Carreg Coetan cromlech, Newport

This is an interesting cromlech, hidden away in a small housing estate on the edge of Newport.  The massive capstone rests on just two of the uprights, and appears somewhat precarious.  It has been quite well investigated by Sian Rees and colleagues:

Archaeologia Cambrensis 161 (2012), 51–163
Excavations at Carreg Coetan Arthur chambered tomb, Pembrokeshire
By Sian Rees

https://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-3493-1/dissemination/161-2012/03_Arch_Camb_161_051-164_Sian_Rees.pdf

This one is not made of dolerite, but of rhyolitic lavas and ash-flow tuffs which outcrop not far away.  So as Richard Bevins says, the rocks are local.  They have probably been moved by ice and then collected up and more or less used where found.

As far as I know, this is the case with ALL of the Neolithic structures found in West Wales.  It is therefore quite surprising when the MPP team (including Richard Bevins) seem to think that the stones at Waun Mawn must have been FETCHED from somewhere else......... and then proposed as having been "sourced" from Cerrig Lladron, high on the hillside near Foel Eryr.  They suggested this without having bothered to sample the bedrock outcrops in the immediate vicinity of the so-called lost circle.  In my view it is 100% certain that the Waun Mawn stones were used where found.

Extract from the report:

Geology (Richard Bevins)

All of the five principal Carreg Coetan Arthur stones (capstone and the four orthostats) are of igneous origin, they are all siliceous (rhyolitic), are typically poorly cleaved and have all been altered by low- grade metamorphism. they are all either rhyolitic lavas or rhyolitic ash-flow tuffs. The stones all bear marked similarities in terms of the composition, lithological make-up, alteration state and style of deformation, to rhyolitic lavas and tuffs of the Fishguard Volcanic Group which outcrop in a belt across north Pembrokeshire stretching from Porth Maen Melyn, on Pencaer, in the west, to the Crymych area in the east. Rocks of the Fishguard Volcanic Group crop out in the area immediately south of Newport and hence a local origin is probable.