Top Ten Pembrokeshire erratics

1. Picrite boulder at Porthlysgi, near St Davids.  A very famous erratic made of "picrite" which appears very different from anything known in Wales -- therefore it is suggested that it might have come from Scotland.    It fell over the cliff and was lost for many years, but then it was hauled back up the cliff again in a heroic erratic rescue.  In May 2023 I found it again and recorded its precise position on the clifftop.

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

2.  The Flimston boulders.  There are seven of them in the old churchyard, gathered up from the surrounding area and intended to be used as grave headstones.  Four of them stand in a row and have memorial tablets affixed -- the other three are lying unused in the long grass.  Origins are unknown, but the best bet at the moment is that the boulders have come from the St Davids Peninsula.

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

3.  The Sleek Stone "super erratics" near Broad Haven.  There are several of them, the largest of which weighs c 75 tonnes.  It is a quartz porphyry, according to Cantrill -- possibly from Ramsey Island.  It rests on a broken platform of Coal Measures sandstones and shales.

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

4.  The Ramsey Sound super erratic, made of grey or purple tuff.  Origin: possibly Ramsey Island.  It has probably not travelled far -- there are outcrops of the same rock type in the vicinity.

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

5.  Druidston super-erratic.  An enormous erratic located in the valley upstream from the beach.  Difficult to get at, but it appears to be a dolerite, probably from the St David's Peninsula.

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

6.  St Bride's Haven erratic pair.  These dolerite erratics are on the beach at the head of the bay -- you walk past them if you are walking on the coast path.  They rest on ORS bedrock -- the colour difference makes the erratics stand out prominently..

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

7.  Llan spotted dolerite erratics, Lampeter Velfrey.  Spotted dolerite erratics incorporated into a ruined burial chamber on the lowlands of South Pembrokeshire (nearly all underlain by sedimentary rocks).

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

8.  Glan yr Afon erratic cluster, near Crosswell.  I'm mystified by these, because they are nearly all spotted dolerites -- but found to the NORTH of Mynydd Preseli within sight of the nearest known spotted dolerite tor called Carn Goedog.  But iceflow here was (or so we think) from the north towards the south. Were the boulders carried north by the ice of a small local ice cap?  Did they come from a spotted dolerite poutcrop that we know nothing about?  Or are they not in a natural position at all -- were they collected up by people in historic time, looking for attractive building stones?

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

9.  The Loving Stone, Loveston Farm, near Kilgetty.  This massive igneous erratic is clearly not in its original position.  It must have been found nearby, but it has been moved around in the farmyard, having been treated as a nuisance.  It has probably come from the St Davids Peninsula.

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

10.  The Russia Stone, near Cwm Gwaun.  This massive monolith is one of a cluster of stones known as the Russia Stones. Some of the stones might have been used in Neolithic stone settings, but the farmer told me that this one was recumbent and was moved a short distance so that it could be used as a gatepost.  This stone has probably not travelled far -- there are a number of dolerite outcrops in the vicinity.

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

Note:  I have resisted the temptation to include perched blocks in this list.  There are plenty of them -- but although they may have been dislodged from their original positions by overriding ice, most of them have only travelled a few metres, so they are not quite erratic enough to make the list.

The Cardigan Bay glaciers

From Patton et al, 2013, modified.  The three big outlet glaciers draining westwards from the Welsh Ice cap were the Tremadog, Mawddach and Dyfi Glaciers.  Separating them are the three sarns ++  long ridges of morainic debris exposed only at low tides.  They have much mythology attached ++ relating to Cantreär Gwaelod or the lost Lowland Hundred.  See other posts on this blog.  The confluence or contact between Welsh Ice and Irish Sea Ice shown on this map must have moved eastards and westwards as the relative strengths of the two ice streams changed over time within and between glaciations.  The survival of the sarns shows that the last significant glacial episode to affect this area must have involved an expansion of the Welsh outlet glaciers. They would not have survived an assault by Irish Sea Ice flowingv from the N and NW.

Rapid marine deglaciation : Asynchronous retreat dynamics between the Irish Sea Ice Stream and terrestrial outlet glaciers
December 2013
Earth Surface Dynamics 1(1)
DOI: 10.5194/esurf-1-53-2013
H. Patton et al

 

Outlet Glacier flowlines and surface velocities, after Patton et al.

The view from high above Pwllheli, as it might have been.....

Irish Sea ice and Tonfanau erratics

With ref to this interesting site report from John Mason:

https://geologywales.co.uk/storms/winter14c.htm

I was intrigued by the images of far-travelled metamorphic erratics found on the beach at Tonfanau -- on the northern coast of Cardigan Bay.  Thanks to John for allowing the use of the pics.

