Carreg Coetan and the Enigmatic Dolerites

Now for another enigma.  Where did the dolerite boulders of NE Pembs come from?  It's very easy to explain the presence of dolerite boulders almost everywhere else in Pembrokeshire as a consequence of glacial erratic transport, but in NE Pembs -- in the area between Dinas Island and Cemais Head, incorporating the country around Newport, Moylgrove and Cardigan -- there are dolerite boulders dotted around here and there which cannot easily be explained by reference to the glacial maps.  Many years ago the geologist OT Jones pondered on their presence.  The ice during the Devensian glaciation came in from the N and NW, having crossed the area now known as Cardigan Bay.  As far as we know, all of the dolerite intrusions associated with the Fishguard Volcanic Series are to the south of Newport (see the BGS map above), and so they could not have been picked up and moved northwards by ice  moving in the opposite direction!!

There are some possible explanations.

Maybe the dolerite boulders in the area have come from dolerite exposures in the Pen Caer - Strumble Head area by ice moving directly west - east.  There are some dolerite outcrops on the peninsula.  We know from striations on the coast that the ice did move in this direction at some stage.

Another possibility is that the dolerites have come from the Cader Idris area, or even North Wales, and were transported by Welsh ice and Irish Sea ice before being dumped in this area.

A third possibility is that they have come from dolerite outcrops which are now beneath the sea bed.  This seems quite feasible, given that the Penyraber and Cwm-yr-Eglwys mudstone formations are right at the base of the sedimentary sequence overlying the Fishguard Volcanics.  There are complex faults in the area, and I am sure there are some igneous rocks exposed in the cliff faces of Newport Bay, although they are not shown on geological maps.

A fourth possibility is that the boulders are really from the Preseli outcrops, and that they have been transported northwards by ice flowing from a Preseli ice cap either before or after the Devensian LGM of Last Glacial Maximum.  There are a few anolalies in the evidence, and I have suggested several times on this blog that at some stage Welsh ice crossed North Pembrokeshire, flowing from the east towards the west.  But it's doubtful that such ice would have carried dolerite boulders, for the prevailing rock types to the east are Ordovician and Silurian sedimentary rocks -- sandstones, gritstones, shales and mudstones.

Then a fifth possibility.  Could the stones have been transported to the locations of assorted Neolithic burial chambers and Bronze Age standing stones by prehistoric people?  OT Jones was convinced that assorted dolerite pillars used as gateposts and in house building in this area were in fact collected from the Preseli Hills by landowners and farmers during and after the Middle Ages;  I would go along with that, and there is indeed much evidence of "modern stone gathering"and maybe even quarrying of dolerite outcrops within historical time -- simply because the stones were easy to transport and use.  But if we go back to the Neolithic, the evidence of stone transport is not strong.  Those archaeologists who have looked at the cromlechs of the area -- like Pentre Ifan,  Cerrig y Gof, Llech y Dribedd and Trellyffaint -- have always concluded that the boulders used have been glacial erratics used more or less where they were found.

Carreg Coetan Arthur, in Newport, is a good example.  It is in a small enclosure not far from the estuary, and it incorporates a huge capstone resting on just two uprights.  It is very precariously balanced, and it is a wonder that it has not collapsed.  Two other uprights almost touch the capstone, but daylight is visible at the tip of each stone.

All of the stones at this site (capstone, pillars and several other fallen stones embedded in the turf) are made (as far as I can see) of unspotted dolerite which looks like the dolerite of the Carningli-Newport area, which outcrops just a couple of km to the south.  The stones are well weathered, and have rounded or abraded edges typical of glacial erratics.  So they LOOK like glacial erratics, and they are most certainly not quarried from anywhere.  It has to be assumed that they were used more or less where found.  The massive capstone also has a worn and weathered upper surface and a less weathered and somewhat flaky lower side;  I assume that (as in the case of the Pentre Ifan capstone) it once lay embedded in the ground and may even have been overridden by ice before being exposed and subjected to 15,000 years of weathering.  Then somebody came along with the bright idea of levering it upwards, placing props or pillars beneath it, and making it into a burial chamber......

Was this also the history of the other burial chambers to the north of the Nevern River?  Watch this space....