The Foel Drygarn quarries

 

I have done a number of posts on the Bronze Age and Iron Age quarries near the summit of Foel Drygarn.  I was up there a couple of weeks ago, and this is one of them.

  I am quite convinced that the quarrymen were quite disinterested in large blocks of stone, and targetted rubble and small stones that could be used in the 3 spectacular burial mounds on the summit and in the defensive embankments around the settlement site.  In most cases all they had to do was to take loose stones from banks of scree -- but in some cases they have clearly taken loose stones from the degraded flanks of the rhyolite and micro gabbro tors.

Nothing much was needed in the way of technology, although I would guess that wooden levers might have been used in order to extract the stones from the ground.

A Foeldrygarn puzzle

This is a photo of one of the microgabbro outcrops near the summit of Foeldrygarn.  The outcrop is a small one, surrounded by grassy banks.  It's a degraded small tor, but here the focus of interest is the semi-circular arrangement of stones set in the turf.  This is quite unlike anything I have seen elsewhere in Preseli.  There are plenty of semi-circular stone banks set against small cliffs in the uplands of Preseli, but these are characteristically composed of curving lines of boulders or low ridges up to 2 m wide.  Some appear to have been low walls that maybe supported roof timbers -- with the upper ends of the timbers supported against the rock face.

Here on Foeldrygarn we see a "band" of large boulders or slabs, many of which seem to have been placed end to end...........

The site lies within the summit hillfort and not far from the triple Bronze Age burial mounds -- all made with locally quarried stones which were carried short distances uphill.  The stones were all small enough to carried by one man -- or sometimes two men working together.  Large boulders and slabs were rejected on the grounds that they were unmanageable -- and maybe this holds the clue to what was going on.  Maybe the "band" of large stones was made entirely of rejected stones, pushed aside while the smaller stones and finer debris fragments were carried away for the construction of bural mounds and later for the fortified mounds that are prominent features of the site?

Has anybody seen anything like this elsewhere?  Any other theories about what was going on here?

Carnalw

 Another of Hugh Thomas's wonderful Preseli images -- this one shows Carnalw, near the eastern end of Mynydd Preseli.

The Blatten Glacier Disaster

Before.........

.... and after

There is a fascinating piece of reportage (with spectacular animation) on the BBC web site, relating to the recent disaster in the Swiss Alps.

https://www.bbc.co.uk/news/resources/idt-c7f929de-96a9-45e5-b1bb-31de82fce72d

Thank goodness there was no loss of life, since the idyllic village had already been evacuated, prior to the catastrophic event.  I thought, before I looked into this, that the "glacier collapse" took place in the upper part of a regenerated glacier, like that of Supphellebreen at Fjaerland in Norway -- but no, this was a very small and very dynamic independent glacier (Birch Glacier) in a high mountain environment.  High rates of snow accumulation, steep slopes, and rapid ice flow under the influence of gravity, on slopes dominated by frost shattering and rockfalls. Permafrost melting and rainfall in the Alps (as distinct from snowfall) were contributary factors in causing additional debris mobility.   

The old photos show a substantial fan of debris close to the village, with no clear terminal or other moraines.

There is no doubt at all that collapses like this will now occur with increasing regularity in all of the high mountain environments where there are thousands of vulnerable glaciers of this type.......

(For very good enlargements of the images, just click on them.)

The other evening.......

 Thanks to Lynne for this photo taken at the beginning of my talk the other evening at Canolfan Bethlehem, Newport.  There was an excellent response from the audience, which included many who have also heard MPP reporting on his "discoveries" during and after his September digs in North Pembs.

Just in -- the following comment:  ".........really enjoyed your talk last week. It was so engaging that I don't think anyone minded the hard pews! We thought the talk was a nicely balanced assessment of the pros and cons of all 'the available theories' about how the Bluestones were transported. You have, as one of my bosses used to say, 'relentless pounding logic' on your side. Very well done.

An Irish erratic near Cardigan?

 

I found this small erratic of Carboniferous Limestone in the Irish Sea till at Gwbert, near Cardigan, today.  It has two beautiful brachiopod fossils in it, and as befits a far-travelled stone its surface is covered in striations.  

Where has it come from?  The best bet is that it has come from somewhere in central Ireland, and limestones rich in braciopods seem to be especially common in the Burren area on the west coast.  Here is an example:

However, from what we know of past iceflow directions, a source somewhere near Dublin seems more likely:

Geological map opf Ireland -- the light blue tint shows the area of Carboniferous Limestone outcrops.

If the erratic really did come from Ireland, there must have been much more mixing of ice streams and erratic material in the Cardigan Bay -- St Georges Channel area than has been previously recognized............

We also have to consider Anglesey and the  North Wales coast as source areas, although the outcrops there are of very limited extent.  This is from the GeoMon Global Geopark web site:

.............highly fossiliferous limestones outcrop all long the east coast southwards from Lligwy to Penmon [and along the coast of North Wales from the Great Orme to Prestatyn]. These rocks were laid down in cycles as the sea transgressed and retreated as many as 11 times over North Wales. On Anglesey, only four of these cycles, which consist of limestone followed by sandy beds and then mudstone were deposited. The rocks are rich in brachiopods and corals and often exhibit palaeokarstic surfaces and rarer sandstone pipes.

An Anglesey origin for the erratic is, on balance, most likely since this accords most closely with the ice flow pattern shown on the map above.

An interesting dilemma.........

Gwbert raised beach platform

 

Down on the Teifi Estuary this morning checking out the situation on the east side of the river mouth. There used to be amazing exposures of Irish Sea till here (back in the 1960's) but they were destroyed with the building of new coastal defences and a new caravan park.  

The Irish Sea till is still there, in patches, resting on the old raised beach platforms which are still visible.  These are the counterparts of the raised beach platforms exposed at Poppit, on the other side of the river mouth.  There are occasional exposures of till, fluvio-glacial materials and slope breccia, but because there has been so much slumping it's almost impossible to see stratigraphic relationships. 

The higher parts of the raised beach platform seem to be well above the influence of current HWM coastal processes, so  I interpret the smoothing and rounding of bedrock undulations to glacial erosion prior to the deposition of the till.