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Saturday 9 May 2015

Devensian Till on Brynberian Moor

After my most recent trek across the trackless (well, almost) wastes of Brynberian Moor in the company of Chris Johnson, I'm more convinced than ever that there is no extensive spread of lake deposits.  So let's forget the idea of an extensive Lake Brynberian impounded against the face of Mynydd Preseli.  I have always been a little concerned about this theory, since if there had been a large lake there should be a deep spillway across the col somewhere near Tafarn-y-bwlch which carried water down towards the Gwaun Valley.  There isn't one, so although some maps from other geomorphologists have shown this lake with a surface altitude of c 285m, let's now forget it........

There do appear to be some thin lake clays, but they are at a much lower altitude, around 130m on the moor near the cottage of Glanyrafon Uchaf.  How deep was the water?  It's difficult to say -- but probably in excess of 10m?  And where was the ice edge at the time?  And where were the main inputs of debris-carrying meltwater?  And was the Brynberian river gorge -- running past the site of Craig Rhosyfelin -- the main meltwater escape route?  Questions, and more questions...... but my working hypothesis is that one or maybe several ephemeral lakes were present here on these lower slopes for maybe just a few decades or centuries during the period of ice wastage around 20,000 years ago.

As far as the moorland is concerned, there are surface exposures of clay-rich till all over the place.  The top photo shows such an exposure on the trackway leading across Waun Brwynant towards Hafod Tydfil.  The middle photo shows about 50 cms of churned-up clay-rich till overlain by about 20 cms of peat.  There is a widespread cover of peat, but I'm not sure that it is thick enough anywhere on this moorland to have been used in the past for peat-cutting "turbaries".  (In contrast, there are a number of sites high on the Presely ridge where the traces of past peat cutting can still be seen.)

The bottom photo also shows churned-up till about 60 cms thick on the moorland below Hafod Tydfil.  The churning suggests that there has been permafrost here, and that periglacial conditions existed at some time following the deposition of the till.  Although the best exposures are in stream cuttings, and are thus difficult to excavate and examine properly, they look to me like periglacial involutions and frost-heave features.  I do not think they are loading features such as we sometimes see in flow tills and other sediments that have behaved like very mobile fluids when additional loads have been dumped on top of them.  In true loading features, we would expect to see a layer (or several layers) of additional sediments on top of those displaying the structures -- and here there are no loads anywhere to be seen!  So let's live with the periglacial hypothesis for the time being.  It would make sense, since almost everywhere else where we find Devensian glacial and fluvio-glacial deposits in Pembrokeshire, we find within their uppermost layers signs of permafrost structures like fossil ice wedges and involutions, or else -- where there are substantial slopes and good supplies of slope materials -- a layer of "upper head."

So I'm going to stick with the idea of an extensive till plain here, with occasional patches where real morainic features project above the level of the wide stream basins.  There is more topography here than we might think when we look at the area from a distance, or look at satellite images.  Actual relief is more than 5m in many places -- and if we examine the mosaic of vegetation we can see areas of sphagnum, grasses and rushes interspersed with drier areas of bracken, with low rises or ridges of morainic material with boulders littered about and with vegetation dominated by gorse bushes.  There are some rock outcrops too.  And erratic boulders everywhere -- none of them very far-travelled, but all showing signs of wear and typical facets arising from breakage in transport.  Much more work -- and time -- is needed to sort out the glacial geomorphology of this vast wilderness which stretches for more than 6 km along the north face of the mountain.......

1 comment:

Hugh Thomas said...

Very intetesting stuff and helpful too in trying to gain a perspective on the area.