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Wednesday, 15 October 2014

Glaciation of Rhosyfelin

I was down at Rhosyfelin this morning with geologist John Downes  -- of which more in due course.
But on the matter of glaciation, the signs are more and more obvious.

I got a better photo of the beautiful ice-moulded surface close to the tip of the spur:

I'm not sure how the archaeologists explain away features like this.  Maybe they had apprentice quarrymen in the bad old days, who were given menial tasks like polishing and smoothing rock outcrops, so that they would be really good at it by the time they got to Stonehenge?  The trouble is that there are smoothed and worn surfaces like this all over the place, and many of the bedrock outcrops in the vicinity of the wonderfully-named "proto-orthostat" are equally well worn.  My guess is that the features have been shaped both by glacier ice and by torrential meltwater during deglaciation.

I'm more and more convinced that the abundant rounded and sub-rounded erratics in the till and the fluvio-glacial deposits have come from the west and north-west, where there are many outcrops of dolerite and other volcanic rocks belonging to the Fishguard Volcanic Series.  The upstanding tors of Carnedd Meibion Owen are only about 3 km away,  and there are more dolerites exposed near Pentre Ifan, about 4 km away to the NW.  That would match up with the suggested direction of Devensian ice movement very well.  Next time I go down I hope I'll also have a geologist with me, so that we can look seriously at the likely provenances for the dolerites and also for rocks like this erratic cobble about 6" long:

The lower picture shows a fragment from the same stone, about 2" long.  I think it's an ash or tuff from the Fishguard Volcanics, but I can't be sure until somebody more expert looks at it.  It's  not from Rhosyfelin, I reckon. 

I'm also coming to the view that the grey clay-rich deposit art the base of the sequence on the valley floor MAY be a deposit from a short-lived proglacial lake.  Watch this space.

Another thing I've been pondering on is the orientation of the meltwater channels at Rhosyfelin.  The little one runs NNE, and the big one runs NNE and then swings towards the NW -- and this latter orientation runs directly counter to the direction of Devensian ice flow around 24,000 - 20,000 years ago.  The channels themselves are much older -- I suspect that they date from the Anglian.   The fluvio-glacial materials in the valley are suggestive a powerful meltwater flow -- probably unconstrained by any ice barrier.  So that means the valley carried a large stream at quite a late stage of deglaciation.  This brings up another interesting question -- could the meltwater have come from the melting of a late Preseli ice cap which remained in existence after the dissolution of the Irish sea ice in the area?  That's an intriguing thought.......

1 comment:

TonyH said...

Seems more and more as though these intrepid Rhosyfelin archaeologist investigators are talking a load of erratic cobblers, mates. Haven't any of them heard of the effects of ice on the Landscape BEFORE, say, Neolithic times?? Not everything in the Landscape is man - made!! Archaeologists are too anthropocentric. I wonder why? Anyone have any ideas?