Thanks to Tony for drawing this to my attention. I have seen references to the glacial deposits of the Nightingale Valley on Portishead Down (Chris Hunt, p 144 of the QRA field guide to the Quaternary of Somerset) but not to any glacial deposits on or close to the beach. I am intrugued by the suggestion that the blocks of gneiss are of "identical mineralogical composition" to the famous Porthleven Erratic.
It is possible that these erratics have come from the nearby Woodhill Bay Conglomerate, which contains far-travelled igneous and metamorphic pebbles and cobbles -- but these tend to be very small, and rather rounded. The reference to "angular blocks" on the beach suggests a quite different origin.
I will check this out directly with Prof Brian Williams. Watch this space........
Portishead Beach -- Royal Hotel section A, near the steps
A further point of interest at this particular stop is the presence, in fair abundance, on the gravel beach of angular blocks of biotite-garnet gneiss. Obviously exotic to this area two suggestions as to their occurrence have been put forward: the boulders may be of glacial origin, erratics produced by southerly moving ice during the Quaternary; the material may simply represent discarded ship’s ballast. A point in favour of the first theory is the presence of a 50 ton glacial erratic of identical mineralogical composition at Porthleven in Cornwall. Against this idea is the fact that the gneiss is totally restricted to this one locality along the Portishead coast and that this locality is in juxtaposition to Portishead Docks. Their occurrence, therefore, is a matter for further debate.
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