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Tuesday, 19 August 2014

Those mysterious pink erratics


Where did these mysterious pink granite erratics come from?  The one in the top photo is on the island of Flat Holm in the Bristol Channel, and the one in the bottom pic is from Saunton in Devon.  Both are quite close to sea level, and both are resting on wave-cut platforms.  The mobile phone in the lower photo gives the scale.

I assume nobody has ever done any petrography on these.... but there are a number of possible sources of pink granite erratics in the UK -- some in Wales, some in Ireland.  But the main possible source is in northern England, where the Shap granite outcrops.  Could that really be the source?  Here is a close-up:


Over to the geologists.......






7 comments:

Myris of Alexandria said...

Hate to say this but it is not unlike the Shap adamellite.
There are no small dark heathens in it?
M

BRIAN JOHN said...

Why do you hate to say that? Thought you were an impartial scientist free of all encumbrances?!!

No idea what either granite looks like close up. But hoping to get over to Flat Holm before long, to take a look.....

Constantinos Ragazas said...

Brian,

Many great posts here! Keep up the good work you are doing. Invaluable contributions for both professionals and laypersons alike! I appreciate the lessons!

I have one small curiosity. How did that pink granite erratic get wedged under that sizable bedrock strata in the photo? What can this tell us about the ice that deposited it there and not on top of the bedrock strata?

Kostas

BRIAN JOHN said...

Kostas -- the pink erratic is resting on a coastal rock platform cut at a time of higher sea-level. How much time elapsed between the erosion of the platform and the arrival of the boulder is currently not known. later on, other sediments began to accumulate and covered the boulder. For many thousands of years the boulder was sealed away, out of sight -- then recent coastal erosion cut the "drift cliff" back and exposed the boulder again. Straightforward sedimentology.....

Myris of Alexandria said...

I hate to pontificate but someone has to do it.
M

BRIAN JOHN said...

Message for Kostas

Comment rejected. Not going there again -- the topic of giant erratics on the SW England coastal platform is well covered already on this blog. Please search and thou shalt find.

Myris of Alexandria said...

The Shap adamellite is highly recognisable and if it Shap I am surprised that that has not been noted before. In addition Ixer and Vince looked at the use of Shap erratics in AS pottery and needed to look at the ice directions moving the erratics. I don't think they mentioned north to south moving ice entraining Shap, but as they were dealing with NE England may have missed it.
M