area, which is not shown at all on the geological map.  How extensive and how thick is it?  Does it run more or less west-east and outcrop again in the Pont Saeson area?  I need to check this out.......

Bedd yr Afanc passage grave lies on the western edge of flattish plateau that runs eastwards for about a kilometre.  All around the monument there are abundant dolerite boulders scattered across the moor, but then they run out and around SN115345 there is an extensive area where there are hardly any erratic boulders to be seen.  There is a thin till, and the bedrock here is tuff and rhyolite, not mudstone as shown on the geological map. Some small gravelly exposures are seen around the steepened edges of the "plateau", and especially where the trackway towards Hafod Tydfil has been cut into the bedrock.

There are good exposures of these volcanic rocks on the east-facing "escarpment" where the streams come together near Glan yrAfon:

Fishguard Volcanics -- rhyolite and tuff -- exposed across the valley 

to the west of Glan yr Afon

Dolerite boulders begin to appear in great abundance near the footbridge near the rhyolite crag on the moor --west of Glan yr Afon

The moor (which is crossed by two north-flowing streams) is littered with dolerite boulders of all shapes and sizes.  Most are heavily abraded and weathered.  Several different dolerites are seen, including some that are spotted.  The frequency of the spotted dolerites increases as one passes eastwards towards the Glan yr Afon trackway, as indeed does the amount of "spottiness" on the boulders.  This is intriguing.  More fieldwork needed.........

 Slightly spotted dolerites from broken boulders on the moorland c 200m west of Glan yr Afon

I am now wondering whether these spotted dolerites have come from the north or the NW, from a previously unrecorded sill.   Interestingly enough, Pont Saeson -- which needs to be investigated -- is only 400m or so from Craig Rhosyfelin, a place which has received a good deal of attention already on this blog..........

Bedd yr Afanc stone sources

 I had a good look at Bedd yr Afanc today, and it was easier to examine than on some previous visits because several gorse bushes have been cleared away.  This revealed the slight mound on which the passage grave is built -- intentionally making the site drier than it would otherwise have been.

Apart from the 16 "standing" or leaning stones there are around 14 which are embedded in the turf.  As mentioned in previous posts, the stones are a mottley collection -- dolerites of various types, ashes, rhyolites and quartz blocks -- all obtained in the immediate vicinity.  there has been no real attempt to find "pillars" -- the stones are highly varied in shapes and dimensions.  I have wondered in the past why this grave site was not built a few hundred metres to the east, on the extensive and rather dry rhyolite bench that runs almost all the way to Glan yr Afon.  I now think it was built where the stones were; around the chosen site there is a litter of gabbro, dolerite, rhyolite and tuff boulders, whereas about 200m to the east there is a sudden break with a gravelly rhyolite terrain on which there are very few erratics.  I can't explain why erratics are abundant here and in the Glan yr Afon area, but not in the intervening area.  Working on it......

Close-up of a broken surface on one of the Bedd yr Afanc upright stones.  I'll call it a gabbro -- it's similar to rocks found close to Brynberian road bridge and also further east near Glan yr Afon.