I have been looking again at the manner in which, in 1923, HH Thomas informed the world that the glacial transport of bluestones from west to east (up the Bristol Channel) would have been impossible. Here is an earlier post of mine:
https://brian-mountainman.blogspot.com/2019/10/hh-thomas-and-his-glacial-blind-spot.html
Thomas was guilty of over-simplification and selective citation of his samples (there were very few anyway) and his rock identifications, in order to flag up the idea that there were just two main sources for the bluestones. It's interesting that in 2018 Bevins and Ixer attacked Thomas's slapdash methods, his sharp practices and his reputation pretty enthusiastically, but they restricted their scrutiny to his geological analyses, and criticised him for homing in on Carn Meini and Carn Alw as the main source locations for the Stonehenge bluestones.
https://brian-mountainman.blogspot.com/2018/06/herbert-thomas-scrutinized.htmlRetracing the footsteps of H.H. Thomas: a review of his Stonehenge bluestone provenancing study.
Richard Bevins and Rob Ixer
Antiquity, May 2018.
https://doi.org/10.15184/aqy.2018.10
Published online: 31 May 2018
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Extract from HH Thomas, 1923 Antiquaries Journal
Having in a great measure solved the problem of the source of the Foreign Stones, we must consider carefully the possible and probable modes of transport of the stones from Pembrokeshire to Salisbury Plain. Two modes of transport have been suggested : one natural, by ice during the great Ice Age ; the other, by human agency at, of course, a later period.
The Hypothesis of Ice-transport.
We have, of late years, considerably advanced our knowledge of the distribution and extent of the British Ice-fields, and also accumulated much information concerning the directions and limits of dispersal of erratic boulders. The geological evidence is such that the idea of a glacial origin for the Foreign Stones will not bear investigation.
Let us consider critically this hypothesis of glacial transport as suggested by Professor Judd. First, there is no evidence of glacial drift on Salisbury Plain such as would of necessity have been left by any ice-sheet capable of transporting the masses of rock in question. Isolated masses of rocks foreign to the district, other than those used in the fabric of Stonehenge are entirely wanting, as also are small pebbles of such rocks from the gravels of the neighbourhood. It has been claimed, without producing any evidence in support of the statement, that such masses did exist but that they have all been collected to make walls, gateposts, millstones, etc. But, as Mr. Stevens of Salisbury has cogently stated, no one can point to a single rock-mass like any of those used at Stonehenge having been put to any such purpose. Mr. Stevens says ' There are many millstones and gateposts in Wiltshire, but where is there one which corresponds in any way to the upright Foreign Stones of Stonehenge ? Unhappily this tangible evidence is wanting ; so, alluring as the Glacial Drift Theory may appear, it must reluctantly be set aside for want of convincing evidence.'
((Comment: It's disingenuous of Thomas to pretend that substantial "rock masses" of exactly the right type on Salisbury Plain are required in order to demonstrate that glacial entrainment and transport might have occurred. There are erratic stones in the records of work prior to 1923, and the Boles Barrow bluestone was already on the record following its discovery by Cunnington in 1801. If you want to wear rose-tinted spectacles rather than dark sunglasses, you might wish to count the Stonehenge bluestone assemblage as quite valid evidence of glacial transport and dumping, and Thomas should have acknowledged this.))
To transport glacially a series of igneous boulders of great size from Pembrokeshire to Wiltshire postulates the existence of an ice-sheet of unbroken character occupying the whole of the intervening country ; and with the ice moving in a direction a little south of east. We have, fortunately, good evidence of the extent of glaciation of Pembrokeshire, and we find that this county was crossed in a south-easterly direction by an ice-sheet that moved down the Irish Sea. This ice-sheet carrying Scottish
boulders, crossed the low plateau of Anglesey and Carnarvon, gathering fresh material as it went, but was kept from passing far inland by the local Welsh ice-sheet that had its centre of dispersal in the highlands of Snowdon, The Arenigs and Cader Idris, and was pressing outwards towards the coast. On reaching the latitude of Pembrokeshire, far removed from the main centre of Welsh glaciation, the Irish Sea ice-sheet was allowed to spread fanwise and to override the plateau-regions of Pembrokeshire and Southern Ireland which offered relatively little opposition. In spite of this there is the clearest evidence, from the distribution of Pembrokeshire and Scottish boulders that the ice-front lay only just south
((Comment: That is not correct. The Pembrokeshire erratics in the Storrie Collection, from the till at Pencoed in Glamorgan, were well known by 1923. Indeed, Thomas himself was one member of the Geological Survey team that described the erratics in the Bridgend GS Memoir of 1904.))
Scottish boulders, however, occur on the north coast of Devon and on the coast of Glamorganshire where their presence, unmixed with Pembrokeshire boulders, indicates that they were not carried by that portion of the ice-sheet which had crossed Pembrokeshire but had been borne by the portion that came down the central region of the Irish Sea. The ice-sheet would probably have a crescentic front and the medial portion would have the furthest southerly extension. It is to be noticed that all the occurrences of Scottish boulders outside Pembrokeshire and its adjacent islands lie at raised-beach level, as at Croyde Bay and in Glamorganshire. There is no evidence of the erratic material mounting the cliffs or extending inland. The inference is, therefore, that these Scottish boulders were deposited from icebergs that had broken away from the central portion of the main ice-front and were stranded on relatively distant shores. The geological evidence proves conclusively that although Pembrokeshire was crossed in a south-easterly direction by a lobe of the Irish Sea ice-sheet the front of this ice-sheet never reached across or far up the Bristol Channel.
Passing to the country intervening between Pembrokeshire and Wiltshire, we find nowhere along the line that an ice-sheet would have to traverse in order to transport Pembrokeshire boulders to Salisbury Plain, any evidence of glaciation of an intense character.
There are no trains of far-travelled boulders, no ice-scratching and polishing of outstanding rocks, and no thick accumulations of boulder-clay. As has been pointed out in a previous communication such a hypothetical ice-sheet, in order to account for the Foreign Stones of Stonehenge would have to gather from Pembrokeshire blocks all of about the same size and mainly of two rock-types. It would have to carry them all that distance without dropping any by the way.
((Comment: Thomas must have known, from his work with the Geological Survey including Cantrill, Strahan, Dixon and Jones, that glacial entrainment, transport and deposition are complex matters which are difficult to predict and which constantly throw up surprises in the field. The apparent absence of erratics from all upglacier outcrops does not (and did not in 1923) demonstrate the absence of glaciation.))
Note: The rocks of the Western Isles, Ailsa Craig, and Galloway are fairly common as erratics. They occur on the Cardigan coast, on the plateau-region of Pembrokeshire and its outlying islands (Skomer, Skokholm, etc.), and on the Glamorganshire coastal regions of the Bristol Channel.