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Saturday, 2 May 2015

Lake Brynberian -- further thoughts




I have speculated a lot in the past (and so have others) about the presence of an ice-dammed lake held up against the north face of Mynydd Preseli.  Well, I wandered about quite a bit yesterday across Brynberian Moor (it's not often dry enough under foot to be able to do that at this time of year) -- partly in search of lake sediments.   The BGS map of the area (Fishguard sheet No 210) shows the whole area as covered with Late Devensian till -- described as gravelly sandy clay.  I think I'd agree with that, at least for the upper part of the gentle sloping depression seen in the top photo.  There are a few gentle sloping shelves of bedrock, and occasional outcrops of ashes, rhyolites and dolerites can be see.  Everywhere there is a litter of boulders and smaller stones -- of the same rock types.  These are for the most part sub-angular or sub-rounded, and are typical of what we would see in an area of morainic litter following a glacial retreat.  I did not see any proper exposures of thick till, or of fluvio-glacial materials.  No obvious foreign erratics either.   There doesn't seem to be much in the way of periglacial slope deposits either -- not surprising since surface gradients are generally less than 2 degrees and since there are no rock faces or steep slopes which might have provided frost-shattered debris for downslope transport.  And not a single pingo to be seen.......

Where there are small exposures as a result of stream erosion, we see this sort of thing:


These are essentially surface exposures of broken bedrock and very thin till with glacially-transported boulders scattered about on the surface.

However, once we drop down towards the 130m contour, things begin to change, and I now think that at about 138m there may be an ancient shoreline.  There is no obvious break of slope, but a subtle change in the vegetation suggests that something is different in the character of the surface sediments, and sure enough, we start to find a bed of clay in the stream cuttings above the cottage called Glanyrafon Uchaf, round about SN119345.  The streams here are the headwater streams of the Afon Brynberian, which flows past a place that goes by the name of Craig Rhosyfelin, a couple of km to the north......

This clay bed is at least 1m thick, and it is not a till layer because it is quite stoneless, grey in colour and very sticky indeed.  That suggests a relatively small amount of silt and sand in the mix -- and also suggests stillwater sedimentation some distance from a sediment source.  Here are a couple of pics of this layer:


The upper one of these two photos shows a thinnish bed of clay about 30cms thick, resting on a thin iron-stained gravel bed.  In the lower photo the clay is clean, with some evidence of darker organic material in places, and with many signs of gleying -- the orange "speckling" is typical. As far as I could see on the basis of a rather hurried examination, there are no laminations or varves to suggest an annual cycle of sedimentation.  But I need to look at more exposures to check that out........  There is no till on top of this clay bed, so it was deposited after the till was emplaced across the greater part of this gently sloping area.  On top of the clay we just see modern peaty soil up to 20 cms thick.

So for what it's worth, my hypothesis for the history of the Preseli north slope is as follows:

1. Glacier ice from the north pressed up against the north face proper, and affected the crags of Carn Goedog and Carn Alw.

2.  Ice wastage, leaving an extensive plain of broken rock outcrops, scattered till and erratics -- and a classic tundra environment. Presumably, at this stage, meltwater was able to escape freely, either along the ice edge or subglacially.

3.  For some reason, the creation of an ice dam -- possibly as a result of short-lived ice thickening and a temporary advance interrupting the general process of ice edge retreat.  Meltwater is impounded and a short-lived Lake Brynberian is created, with a surface level somewhere near the 138m contour.  The ice edge might have been at least a kilometer away from the exposures at Glanyrafon Uchaf,  which might place it somewhere near Rhosyfelin!

4.  Continued deglaciation and drainage of the lake, probably via the Brynberian gorge and into the Nevern Valley.  Thus might have been a catastrophic process, and might explain the evidence of very turbulent and powerful meltwater flow at Rhosyfelin.

5.  During the post-glacial period, development of the drainage system that we see today, and accumulation of peat beds on the lower parts of the slopes.

Now I've got the bit between my teeth -- watch this space......

 

6 comments:

Hugh Thomas said...

Im so glad you have had a chance to have a look across Brynberian moor , I have often wondered if there was indeed once a lake here. The lie of the land is quite deceiving as it is not until you really get down into it can it really be possible to see just how it undulates then dips to the north .
When you really get down into this area apart from the streams the lie of the land seems upwards in every direction . It was only after seeing this for myself I began to wonder what would have held the water back , a moraine or ice dam I wondered in my non expert head. This told me the ice HAD to have reached the outcrops on the northern slope at least once , all makes perfect sense once we actually get out there and have a look.. Is there any way of knowing which period the ice flow was intense enough to pick up and move the stones south and eastwards ? Whenever I read about it , it seems to almost always be simply described as "during the ice age"... Would be interesting to know roughly when if that was possible..

BRIAN JOHN said...

Yes, Hugh -- there is much more variation on the moorland than one thinks when viewing from a distance. I think the crags were affected by ice during the Late Devensian -- including Carn Goedog and Carn Alw. There are moulded surfaces everywhere, and I don't think they can have survived, looking quite so fresh, if they were Anglian features created 450,000 years ago. Two glaciations, one in the Anglian and the latest in the Devensian, around 20,000 years ago.

Evergreen said...

Brian, what I know about Geology you could write on the back of a particularly small stamp, what makes something like the parallel roads of glenroy so obvious but the shoreline here more subtle? Or am I barking up the wrong tree completely with that line of enquiry?

BRIAN JOHN said...

Perfectly sensible question. The extent that a shoreline will leave a real trace in the landscape depends on many factors. Probably the two most important are the length of time over which a water level is maintained, and the type of shore processes operating. If there are big waves and a long fetch, shore processes might be quite dramatic. If a lake is small, and frozen for a lot of the time, shore processes might be ineffective. If a shoreline is on solid rock, even a long "stillstand" of water-level might leave little trace -- but if a shoreline is on thick unconsolidated sediments (such as till or fluvioglavial gravels) a distinct notch might be cut, since even small lapping waves might be able to remove finer materials from the matrix. If sea ice or lake ice is present, all sorts of other processes come into play.... I did a big report once, with David Sugden, on high-latitide coastal processes. It's free on Researchgate, if you want a link....... fascinating subject!!

BRIAN JOHN said...

.... or you can of course read "Acts of God" (available for a small fee, from Kindle) in which some of these processes are described, with rather dramatic consequences! End of plug.

Evergreen said...

Thanks Brian.