To summarise the contents of earlier posts, I am now convinced that in the Stonehenge debitage (as explored in excavations across about 50% of the ground area within the stone settings) there are traces of at least four different sandstones. Although Ixer and his colleagues are understandably reluctant to admit this, the evidence presented in their "sandstone" papers does NOT suggest one Devonian sandstone source and one Lower Palaeozoic source for bluestone material at Stonehenge. We still do not know where the Altar Stone came from, or stump 40g, or stump 42c. Mill Bay and the Cosheston Sandstone appear now to have been eliminated from the frame, and that represents good progress! Although the papers contain accurate descriptions of a number of sandstone samples from assorted collections, we are short of tabulated information and short of graphic representations which might help the verify the suggestions of the geologists involved in these studies.
So there are at least two Devonian sandstones and at least two from the Lower Palaeozoic (Upper Ordovician or Lower Silurian). It is quite possible that none of the rocks examined has come from Pembrokeshire -- more research is needed to work out where they have come from. Possibilities include the Ordovician outcrops of the Tywi Valley, and the Senni Beds on the fringes of the South Wales Coalfield.
As I have said before, the more detailed the research on the geology of the bluestones becomes, the greater the number of convincing provenances are identified or suggested. At the same time the idea of glacial entrainment and transport becomes more and more convincing, as the idea of Neolithic bluestone quarrying in West Wales appears more and more absurd.