I think I forgot to mention this little note before. It has been posted on the Academia web site, but there is no citation or date. So it looks as if it has never been published. But the info contained is interesting!
Palynology, age and provenance of the Lower Paleozoic Sandstone of Stonehenge
Stewart Molyneux, Richard Bevins, Rob Ixer and Peter Turner
Shouldn't that be "Palaeozoic"??
The bluestones comprise a variety of volcanic, intrusive and tuffaceous igneous rocks, along with rarer sandstones. The last include the ‘Altar Stone’, two buried orthostats and a number of sandstone blocks in the debitage. The Altar Stone is petrographically similar to fine- to medium-grained calcareous sandstones in the Lower Devonian Senni Formation of South Wales. Sandstone fragments from the debitage, however, include specimens of greenish-grey, indurated, fine-grained, feldspathic sandstone that have been subjected to low-grade metamorphism, with a suggestion of a spaced cleavage. They are more deformed than the Devonian sandstones exposed in South Wales, form a coherent lithological group, now referred to as the ‘Lower Palaeozoic Sandstone’, and contain characteristic clasts of dark mudstone.
The palynological data, coupled with petrography, show that the Lower Palaeozoic Sandstone of Stonehenge is not older than Late Ordovician, and it is most probably from a Late Ordovician unit in the Welsh Basin. However, the possibility that it is from a Welsh Basin Silurian unit cannot be discounted without more information on acritarch assemblages from Silurian sandstones, including the nature of any recycling.