As we know, it's now assumed that the Altar Stone (or at least, some bits of rock assumed to have come from the Altar Stone) has come from the Senni Beds, somewhere in mid Wales. Much more provenancing work needs to be done, but here is one candidate site. I received these interesting photos from one of the faithful readers of the blog, with the query "Could the Altar Stone have come from here?"
The site is on a NE-facing mountainside, at an altitude of 500m, on the flank of the mountain called Y Gelli. There is an old quarry there, which appears to have been worked over a long period and which exploits the Senni Beds sandstone formation. There is thick bedding, and the rock seems to naturally break down into large angular blocks. The precise location is SN 91417 22163. The quarry lies c 1km SW of Heol Senni and just over 2 km SE of Cray. Sennybridge is to the north.
What interests me about this site is the rock outcrops above the quarry on the mountainside -- on the old maps these show up quite prominently, and we can see them in the middle photograph. They appear to be on the headwall of a small nivation hollow or incipient cirque, but I don't think it was ever deep enough for a body of ice to have generated its own terminal moraine.....
These outcrops might be considered perfect candidates for entrainment by overriding ice. However, a word of caution. During a local glaciation the "natural" direction of ice flow here would have been towards the NE. On the other hand under a Welsh ice cap the direction of ice movement would have been across the main ridge of the Brecon Beacons, through a col and then southwards into the Neath River catchment. I don't understand ice movement directions in the area well enough to make sensible suggestions right now -- but at least we have an intriguing possibility that the Altar Stone MIGHT have come from Heol Senni or from somewhere nearby.
Richard E. Bevins, Duncan Pirrie, Rob A. Ixer, Hugh O’Brien, Mike Parker Pearson, Matthew R. Power, Robin K. Shail
Journal of Archaeological Science 120 (2020), 105188