Geology of Brynberian Moor

The surface of a possible moraine 200m from Brynberian Bridge

In my hunt for spotted dolerite sources, I wandered about a bit on the moor today, starting near Brynberian bridge and wandering eastwards as far as Glan yr Afon.  I confirmed that the geology map is not all that accurate.  

In the headwaters of the Afon Brynberian, where several streams flow down to Penllyn, therse is one exposure on the river bank which confirms that the bedrock there is Abermawr Shale, but mostly the bedrock is buried beneath a thick layer of stony till.  Around SN1046348 there is undulating terrain with many dolerite boulders; I think this is a morainic accumulation.  The boulders are heavily abraded and weathered, and most of them are dolerites -- some fine-grained and others coarse-grained and maybe best described as gabbro.  Some appear to be similar to the dark-coloured porphyritic rock with large phenocrysts which outcrops at SN103352 in the old Brynberian Quarry:

Because of the distinct whitish spots which are obvious on the outer exposed surfaces of this rock, I think we could refer to it as spotted dolerite, but the matrix is very fine-grained and almost flinty -- and on balance I think it's probably a lava..........but just a few metres away there is another outcrop of dark blue unspotted dolerite.  As at Cilgwyn, there seems to be a close juxtaposition of rhyolites / lavas and dolerites.  So there must be a sill of dolerite in the Brynberian area, which is not shown at all on the geological map.  How extensive and how thick is it?  Does it run more or less west-east and outcrop again in the Pont Saeson area?  I need to check this out.......

Bedd yr Afanc passage grave lies on the western edge of flattish plateau that runs eastwards for about a kilometre.  All around the monument there are abundant dolerite boulders scattered across the moor, but then they run out and around SN115345 there is an extensive area where there are hardly any erratic boulders to be seen.  There is a thin till, and the bedrock here is tuff and rhyolite, not mudstone as shown on the geological map. Some small gravelly exposures are seen around the steepened edges of the "plateau", and especially where the trackway towards Hafod Tydfil has been cut into the bedrock.

There are good exposures of these volcanic rocks on the east-facing "escarpment" where the streams come together near Glan yrAfon:

Fishguard Volcanics -- rhyolite and tuff -- exposed across the valley 

to the west of Glan yr Afon

Dolerite boulders begin to appear in great abundance near the footbridge near the rhyolite crag on the moor --west of Glan yr Afon

The moor (which is crossed by two north-flowing streams) is littered with dolerite boulders of all shapes and sizes.  Most are heavily abraded and weathered.  Several different dolerites are seen, including some that are spotted.  The frequency of the spotted dolerites increases as one passes eastwards towards the Glan yr Afon trackway, as indeed does the amount of "spottiness" on the boulders.  This is intriguing.  More fieldwork needed.........

 Slightly spotted dolerites from broken boulders on the moorland c 200m west of Glan yr Afon

I am now wondering whether these spotted dolerites have come from the north or the NW, from a previously unrecorded sill.   Interestingly enough, Pont Saeson -- which needs to be investigated -- is only 400m or so from Craig Rhosyfelin, a place which has received a good deal of attention already on this blog..........