More on the Glan yr Afon puzzle

 

One of the tors of Carnedd Meibion Owen.  Could the Stonehenge spotted dolerites have come from somewhere near here?

In trying to explain the presence of all those spotted dolerite boulders at Glan yr Afon, in a place where they are not supposed to be, I have been doing some more investigations of the local geology.  The geological boundaries are somewhat fuzzy because of the scarcity of outcrops, but the "boulder scatter" occurs in an area close to the junction between Abermawr Shales -- actually mudstones for the most part -- to the south and Fishguard Volcanics to the north.  The actual rock types here are supposed to be "rhyolitic and dacitic lavas, spilitic breccias, acid tuffs and intercalated volcaniclastic sediments."  My own observations confirm that, although the boundary should be placed further south and south-west from that shown on the map.  But there are no records of spotted dolerite outcrops anywhere in the vicinity.   As mentioned in my last post, there may be an unrecorded sill of spotted dolerite somewhere -- or a small fragment of a sill, given the extent of local faulting and displacement.

As mentioned in the last post, I am rather more sceptical about the boulder scatter being transported northwards from the outcrops of the Carn Goedog sill by ice associated with a Mynydd Preseli Ice cap.

The other thing that we have to think about is the direction of flow of the Devensian Irish Sea Glacier.  Nearly all of the evidence suggests an ice flow from the NW towards the SE. At some stage the ice may have come from the north, but let's ignore that for the moment. So could the ice have carried all those boulders to Glan yr Afon from the NW?  Well, yes -- and this ice would have passed over Ty Canol Wood and the four tors of Carnedd Meibion Owen.

But the tors are made of unspotted dolerite, not spotted dolerite.  So if this did really happen, the ice must have plucked up chunks of bedrock from an unrecorded spotted dolerite outcrop somewhere along the line shown on the map above.

Whatever the truth of the matter, the geologists Ixer and Bevins do now have a problem.  There is a source of spotted dolerite somewhere within striking distance of Glan yr Afon that has not been considered as a possible provenance for the spotted dolerites - or some of them -- that ended up at Stonehenge.  In turn, until the Glan yr Afon boulders are analysed, and their petrography and geochemistry compared with those of other analysed samples, they cannot be certain that their analytical conclusions about the provenancing of the Stonehenge spotted dolerites are at all reliable. I have always said that the precision of the provenancing is much less secure than claimed.  Might these interesting boulders be matched with the Stonehenge spotted dolerites even more closely than the outcrops at Carn Goedog?

And the archaeologists have an even bigger problem, having been singing the praises of the Carn Goedog "quarry" for the past five years or so to anybody prepared to listen.  In my view there is no Neolithic quarry there anyway -- but if it turns out that the "mystery source" of the Glan yr Afon boulders is a closer match for the Stonehenge spotted dolerites than Carn Goedog, the quarrying hypothesis will sink without trace.

Some more geology is needed.  Over to the geologists.

Outcrops of unspotted dolerite at Carnedd Meibion Owen

Rock outcrops in Ty Canol Wood.  Most of the outcrops are of rhyolites, lavas and "volcaniclastic" rocks -- but are there spotted dolerites somewhere as well?