John says that these rocks are not local, and that they remind him of some of the rocks in the Lewisian / Torridonian / Moine sequence in northern Scotland. In the text of his blog, John says: 

"All sorts of other rock-types are to be found here (ie on the beach): perhaps the most exotic are the rare boulders and pebbles of high-grade metamorphic rock, reminiscent of the ancient, 2-3 billion year old rocks of NW Scotland. They consist of quartz, pink and white feldspars, glittering spangles of white and black mica and, in some cases, garnets - small examples of which are visible (small, intense red areas) in the photo with the 50p piece."

"There is one area on the beach, usually covered over by sand, where a number of large blocks of these metamorphic rocks lie embedded in the moraine, like the one in the image below (found in 1998), which is about half a metre long. Its more angular-looking underside is where it was embedded in the clay matrix of the moraine. The general scarcity of high-grade metamorphic rocks, and the occurrence of so many of them in the one spot, has led me to suspect that it all arrived together in one mass of ice - perhaps an iceberg, calved off some far-distant glacier and incorporated into the ice-sheet - that subsequently grounded here and, melting away, released its payload of rocks that it had brought from far away." 

Interesting stuff.  It's known that at Tonfanau, in the Devensian Glaciation, Welsh ice from the uplands of Snowdonia and Cader Idris flowed out into the bay across the coast -- but later this ice was displaced by the ice of the Irish Sea Ice Stream, which must by then have been immensely powerful.  I don't accept that these boulders can have been carried by floating ice -- sea level was far too low at the time, near the time of the glacial maximum..

Of course, we know of other assumed Scottish erratics in Pembrokeshire and on the Bristol Channel coasts, but if any of these have come from these ancient rock outcrops in NW Scotland, that means they must have come from north of the Highland Boundary Fault -- but this area was (according to all the text books and learned papers) affected by ice flowing west or north-west, ie on the northern flank of the ice shed.  On the southern flank of that same ice shed the ice fed the Irish Sea Ice Stream and flowed southwards.

This is an intriguing dilemma.  I wonder what the truth of the matter might be?  This might of course be of some relevance to the current Altar Stone debate.......

Patton, H., & Hambrey, M. J. (2009). Ice-marginal sedimentation associated with the Late Devensian Welsh Ice Cap and the Irish Sea Ice Stream: Tonfanau, West Wales. Proceedings of the Geologists' Association, 120(4), 256-274.

 https://doi.org/10.1016/j.pgeola.2009.10.004

BRITICE Devensian ice sheet animation: a model for the Wolstonian?

https://www.youtube.com/watch?v=oABxYza9ELM

This BRITICE animation, from 6 years ago, is suddenly relevant again because of the latest research showing that the pattern of glaciation assumed to be more or less correct for the Anglian glaciation may in fact accurately represent the ice extent of the Wolstonian British and Irish ice sheet.  In turn, the "extreme" model generated for the Devensian seems to fit rather well, with ice extent somewhat greater than that of the Late Devensian.

The model at 21 K shows approximate maximum ice extent. This is really interesting, as it shows rapidly streaming ice crossing western Pembrokeshire, with more sluggish ice in the east.  It shows the thin local ice caps of Exmoor, Dartmoor and Bodmin Moor incorporated into the main body of ice, with Irish Sea ice flowing between them and affecting the whole of the SW Peninsula.  That makes glaciological sense.  Then -- and this is really interesting -- the model throws up a stream of rapidly moving ice pushing far into the Somerset Levels depression almost as far as the English Channel coast.  I had not noticed this feature before -- but of course it backs up what I have suggested, and also the claim (made by Gilbertson and Hawkins many years ago) that glacier ice penetrated as far east as Salisbury Plain.

Watch this space......

The view from Pwllheli

There are always parallels.  I was looking for an illustration of what things might have looked like at the northern end of Cardigan Bay around the time of the LGM  and I found this splendid image of the Lower Skelton Glacier in Antarctica....

Just imagine that the photo was taken above Pwllheli, looking south.  On the right is Cardigan Bay, filled with the streaming ice of the Irish Sea Ice Stream.  On the left are the uplands of north Wales, with mountainous headlands separated by deep troughs carrying ice from the Welsh Ice Cap.  The junction between Welsh ice and Irish Sea ice runs along near the centre of the image, near the line of the present coast, with areas of dead ice separated by tributary streams of Welsh ice which are diverted southwards.  At first the Welsh ice dominates, with the contact zone pushed over towards the right (west) edge of the photo.  Any deposits laid on the coast are related to this Welsh ice.  But then the Irish Sea Ice dominates, pushing the contact zone in towards the mountain front.  Deposits laid along the position of the modern coast are nor related to this powerful ice stream carrying erratics from the north, including some from Scotland.......

There's nothing new under the sun.......

An erratic work of art.........

 

 

One of the family took this photo back in the jolly month of July, showing that erratics are not just of scientific interest, but have an artistic side to them as well.............

