I have been looking again at those red sandstone boulders and cobbles found in the roadworks near Penblewyn, near Narberth. There are quite a lot of them in the glacial deposits -- mostly quite small, less than 50 cm in diameter. They are sub-rounded, but they do have glacial facets on them, and some are clearly striated. HH Thomas (yes, he of the bluestones) recorded other red sandstone boulders in the area, and assumed they had come from the ORS red marl beds -- which are exposed around 3 km to the south of the find site.
This is what I said in another post:The biggest puzzle relating to the glacial deposits at Penblewyn is the relative abundance of boulders and cobbles of red marls -- coloured bright red and pink. What on earth are they doing here, 3 km to the north of the ORS outcrop? Are they derived from ancient river gravels that have been carried northwards from the outcrop and then incorporated into glacial deposits? That would be vanishingly unlikely, because there is no reason to assume that local drainage ever flowed northwards from the outrop, across the Afon Marlais valley (Lampeter Vale) and up the south-facing slope of the ridge. I checked the old Geological Survey Memoir for the Country around Haverfordwest (1914) and found that similar deposits from nearby are described on page 221: "The presence of red marl from the Old Red Sandstone in this drift.........shows that the transportive agency had a certain amount of northerly direction." The author? None other than our old friend HH Thomas. He was confused, and so am I........
All in all, I incline to the view that the red, pink and purple clasts found in glacial deposits in the Penblewyn - Llanddewi Velfrey area have come from the St David's Peninsula -- but I doubt that they were freshly entrained from bedrock outcrops during the Devensian glaciation. I am pretty sure they have come from an ancient till deposit somewhere to the west or north. The boulder surfaces are heavily weathered or rotten, and they still have residues of iron oxide / manganese oxide cement, providing a rather intriguing kaleidoscope of colours! Patches of concreted gravelly debris still remain, especially in weathering pits on boulder surfaces. And on the larger boulder collected there are faint traces of striations which cut through the veneer, as seen on the smaller cobble featured in earlier posts:
There are hairline cracks all over the boulder, so it is something of a miracle that it is still intact.