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Thursday, 26 January 2012

OK -- I eat my words.....

I have often said in the past that I think the dolerites, spotted dolerites and gabbros which are found outcropping in North Pembrokeshire are too coarse-grained to respond well to glacial abrasion  processes and to reveal striations.  I have based that assumption on the fact that all of the rock outcrops and the tops of erratic boulders projecting above the ground surface are weathered -- some subjected to weathering for 23,000 years or so, and some for far longer than that.  These weathered surfaces are generally stained and pitted -- and although I have seen ice-moulded surfaces, none of these surfaces has carried striations.  Rhyolites are a different matter -- I have seen striations, for example, on the outcrops at Carn Alw.

Well now -- one of my neighbours has been doing some landscaping with some heavy machinery, and this has involved cutting into a morainic mound and hauling out masses of boulders up to 3m long -- some of them probably weighing over 10 tonnes.  Some of these boulders that have been thrown up into a great pile are shown in the top photo.  On looking at the dolerite and gabbro boulders which have been buried up to 3m beneath the ground surface, and which have now been washed by the rain, you can see how beautifully smooth some of them are -- classic abrasion by fine-grained sediments on a rather fluid glacier bed.  And there are striations!  Lots of them......  When one ignores the damage done by heavy machinery, you can see quite deep gouges and abrasions even on the surfaces of some coarse-grained boulders -- the lower photo shows a boulder with two sets of striae, one set running up and down, and the other running across the face of the boulder beneath that patch of muddy turf.

OK -- revised opinion.  There is in principle no reason why the spotted dolerites, dolerites, rhyolites and ashes used as orthostats at Stonehenge should not carry striations resulting from glacier transport.  That would happen only if the stones were carried on the glacier bed -- although as I have argued on this site before, I doubt that that was the case.  Again, if the surfaces of the stones have been exposed to the atmosphere for just a fraction of the 450,000 years that have elapsed since the Anglian Glaciation, the chances of striations surviving would be virtually zero. 

Oh, for some cosmogenic dating of those bluestone surfaces at Stonehenge........

3 comments:

Confused said...

Brian,
You say:
"Again, if the surfaces of the stones have been exposed to the atmosphere for just a fraction of the 450,000 years that have elapsed since the Anglian Glaciation, the chances of striations surviving would be virtually zero."

However, on the 8th January 2011 you posted your updated Anglian Glaciation map which showed the closest the ice edge reached to Stonehenge was the Mendip Hills. Perhaps this shortfall is a more likely reason for the absence of striations on the bluestones of Stonehenge?

BRIAN JOHN said...

Wherever the stones were dumped -- whether 50 miles from Stonehenge or at Stonehenge -- if they were exposed at the surface, they would have been subject to weathering.

I have played around with lots of ice edge positions -- work in progress!

Tony H said...

Glad to see you're maintaining a "watching brief" on your local geomorphological features (in this case a local moraine).

Archaeologists do (or,at any rate, used to do) the same thing on behalf of the various U.K. Councils, on development sites where previous human activity was deemed to have been very likely. Probably badly affected by the recession now.