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Sunday, 16 October 2011

Super-erratics -- a picture gallery


Vast Finnish erratic on the coast of Estonia

 
Part of the Big Rock erratic complex, Okotoks, Alberta


Large erratic in Manitoba, USA

Isolated "giant erratic" -- Patagonia

 
Cluster of large erratic blocks, Tierra del Fuego



Single large erratic block, Kjove Land, East Greenland

 Granite erratic, Newfoundland

The House and Barn erratics, New England

The 5,000 tonne Madison Boulder, USA.  Note the person for scale.

I was asked the other day whether my post about the entrainment of vast blocks of rock (maybe including whole quarries!) was a wind-up.  I replied that it was slightly tongue in cheek -- but that there was a serious point to it, in that huge blocks or slabs of rock like the above are found on all continents where glaciation has occurred on a large scale.  So there is absolutely no reason why these "super-erratics" cannot also have been entrained in Western Britain, under the influence of the Irish Sea Glacier during the Anglian and other glaciations.

For some of the time, erosion on the glacier bed seems to be by a "nibbling" process, with smallish boulders, blocks or slabs being lifted off the bed over and again, gradually increasing the load being transported.  However, occasionally the glaciers do seem to take "bite-sized chunks" -- and glaciologists are still not sure how frequently this happens, and why.  There is a reasonable theory to explain this dramatic process, but for obvious reasons observations beneath present-day glaciers are difficult!

I like the reference in one of the specialist sites to erratic blocks like these being "in temporary residence" -- on the assumption that next time round (if there is one) they will be moved onwards, sideways or even broken up by glacier ice.

And I am still quite attracted to the idea that vast chunks of rock might have been entrained from sites like Craig Rhosyfelin and Carn Goedog -- and then broken up either during transport or else following dumping at some distant location.




5 comments:

Tony H said...

Has there not been some suggestion, perhaps from documented medieval history sources, that Glastonbury Tor - only 30-odd miles from Stonehenge as the proverbial crow flies - was once the home of a fair number of fairly large boulders, origin unknown? If so, wonder if the farmers/ monks removed them? Anyone heard of this?

Alex Gee said...

Brian
I can't remember where (it may have been here), but I've read about a huge rock, that was located on the Somerset levels? I think it was called the Upping Stock. Apparently it was of a similar size to those in your photographs, and was broken up for building stone, sometime in the late 1800s or early in the 1900s?.
perhaps other posters know more details?

BRIAN JOHN said...

Pete G might have records that are relevant here? He has a very good knowledge of strange stones, and where they are.....

Tony H said...

Alex

I note that the Upping Stock was near Chedzoy, from a quick Google search.

I bet Mick Aston of TimeTeam knows about it, as he was formerly Somerset County Archaeologist.

Alex Gee said...

You could be on to something here. Having looked into this further.
It appears that large rocks and boulders crop up quite regularly on the levels.

Nynehead church for example, has a large "upping stock" boulder used for mounting horses;it seems that this is the common usage .It also has a 16th century red sandstone dole table; across which the poor received alms. On the levels, other large rocks seem to have been commonly used as small bridges to cross streams.

All these boulders must have come from somewhere. Its a great pity that someone hasn't found one in situ.

Maybe some ferreting around the levels is called for?.