Spotted dolerite provenances -- a spanner in the works?

Deeply embedded spotted dolerite boulders on the common near Glan yr Afon

Geology of the North Preseli area, showing the known areas of Abermawr Shales, dolerite sills and Fishguard Volcanics including tuffs, lavas and rhyolites.  The area shown as "Abermawr Shales - mostly mudstones" is covered with till, and the geological mapping is inadequate.

As fellow bloggers will know, I have no problem with the idea that most of the spotted dolerite monoliths and fragments at Stonehenge have probably come from the northern flank of Mynydd Preseli, since that is where the best "sample matches" are to be found.  In saying that, I remain convinced that there is no bluestone quarry at Carn Goedog, and that the provenancing of the Stonehenge spotted dolerites to Carn Goedog is a good deal less certain than the geologists would have us believe.  As I have said many times, the geological work seems to narrow down the "most likely source" of some of the spotted dolerites to somewhere on the Carn Goedog sill which runs E-W for several kilometres -- but I would put it no higher than that.

Anyway, I have been out today on a St Davids Day walk, to take a look at those problematic spotted dolerite boulders around Glan yr Afon -- and I am seriously confused, since this is supposed to be an area where there are no spotted dolerites.  There are thousands of the boulders, spread across an area of at least 90,000 sq m, and although there are no obvious outcrops visible at the ground surface I am seriously considering the possibility that there is spotted dolerite sill here, with a veneer of till and what appear to be morainic hummocks.  There are some flattish rock exposures just visible through the turf -- these might be ice-smoothed bedrock slabs.

The dashed line -- after Williams-Thorpe and Thorpe -- shows the known extent of spotted dolerites on Preseli.  Note that Glan yr Afon lies well outside this area, around 3.5 km NNE of Carn Goedog.

As I have mentioned in previous posts, a litter of thousands of spotted dolerite boulders, mostly heavily weathered and abraded, and up to 10 tonnes in weight,  cannot have been carried northwards and dumped by human beings.   I thought initially that they might have been carried northwards from the Preseli ridge by the ice from a local ice cap -- but I am now having doubts about that, since Preseli does not have an extensive plateau that might have nurtured a large body of ice capable of spilling northwards, eroding and transporting large amounts of debris from tors such as Carn Goedog.  Normally one would expect ice capable of eroding and transporting to be streaming and contained within a channel of some sort -- but those conditions are not satisfied here.

I'm not having a go at the geologists here -- I am genuinely puzzled.  But I am gradually getting to understand the local geology, and I am on the case....... 

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Note:  The greatest expanse of spotted dolerite boulders and outcrops (?) occurs around the Glan yr Afon trackway and on the moor to the east of it.  To the north of Glan yr Afon there is a hillside exposure of volcanic ash and meta mudstones.  To the west of Glan yr Afon on the open common, there is a small upstanding crag of rhyolite with large quartz inclusions, and more rhyolite is exposed at the surface as it rises to a flat-topped ridge to the west.  

The greatest spotted dolerite boulder concentration is in a hollow with many small streams: these converge as they flow northwards, and the valley deepens to the north of Glan yr Afon and as it drops towards the confluence with the Afon Brynberian river near Pont Saeson. This valley must have been a major discharge route for glacial meltwater.

Geological notes on a satellite image.  Junctions between the various outcrops 

are impossible to discern.

Large rhyolite slab used to bridge a small stream.

Outcropping spotted dolerite to the east of the Glanyrafon Uchaf track.

Peaty area to the east of the track, with many exposed spotted dolerite boulders.