Unmapped dolerite outcrops

 On the River Clydach, which flows across our land, there are a number of white-water cataracts, and from careful observation I can say that each one coincides with a small dolerite sill.  The quieter stretches in the river coincide with the areas of Ordovician shales.    This is an obvious relationship -- but the interesting thing is that the dolerite sills are not shown on the BGS definitive map, and neither are a number of rhyolite outcrops in the area.  All we see is a single large dyke up to 200m wide.  So the "micro geology" of the area is not very well known, except in areas like Tycanol Wood where mapping was done in detail as part of a doctorate project.   The same is true of most of North Pembrokeshire, including Mynydd Preseli where extensive moorlands make the defining of geological boundaries very difficult.  At one stage I was quite good at geological mapping, but where there are no outcrops at all to work with, one has a problem........

That's why I have always had a problem with Richard Bevins and Rob Ixer, who claim to know that certain spotted dolerite monoliths at Stonehenge have come from Carn Goedog and that certain bits of the foliated rhyolite debitage at Stonehenge have come from Craig Rhosyfelin.  Even more staggering, they claim to have identified the source of one fragment sample to "within a few square metres" on the Rhosyfelin rock face.  They have no perfect sample matches, but since they believe that there were bluestone quarries at both Carn Goedog and Rhosyfelin, scientific objectivity has been replaced with a degree of complacency which has attracted a few unmentionable comments from other geologists.  The main thing that worries them is that Ixer and Bevins do not know enough about the local geology to say with any certainty that their spotted dolerite and rhyolite samples at Stonehenge could not have come from anywhere else.

I was reminded of this a few weeks ago when I was hunting for the spotted dolerite source of the boulders scattered about near Glanyrafon, Crosswell.  When I get over this Covid bug, I will go for a walk and have another hunt.  Suffice to say that there are still spotted dolerite sources out there, waiting to be discovered........