Bwlch-gwynt and the LGM trimline

Stony till made of local materials, exposed in the footpath to the west of the main road at Bwlch-gwynt. At about 420m this till fades away and is replaced with slope breccia at the surface.

 I have speculated previously on the possibility that glacier ice from the north has "spilled over" the  Bwlch-gwynt col, leaving thin patchy till and other traces -- up to a certain level.  The other day I climbed up the the summit of Foed Eryr and my suspicions were cemented, if not confirmed.  As one walks westwards up the path from the car parking area, there is a great deal of variation in the cobbles and fragments exposed in the footpath and in small cuttings.  Fragments are mostly of shale or slate from the Aber-mawr Formation, but there are also fragments of rhyolite, dolerite and vein quartz -- all rather local, but jumbled up together in a fashion that makes me think "till" rather than "slope breccia".  There is also a silt/clay matrix, whereas slope breccia is usually clast-supported.

There is slope breccia too, and in some exposures it is seen to be churned and also iron-stained.  In some places a thin till is seen above this slope breccia, and in other places till is absent.

A distorted bed of churned and iron-staned slope breccia with small flakes of shale and mudstone with some larger bedrock bouulders, overlain by c 10 cm of silty clay hillwas or colluvium, overlain by up to 20 cm of peat, with signs of erosion and downslope redeposition closer to the surface.

The tor of Cerrig Lladron (made of slightly speckled dolerite) is in a ruinous state, with the crags now broken down into a series of huge displaced blocks.  But there are a number of solid bedrock slabs still more or less in situ, with clear traces of ice moulding at about 440m.

Ice-smoothed surface on Cerrig Lladron.

On the eastern flank of the col, stony till is specially well exposed in the footpath leading to Foelcwmcerwyn, with a greater concentration of quartz fragments but otherwise a similar range of rock types to the exposures to the west.

On the path top the east of the car parking area,  stony till is exposed in abundance, up to an altitude of c 425m, alongside the forestry fence.

The other feature of interest in the col is an expanse of broken bedrock and boulders, measuring c 100m x 100m, in a slight hollow to the SW of the Foel Eryr summit. This looks like a destroyed dolerite tor, and the blocks are severely weathered and covered with lichen.  Beneath it, there is a slight grassy bench with traces of hummocks that appear to be small moraines, at an altitude of c 365m.

Once again, we are seeing signs of sediment changes and landform changes between 340m and 440m, depending on precise locations and topographic circumstances.  Till, apparent morainic landforms and ice-moulded surfaces all point to glaciation of the depressions and valleys and lower mountain slopes, and maybe no glaciation at higher levels and on the summits.  But what about the Preseli ice cap?  Did it exist at the time of the LGM, or was there a Preseli nunatak?  I admit to being confused!!  More work is needed.

More work is needed, too, on the nature of the till I have been finding.  It is just like the tills found around the Pembrokeshire coast and in the lowlands, except for the absence (in the case of the "high-level tills") of faceted, rounded and striated pebbles and cobbles.  I speculate that this is all down to the fact that the glacier ice responsible for its emplacement was thin, not very active, and present for maybe not more than a few centuries.  And none of the entrained debris was moved very far.

I'll continue to make observations and to record them on this site.....