THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Tuesday, 21 October 2014

Pre-Devensian glacial deposits south of Preseli


Fluvio-glacial sands and gravels (shown in pink) in the area around Llangolman and Maenclochog, to the south of the Preseli upland.

 Some more original field research here, for those who may be interested.......

Thanks to Huw Absalom of Bush Farm, Llangolman for the opportunity to look at his splendid gravels today.  Very interesting indeed.  I have been frustrated for some time because although there are plenty of patches of glacial and fluvio-glacial deposits in the central and southern parts of Pembrokeshire, as shown on the geological maps, they are very seldom exposed for proper examination.  They should be pre-Devensian if we have got our ice limits in the right places -- but hard evidence of great age was required.  

So when I got a message from Sid Howells telling me that Huw wanted somebody to take a look at the gravels in his small pit close to the Church of St Colman, I got onto the phone and organized a visit.  So I have been over there today, and things are starting to look good.

The pit (grid ref SN 115268) is not a commercial one -- it's just used for providing hardcore and concrete ballast for use on the farm.  It's about 30m across and 10m deep, exposing fluvio-glacial gravels and sands in the full height of the face.  It's cut into the top of an undulating terrace which has clearly undergone a long history of erosion, especially on its northern flank, where the slope drops down into a small tributary valley which is part of the Eastern Cleddau river system.  The valley is about 25m deep.  Huw tells me that clay is exposed on the floor of the valley, and that there are also exposures of sand not far from the stream.  It's a reasonable assumption that clay-rich till underlies the sands and gravels.

The exposed gravels are relatively  fine-grained, with no cobble-sized beds or boulder beds -- indicating turbulent but not violent torrential flow.  There are a few interbedded sand horizons, suggesting periods of quieter flow.  The bedding dips quite steeply from NW towards SE -- so this may indicate the presence of an advancing delta front close to an ice edge.  This is supported by the presence of some larger stones in the gravels, up to football size, and quite angular.  There is one large chunk of bluish rhyolite which looks as if it has come from a larger boulder.  I didn't see any striated pebbles, but I would not be surprised if some were to turn up......  But the gravels are packed with erratics, including assorted Fishguard Volcanics, rhyolite, dolerite and one quite distinctive pebble of Carboniferous Limestone.  (Where on earth did THAT come from?)

Typical gravels exposed in the Llangolman quarry face.  Note the overall alignment of long axes from top left to bottom right.  Note also just how many pebbles are heavily weathered if not rotten.

One of the black layers cemented with manganese oxide, above a sand bed about 20 cms thick.  


Fragments of the lowest black "manganese concrete" layers in the quarry, which was so hard that it had to be broken up with a tractor grab.  Note also traces of foxy-red "iron oxide" concrete.

I have never seen so many layers of manganese concrete in a single small quarry face.  There are at least six of them, including one just a metre or so beneath the ground surface.  This fact, together with the extremely rotten character of many of the pebbles in the gravels, is suggestive of great age.  Maybe we are looking at deposits from the Anglian glacial episode of about 450,000 years ago.

Great age is also indicated by the presence of "churned gravels" near the ground surface which have lost all trace of their original bedding, and several distinct fossil ice wedges and frost fissures within which many of the pebbes are "standing" with their long axes vertical.  That is indicative of pebbles falling or sliding down into an open frost track or wedge from which the ground ice has temporarily melted.  The assumption?  Thick and long-lived permafrost -- maybe for many thousands of years.



The two photos above have been annotated to show the outlines of the ice wedge casts.  Each one is between 2m and 3m deep.  Note how the gravels and sands are "churned" within the casts and normally bedded on the flanks.  I have also annotated the surface layer of gravels, where signs of frost heave are abundant.  This layer is sometimes just 50 cms deep, and in other places well over 1m deep.

Next time I visit the pit, Huw has kindly offered to be in attendance with his digger -- so we can go down deeper, and maybe find some till........

And speaking of till, it is exposed on the flank of a small stone quarry at Plascwrt, at SN 118274.  It's clay rich, with smallish pebbles and cobbles some of which are well rounded far-travelled erratics.  It is friable to the touch, and foxy red in colour.  So again the impression is that it is severely weathered as a result of many thousands of years of exposure to weather of many types.....


The reddish till layer above fractured bedrock in the small stone quarry at Plascwrt.  It is clay-rich and contains abundant erratic pebbles.


Tentative conclusion: these are very old glacial deposits which display a quite different combination of features from those exposed on the northern side of Preseli, which are deemed to be of Devensian age.  My guess is that these are of Anglian age -- or maybe Wolstonian.  (That's a glacial episode that we know very little about, at least in Western Britain.)





15 comments:

Myris of Alexandria said...

As before it is difficult to see some of the detail, not difficult to believe you see it, or that it is there. You show again that as with many aspects you need years of experience before seeing anything.
These are amongst your most valuable and best posts even those we see them but dimly.
You have to know what to look at and for.
Lovely post.
M

BRIAN JOHN said...

