Time for a short history lesson. Let's just remind ourselves what the old geologists thought about the Stonehenge bluestones.
It was suggested as early as 1858 by Ramsey that the foreign stones built into the Stonehenge monument had come from Wales, and in this he was supported by Maskelyne (1878) following the first petrological study of the stones. This early work established that the bluestones were anything but uniform, and showed that the stone group included spotted and unspotted dolerites, various rhyolites (lavas and ignimbrites), sandstones and volcanic ashes. As many as ten different sources were suggested, and this number was pushed up substantially when other rock types (including limestone, siltstone, greywacke, flagstone and slate) were recognized in the assemblage of rock flakes and chips on the site, and were assumed to have come from the destruction and working of some of the bluestones used in the stone settings. This sheer variety of rock types is striking, and is sometimes ignored in discussions about the supposed “bluestone quarries” in Pembrokeshire, at Carn Meini, Rhosyfelin and Carn Goedog.
Far to the west of Stonehenge, Hicks (1885) recorded striations and erratic movement in the St David's area (where he lived) that could only be explained by ice coming onshore from the north-west. In 1891 he also demonstrated that the lowland parts of Pembrokeshire had been inundated to a great depth by ice coming in from St George's Channel. Geikie (1894) proposed that the ice coming in from the sea extended all the way up the Bristol Channel and pressed well across the present coastlines of Devon and Somerset. The term "Irish Sea Glacier" was coined by Carvill Lewis (1894) to describe the great ice stream that extended all the way from Scotland, passing between Wales and Ireland and extending at least as far as St George’s Channel.
In 1902 Judd suggested that the bluestones at Stonehenge were erratics of glacial origin. He argued that the assemblage of debris at Stonehenge had come from North Pembrokeshire or North Wales. He also observed that in areas affected by very ancient glaciations, most of the till had been eroded away by natural processes after hundreds of thousands of years, leaving only a thin scatter of erratics here and there. Further, he observed that hard stones (including bluestones) left behind on Salisbury Plain would have been targetted down through the centuries for building purposes simply because neither chalk nor flint makes good building material. This was a point also made quite forcefully by Thorpe et al (1991) following a large bluestone research project under the auspices of the Open University. Intriguingly, Judd concentrated not on the 43 known bluestone monoliths or orthostats themselves, but on the debitage or debris in the Stonehenge soil layer. He found an extraordinary assortment of soft or fragile stones including fissile sandstones, micaceous sandstones, greywackes (argillaceous and easily broken down), flagstones, slates and "clay-slates", and fine-grained glauconitic sandstones. He made the point specifically that this material did not seem to be very closely related to the remaining standing bluestones -- so he concluded that only the hardest stones had survived to the present day, with all the other material breaking down and becoming incorporated into the soil layer over many thousands of years. Judd suggested the presence of a “Stonehenge moraine” incorporating an abundance of foreign stones which would have been readily available to the builders of Stonehenge. He also argued that “stone availability” (of both bluestones and the larger sarsens) might have actually determined the precise position of the monument -- an idea which has subsequently been largely forgotten.
In 1904 TJ Jehu worked out the Pleistocene stratigraphy of North Pembrokeshire. He was a meticulous field worker who lived in wales before moving to Edinburgh, and his records of Pleistocene sections and stratigraphy remain valuable to researchers to this day. He agreed with Hicks that the traces of ice action (striations and erratic transport) all showed a dominant ice flow from north-west towards south-east.
In 1908 the geologist Herbert Thomas speculated on a Preseli origin for the Stonehenge bluestones, and in 1921 he gave his famous lecture that changed everything because of his bizarre and irrational belief that glacial transport would have been "impossible"........