On comminution and giant erratics

Igneous geology of the west of Scotland (Wikipedia)

On a walk near the edge of the Nevern estuary (Parrog, Newport) yesterday, I found a couple of small pebbles which I think have come from Ailsa Craig.  They are microgranites with small bluish speckles and signs of some larger lighter coloured minerals, but the matrix is slightly pinkish -- rather than the pure white of genuine fresh Ailsa Crain riebeckite or micro granite.  

I'm not sure how unique the Ailsa Craig microgranite is, since related rocks also occur on the island of Arran -- no matter, since we can be reasonably sure that the pebbles have come from that general area of the Firth of Clyde.  There is nothing similar, as far as I know, on the Isle of Man or in North Wales.

The "curling stone granite" from Trefor Quarry in North Wales looks very different:

The blue, grey and pink granite used for curling stones -- from Trefor Quarry in North Wales.

Wikipedia:

Ailsa Craig is a spectacular, conical island in the Firth of Clyde about 20 km south of Arran (P914119). It is formed by a boss of peralkaline microgranite intruded into Triassic rocks. The microgranite is characterised by riebeckitic arfvedsonite and Zr-rich aegirine (Harding, 1983; Harrison et al., 1987); aenigmatite also occurs (Howie and Walsh, 1981). This distinctive rock-type is a widespread glacial marker southwards on either side of the Irish Sea (p. 160). It has traditionally been a favoured lithology for the manufacture of curling stones (p. 173).

Ailsa Craig curling stone quarry -- human being for scale

Anyway, the really interesting thing about these small pebbles with bluish spots is that they are all very small.  I have not seen one which is larger than a human fist.  Many thousands of tonnes of Ailsa Craig rock must have been removed  by overriding ice, and the original entrained blocks must have been of all shapes and sizes.  There is no reason to think that they were uniquely small before they started their journeys southwards towards Wales or westwards towards Donegal.

One of the assumptions in glacial geomorphology is that as large erratics travel within or under a glacier they are subjected to an assortment of processes which combine to cause comminution -- the gradual reduction in size as the rock mass is broken, broken again and then broken again until there is not much left apart from small pebbles.  These pebbles may be sub-angular, sub-rounded or rounded, and if water is involved towards the end of the journey they may even be well-rounded.

Different rules apply to supra-glacial transport because debris on a glacier surface is not subjected to abrasion or pressure-induced fracturing.  As Lionel Jackson and I explained many years ago in our article on the 930 km long Foothills Erratic Train in Alberta, Canada and the "Big Rocks" erratic cluster near Okatoks, huge rock masses that fall onto a glacier surface as a result of cliff collapse can be carried hundreds and even thousands of kilometres with relatively little modification.  The angularity of the giant erratics and their related debris may actually be increased as a result of frost (freeze-thaw) processes.

https://www.earthmagazine.org/article/stonehenges-mysterious-stones

The biggest erratic at Okatoks -- calculated to be 16,500 tonnes in weight.

Closer to home, we have other giant erratics on the shores of the Bristol Channel, including those at Limeslade, Lydstep, Freshwater Gut, Westonzoyland (now destroyed), Saunton and Shebbear, and on the tip of the South-West Peninsula at Porthleven.  

The famous Freshwater Gut (Baggy Point) erratic, made of granulite gneiss from Western Scotland (photo: Paul Berry).  It is reputed to weigh 50 tonnes.

So why is it that some clasts are comminuted down to pebble size over a glacial transport distance of 500 km, while other giant erratics survive?  Well, it has to be admitted that the great majority of clasts are broken down, while the giant erratic survivors are the great exceptions.  I have speculated before on this blog about the preferential survival of dolerite boulders in transport, and it seems that igneous boulders have a better chance of long-distance survival than sedimentary or metamorphic rocks.  If you look at a typical Pembrokeshire storm beach you will find that the great majority of pebbles and boulders have come from degraded or destroyed glacial and glaciofluvial deposits; maybe 90% of the clasts will not have travelled far, and maybe 10% will be from sources far away.

The clasts found in glacial deposits are typically of all shapes, sizes and lithologies, with variable surface characteristics as well.  Some will be polished and striated, and others will not be.  Some will have the "ideal" bullet shape, like the famous Newall Boulder found at Stonehenge, and others will be roughly rectilinear or even roughly rounded. 

 

So to answer the question raised above, I will have to say that we currently do not know why some giant erratics survive while others are broken down into small pebbles.  My best guess is that every clast undergoes a unique journey, related to its changing position in, or on, or under the ice; related to ice temperature and velocity and other glaciological conditions; related to rock type and internal structure; and related to distance travelled.  Is all of this random?  Well, not really -- the laws of physics apply, but as yet we do not fully understand them.  But "chance"factors come into play  -- for example when one clast in a vulnerable position is suddenly assaulted by something harder, sharper and heavier........

One final point.  Giant erratics are NOT restricted to the intertidal zone around British coasts.  That is a myth repeated over and again, even in learned publications.  So their distribution has nothing whatsoever to do with transport by floating ice.