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Sunday, 22 March 2015

More on the Sleek Stone Super-Erratics


 The two biggest chunks of the "super-erratic" near the Sleek Stone, resting on a rough platform of Coal Measures sandstones and washed by the sea at high tide.  The chunk in the foreground is the smaller of the two; the larger one is immediately beyond it.

 The hammer rests on a third chunk of this same erratic, broken up by the pounding of the waves over thousands of years.  There are visible fractures in all of these chunks -- so further breakages and diminution are inevitable.

Been over to the Sleek Stone at Broad Haven again today, hunting for more bits of that "super-erratic" which has come -- probably -- from Ramsey Island.  In addition to the two large chunks which I think have come from a single "super-erratic" weighing over 100 tonnes, I'm now pretty sure that there is a third chunk of the greenish volcanic rock about 10 m away, at a slightly lower level.  I calculate that this chunk weighs about 12 tonnes -- it's smaller than the other two.  There's a fourth boulder as well in the vicinity, about half that size and weighing maybe 5 tonnes.

About 50m towards the north there is a second very large erratic now broken into three chunks, one weighing about 17 tonnes, another about 23 tonnes, and another abouy 10 tonnes -- a total weight of about 50 tonnes.  Again the rock type is a bluish-green "quartz porphyry" presumably from Ramsey Island.



The second giant erratic about 50m to the north of the "super erratic" -- in the rock platform embayment north of Sleek Stone.  This erratic block has broken into three pieces.  The hammer rests on the largest of the three.

Then there is a third giant erratic, not far from the green drainage pipe that carries water down from the cliff top.  This is made of a similar rock type, and weighs about 21 tonnes.  It is shown in the photo below.


These are the three biggest erratics in the vicinity; I suspect there may be others that I have not yet spotted.  In addition, there are scores of boulders which are in excess of 1m in diameter and weighing around 3 tonnes each.  And many smaller cobbles as well, littered all over the place.  The commonest rock type by far is the greenish or bluish quartz porphyry.  There are literally thousands of tonnes of it, just scattered about in the small embayments in the vicinity of the Sleek Stone.  Some quite large chunks of Ramsey Island have clearly been transported across St Bride's Bay from Ramsey Island.  I am still unsure whether this transport was done in the Devensian glaciation, or in earlier glacial episodes. 

7 comments:

Jon Hudson said...

I would be interested to see a good high res image of this distinctive rock.....I am sure I have seen lots of it all along from braodhaven, madocs haven
, and Nolton.

BRIAN JOHN said...

Images already put onto the blog, Jon. Yes, this is widespread along the west-facing coast of St Bride's Bay......

BRIAN JOHN said...

Images already put onto the blog, Jon. Yes, this is widespread along the west-facing coast of St Bride's Bay......

Constantinos Ragazas said...

Brian,

What does any of this have to do with the glaciation of Salisbury Plain?

You are as far off from Stonehenge on this as you argue Neolithic people were hauling bluestones from afar!

Kostas

BRIAN JOHN said...

Don't understand what you are saying, Kostas. Since when am I arguing for long-distance haulage of bluestones from afar? I am seeking to understand the sequence of glacial episodes and how big they were -- and that is very relevant for the Stonehenge bluestone debate.

Myris of Alexandria said...

Kostas
Stonehenge is but a small part of this new blog that is teaching us all much Pleistocene/Holocene geology.
Some of the best blogs have not mentioned the Druidical Plains.
M

Constantinos Ragazas said...

Brian,

To be clear, of course you have not argued in favor of long distance haulage of bluestones. To the contrary.

I was only relatively comparing the far off distance from Stonehenge your recent posts have with the far off distance purported for bluestone haulage you argue against.

Kostas