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Thursday, 23 February 2012

Mystery erratics on Newport beach

Now then -- a little puzzle.  These are samples from the erratic boulder found on the beach in Newport estuary a couple of weeks ago.  Each of the fragments is about 8 cm long.  The boulder is sitting on the beach between the tide marks, in the middle of quite an extensive area of till which was previously covered by estuarine mud.  A few yards away from the boulder, on the surface of the till, I found a small coin last Sunday -- which turns out to be a George III halfpenny -- the date seems to be 1798.  Does that date the glaciation that deposited the till?  I doubt that -- so let's move swiftly on......

I don't recall seeing a boulder of this type before -- it is certainly not local.  To me, it looks like a felsite -- partly because of the texture, but also because of the characteristic "dendritic" or fern-like patterning on the weathered surface.  The early geologists (including HH Thomas!) found felsites in the Newport area which they interpreted as "Cader Idris or Arans felsite."   Whether these fragments can be given the same provenance remains to be seen.

Opinions, Rob or Richard or anybody else who knows igneous rocks?

If this really is a felsite boulder from the Cader Idris area, that would be quite interesting, because the movement of such a boulder must have been in at least two stages -- first, movement broadly westwards out from the Welsh uplands by a Welsh ice stream, and second, movement southwards by Irish Sea ice which crossed Cardigan Bay and impinged on the North Pembrokeshire coast.

That, as you keen observers will have noticed, is precisely the mechanism I have proposed for the transport of the Altar Stone towards Stonehenge......... except that we need broadly southwards movement by Welsh Ice and broadly eastwards movement by the Irish Sea Glacier.  From what we know about ice movement directions, that is perfectly feasible.

5 comments:

Tony Hinchliffe said...

Could it have come from Ireland? I'm remembering the Irish provenance of a large erratic at Kenn, North Somerset.

BRIAN JOHN said...

I suppose it's possible -- but maybe this is too far east. If there are Irish erratics in Pembrokeshire I would expect them further west -- maybe on the St Davids Peninsula or Castlemartin Peninsula. The Kenn "white limestone" erratic is reputed to have come from Ulster.

chris johnson said...

Look to me like some rocks I collected from a Colorado gold mine, deep in the Rocky Mountains.

BRIAN JOHN said...

Well, that's very interesting, Chris....

chris johnson said...

I hoped you would be amused. True nevertheless.