The Late Devensian trimline on the north face of Preseli

Brynberian Moor and the north face of Preseli

I have mused on many previous occasions on the extent of Late Devensian ice in the great basin on the northern flank of the Preseli upland ridge.  The "reading of the landscape" and the interpretations have to be very subtle indeed, because this is a landscape free of great dramatic landforms and thick sediments.  I freely admit to being confused!  But I think things are gradually coming together....... and today I had a fabulous walk from Tafarn y Bwlch to Carnau Lladron, on which I discovered many new things.  

Here are some of my earlier observations, partly made on a walk with Chris:

 https://brian-mountainman.blogspot.com/2016/08/the-devensian-preseli-ice-cap.html

On previous visits I have come to the view that around 340m altitude, on the mountainside, there is a subtle change from blockfields with thin till and many dolerite rock outcrops to thicker till with some surface expression.  This till extends right across the depression in an unbroken sheet, with some higher mounds and ridges and many small stream valleys cut into it.  There was, I think, no big "Lake Brynberian" between the edge of the Irish Sea ice and the upland ridge -- but there are intermittent thin clays that look like lacustrine deposits, so there may well have been short-lived bodies of impounded meltwater during the ice wastage process.

The Hafod Tydfil trackway, cut across clay-rich till on the floor of the depression.

Striated cobble of rhyolite (?) found in fresh till adjacent to the Hafod Tydfil track, SN114341

Today, in examining the stream cuttings on the slope above Banc Llwydlos,  I came to the view that the higher exposures in the cuttings are cut into bedrock, with a thin veneer of till that seems to be local (from the Preseli ice cap?) but that there is a substantial change in the nature of the sediments at about 335m altitude, or a little above.  That's very close indeed to my estimate of a few years ago -- and it matches, too, with my observations from the Carn Goedog - Carnalw area, and with my conclusions that both of those tors were inundated by Irish Sea ice coming in from the north.

My working hypothesis of a trimline at around 340m seems now to be supported by additional evidence, and I see no reason to revise it.  

So the ice margin postulated by the Geological Survey (above) is right in general terms, but not in detail.  On the north face of Preseli it is too low, and needs to be pushed further south to approx the 340m contour..  Also, in the light of the morainic accumulations around Gernos Fawr, at Cilgwyn and other locations I am assuming that the Carningli - Dinas Mountain upland was overridden by Devensian ice, so the ice margin in that area needs to be pushed to the south of Cwm Gwaun, as shown in this map:

Watch this space..... work in progress......