On hunting through some old box-files I have rediscovered photocopies of the "superficial deposits" chapters from the Memoirs of the Geological Survey, published between 1909 and 1921. The surveyors working in West Wales were A. Strahan, TC Cantrill, EEL Dixon, OT Jones and HH Thomas -- all very famous geologists who knew their stuff.
The map reproduced above is from the Haverfordwest Memoir (1914) which shows erratic distributions across SE Pembrokeshire. Note the two "erratic fans" marked. The authors do not suggest that there are no relevant boulders dotted about outside those fan limits -- but the lines show where the greatest concentrations are. The spotted "diabase" (dolerite) is of course from eastern Preseli and the "hornblende - porphyrite" is deemed to have come from the Portpatrick district on the Mull of Galloway in western Scotland.
There is a huge amount of detail in these Memoirs -- and I'm still quite confused as to how much of South Pembrokeshire might have been affected by the Irish Sea Glacier in Devensian times. watch this space......
Another thing that intrigues me is that HH Thomas was part of this team, which produced its final report in 1921 (The Pembroke and Tenby Memoir) -- that was the same year as Thomas's famous lecture about the bluestones to the Society of Antiquaries. He claimed of course that glacier ice could not have carried the bluestones to Salisbury Plain -- but he and his colleagues must have known that "diabase" and many other erratics were transported by ice far to the east of Pembrokeshire, since the Memoirs mention Irish Sea erratics in the Swansea area and near Pencoed in the Vale of Glamorgan. I wonder what conversations went on behind closed doors between these guys, who must have spent hundreds of hours together on their fieldwork and in local bars and hotels..........
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