Thanks Myris. I have mixed feelings about putting red lines on photos, because one is "interpreting" reality -- but I think they do help those who may not be specialists. If you look very carefully the stones in the active layer and in the wedge are "churned" whereas the stones elsewhere are more or less aligned as they should be, or as they were laid down. I say "more or less" because some of those stones are churned too -- but one has to draw a line somewhere. When I get the face freshened up by the farmer's JCB, more will surely be revealed......

TonyH said...

My brother and I did see glacial till (which is much more recent than Pre - Devensian) exposed in a hedgebank to the west of Glandy Cross and the Meini Gwyr embanked stone circle in 2012.Glandy Cross appears on the map you have shown.

BRIAN JOHN said...

Now that's rather intriguing, Tony. On what basis do you say it's more recent? (Sounding like a crusty old tutor here!!) But interestingly enough, the geomorphologists who were looking at the glacial features in the Teifi Valley were pretty convinced that a lobe of ice pressed southwards through the gap near Crymych during the Devensian, reaching somewhere around Glandy Cross. I'll check out their map......

Alex Gee said...

I presume that the Carboniferous Limestone clast came from a neolithic quarry in Eire?

Manually transported to Pembrokeshire because of its distinctive bright colouring.

Bet it was found in the pool of a healing spring!

Admittedly this hypothesis is pure conjecture and could possibly turn out to be a load of old fanny!

TonyH said...

I bow to your vastly superior geomorphological knowledge, Brian! I was merely attempting to interpret what you've said in your Post, obviously I misunderstood. My comment was meant to acknowledge that my remarks were not central to your main point. Will go away and read your Post again, more slowly! I am an OAP now, perhaps that is the problem.

BRIAN JOHN said...

I'm not questioning what you assert, Tony! You may well be right that there are fresher tills in the Glandy Cross area -- would be good to know an approx grid reference. Next time I deliver some books to Caffi Beca I can take a look!

BRIAN JOHN said...

Alex, I'm not sure it is Carb Limestone, in the absence of a bottle of HCl at home! Have sent a fragment to Myris for checking -- developments awaited.

Myris of Alexandria said...

Channelling Dr Ixer I can confirm the lithic is a fine grained shelly limestone.
Qtz will not scratch but lmst will when a steel blade is drawn across the surface.
Cannot confirm Carb Lime but def a lmst.
Usually Dr Ixer charges 40 pounds sterling as that is the price of a good whinge. But this time real data (as ever) is FREE and despite the temptation no bleating.
I think Dr Ixer is about to put more papers on line - you know the price.
M

M

Myris of Alexandria said...

Oh and the possible porphyry is a meta-sediment a conglomerate with little amethyst quartz pebbles.
M

BRIAN JOHN said...

Ah, thank you, Myris -- glad =to see that my geology is not entirely up the creek. So where might Carb Limestone have come from? (This looks very different from the Ordovician & Silurian limestones that occur in thin beds in various parts of Wales). Has to be either Ireland or Anglesey.......

And the other bit, from Abermawr -- so not volcanic, but sedimentary. Interesting....... and where on earth might THAT have come from?

Myris, your real reward for professional scrutiny will be in heaven, but who knows? "Acts of God" might just come winging its way to you in time for Christmas......

TonyH said...

Grid reference near Glandy Cross was west of Meini Gwyr embanked stone circle which N.P.Figgis gives as SN 14172658 in Prehistoric Preseli: a field guide [revised 2010]

This is of course very close to the cross roads where stands the petrol station to the east.We observed the till some distance to the WNW, from memory. I may have a photo or two. Just walk around beyond the stone circle's remains up to 100 metres roughly WNW.

Penrodyn said...

I accidently came across your post on the gravel pits next to St Colman Church. I was a boy growing up in Llangolman Farm in the 60s and 70s and I know those pits well, I now live in the USA. Here is some additional information, there is indeed a lot of clay on the valley bottom, I used to dig it myself to make pots. There are still large pits in the wood next to the Farm, we were told these were minded for the clay (blue boulder clay). Further upstream there is also another location at LLyn where clay was also dug. At the base of the hill where the Absalom pit is (are grew up with Huw) there is a fine water spring that supplies Llangolman Farm - I do miss that water. I've always been curious about the gravel pits and suspected glacial origin. I was therefore very pleased to see someone write about these. If you have any further information, I'd be happy to hear. I maintain the Llangolman Wikipedia page so adding a bit more would be worthwhile. My email is hsauro@gmail.com should you want to get in contact.

BRIAN JOHN said...

Thanks for the contribution, Penrodyn! Good to get some extra info here, and it's interesting to know that there is indeed clay around the place, stratigraphically beneath the gravels. This is an interesting question -- is the clay laminated, with recognizable stripes or layers in it, or is it massive and homogenous with occasional stones that had to be extracted before it could be used for pottery? The former would make it lacustrine, and the latter would make it till. In reality, it's not always possible to distinguish the one from the other......

Penrodyn said...

From what I remember the clay was fairly homogenous, don't recall layering.