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Wednesday, 7 October 2020

Erratic boulders at St Brides Haven, near Dale

 We had an excellent day out at St Brides today, including picnic lunch on the beach.  Naturally enough, I managed to do some geomorfin' while my wife searched for interesting pebbles, and I came away duly impressed.  More of which anon -- but for a start here are some photos of the erratic boulders on the beach.  They are easy to spot because the bedrock here is Old Red Sandstone -- and everything "foreign" is very prominent against the dark red background.

Two pieces of what must previously have been a rather impressive boulder of greenish lava -- possibly from Ramsey Island.

Buff-coloured sandstone, possibly from the south coast of the St Davids Peninsula.

I think all of the above are igneous (difficult to be sure because not all have fresh 
facets or broken edges that can be examined).


Finally, my wife's collection of interesting pebbles from the beach -- sandstone, rhyolite. flint, dolerite, granite, ash, quartz and some I don't recognize.  Some of these are not very well rounded -- which means that they have been eroded out of the till at the head of the beach relatively recently.

It's obvious here that these erratics have been carried in onto the coast by glacier ice travelling broadly from NW towards SE.   Much of the erratic material has come from Ramsey Island and the western part of St Davids Peninsula -- but we knew that already.

When were these boulders transported and emplaced?  Probably during the Late Devensian glaciation -- there is no reason to assume that they are older, although of course they might simply be "recycled" from older and pre-existing boulder beds or glacial deposits.

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