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Monday, 23 April 2018

Even more South Pembrokeshire Till



Over the course of three long walks on the South Pembrokeshire coast, I have been finding till all over the place.   At the risk of boring readers, here are some more exposures.  I want them to be recorded for posterity, because South Pembrokeshire is bound to become a focus of activity in the future, as people renew their efforts to define the West Wales Devensian ice edge.  Here we go:


Reddish till made up mostly of shattered limestone fragments with some rounded and faceted erratic pebbles, on the clifftop near Huntsman's Leap.


Rounded erratic cobble in fresh till near Mewsford Point


Till exposure on the clifftop near Bullslaughter Bay


Fine-grained till with mostly small erratic pebbles -- clifftop near Bullslaughter Bay


Till exposed on the clifftop on the western flank of Flimston Fort.  Here again the till is packed with pebbles of quartz -- from ancient Pliocene river gravels or beach deposits?


Another exposure of till near the western edge of Flimston Fort -- on the clifftop.


From an old post:


I have been digging around in some of the old records of the Geological Survey -- from about 1905-1938 -- and have found that the old geologists responsible for the fieldwork in west Wales (Strahan, Cantrill, Dixon, Thomas and Jones) knew all about the glacial deposits scattered across South Pembrokeshire. In their publications they describe many locations where thin till and glacial erratics are to be seen -- including Bullum's Bay on Caldey Island. So I'm not the first to describe this till, by any means. In 1905 EL Dixon wrote of Bullum's Bay: "..... the glacial deposit appears to overlie the raised beach, although the exposure is obscure, and the evidence of superposition is not so conclusive as in Gower." (Summary of Progress for 1905, Mem Geol Surv, p 70). He and his colleagues described glacial deposits at Landshipping, in the inner reaches of Milford Haven; till about 7 ft thick at St Florence; glacial sands and gravels at Bubbleton; gravel and sand at Norchard; sandy loam with erratics at Lamphey; and glacial deposits in many pipes and solution hollows at Catshole Quarry, Pembroke, Sandtop Bay on Caldey, and in other places where Carboniferous Limestone is found. Generally the till in such places is coloured red or pink -- with an obvious association with ORS rocks, and striated sandstone pebbles are also recorded in the deposits, as are pebbles and larger stones of greenstone and felsite.

The Geological Survey maps for the Tenby districts show patches of glacial deposits, as do the maps for the Pembroke and Carmarthen areas.

These are the Memoirs on the Geology of the S Wales Coalfield -- Part XI Haverfordwest - 1914; Part XII Milford - 1916; and Part XIII Pembroke and Tenby - 1921. All are out of print long since, but you will find them in libraries.

The only patch of diamicton or till shown on the latest geological map for  in this large coastal tract is just to the east of the eastern arm of the Bosherston Lily Ponds.

In the memoirs the tills or diamictons of South Pembrokeshire are generally referred to by the old geologists as belonging to the "older Drift" glaciation.   They may be right that many of the inland deposits are very weathered and very old -- but I am convinced that the coastal tills are fresh, and that they must be assumed to be of Late Devensian age.

At the moment, I see no reason to make any great alterations to this map:









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