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......