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Tuesday 11 June 2024

On glacial erratic transport


It's not easy, being an geomorphologist trying to communicate relatively straightforward concepts to people who have no background in the earth sciences.  One has to communicate with members of the public in language which is understandable and yet tight enough to pass muster with the referees and editors of scientific journals.  So I must be patient with those who have a problem in understanding the text of my latest article (on the Newall Boulder) around pp 8 and 9 where I explain why I think the boulder was transported by ice.

Our old friend Tim Daw, on his blog and on Twitter, seems to be suggesting that if the boulder found at Stonehenge has indeed been glacially transported over a great distance, it must look demonstrably different from similar boulders (of many sizes) that are found at Rhosyfelin.  He says that unless I can demonstrate that to his satisfaction, my research and my conclusions are meaningless.  Sadly, that demonstrates a misunderstanding of how natural processes operate.  Transport distance is indeed one of the factors that influences clast shape and surface characteristics, but as I explain in my text, it all depends on where and how a clast is being transported.  Clasts carried supraglacially or englacially (ie either on or within a glacier) may not be affected at all by abrasion or crushing, even after hundreds of kilometres of transport.  On the other hand clasts carried subglacially may be dramatically modified -- if bed conditions are right -- over a transport distance of just tens of metres. Clasts may become trapped or stuck on the basal ice-sediment interface, or they may be rolled over or broken, leaving abraded facets or percussion scars such as those described on the surfaces of the Newall Boulder.

If you look at the clasts featured in my Figure 7,  you will see that each one has a unique combination of features and a different history.

And as indicated in my paper, far-travelled clasts tend to follow zig-zag paths over hundreds of thousands of years, as a result "re-entrainment" and "re-mobilisation" in successive glacial episodes.

There is no way you can look at one clast at Rhosyfelin and another at Stonehenge and say "This one has been subjected to glacial transport and this one has not".........

That having been said, there are certainly abundant glacially-transported clasts at Rhosyfelin that do display typical "diagnostic" surface features, as described in earlier blog posts.

My new paper makes a very simple point:  namely that the Newall Boulder displays a number of features that are characteristic of glacially transported clasts.  I cannot understand why that should be such a problem for some people......

And by the way, we do not know that the Newall Boulder has come from Rhosyfelin.  I am pretty convinced that it has not.  It's all explained in the text.

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