Thinking about the cave in Tycanol Wood, not far from Newport (Pembs), I got to wondering why no erratic material from this area has yet been discovered on Salisbury Plain. Material has been found from near Brynberian (Pont Saeson) and from Carn Alw and Carn Goedog on the north flank of the main Preseli ridge. These are classic entrainment environments for ice moving from NW to SE, as I have explained previously on this blog. Ice which is moving upslope in order to cross a barrier which is transverse to the direction of movement would be expected to shear over the summit, with erratic material entrained and then sheared high into the heart of the glacier. Lionel Jackson and I explored this idea in our EARTH magazine article in 2008:
Rob Ixer and Richard Bevins (and maybe others as well) are examining some of the other "erratic litter" material from the Stonehenge environs, concentrating on the rhyolites which the OU team long ago identified as having potentially great importance. Where did these rhyolites and other complex igneous / metamorphic fragments come from? It's already been suggested by geologists in print that they might have come from the outcrops of the Fishguard Volcanic series which outcrop on the N Pembs coast from the Pen Caer Peninsula to the Dinas area. But there are also igneous inclusions (including ash beds) in the Ordovician sedimentary rocks on the coast between Dinas and Newport. Those rocks also extend well inland, and indeed the vast igneous province in this area (with intrusions and extrusions, and lots of sediments as well) incorporate the dolerite and other outcrops on the eastern parts of the Presely Hills. It's these outcrops (seen best in the tors on the ridge) which have attracted most attention in the past.