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Tuesday, 24 November 2015

Weathered and unweathered bluestones at Stonehenge


 Typical weathering crust on a dolerite boulder on Carningli, N Pembrokeshire

 Tim Daw's excellent plan of the stone monument, showing the main stone types.  (Very generalised -- not all of the dolerites are spotted....)

In 2002 Geoffrey Kellaway argued that the bluestones at Stonehenge were dumped on Salisbury Plain during a very ancient glaciation -- even earlier than the Anglian Glaciation which we frequently talk about on this blog.  That's a possibility that we cannot dismiss.

"Glacial and tectonic factors in the emplacement of the bluestones of Salisbury Plain",  Survey of Bath and District 17 (Nov 2002), pp 57-72.

But leaving the dating issue on one side, he does make some interesting points in his article.  He claims that the abundant chips and flakes of bluestones (of various types) found in the debitage at and around Stonehenge are partly the result of the stone-masons' attempts to remove the brown weathered layer from favoured stones -- presumably to enhance their "blueness" and make them more attractive for placing in the bluestone horseshoe.  This is an interesting point in itself, and I don't know the answer to it.  Are most of the chips and flakes in the debitage fresh and blue, or are they brown and weathered?  Perhaps Rob can answer that one, since he knows the debitage rather well!

So we have the possibility that the dolerites used in the horseshoe have been fashioned and "enhanced" whereas the rougher boulders and slabs in the bluestone circle might have been used as found. (There are apparently just two exceptions, which are assumed to have been shaped and used at one time as lintels.)

 Some of the "natural" bluestones in the circle, overshadowed by the sarsens.  Are there weathering differences on the faces we can see today?

Of course, if you are a megalith builder wandering around on Salisbury Plain looking for interesting stones, you would find them bedded into the turf with one (weathered) face upwards and with other faces relatively fresh, depending on their precise positioning on or in the soil layer.  A boulder lying on the ground surface might have three weathered faces and one fresh face; a boulder deeply embedded might have just one weathered face and three faces protected and unweathered.  Cosmetic work on the collected stones might well be partly for shaping purposes, and partly for the purpose of removing weathered crusts.

So here we have another question:  as far as the standing and leaning dolerite bluestones in the horseshoe are concerned, are all faces equally weathered, or are some more weathered than others?

And another question:  for the bluestones of various types in the bluestone circle, are all faces on these stones equally weathered?

This would make a nice little project for somebody -- and it would be essential if ever a proper programme of cosmogenic dating should come into play, since different surfaces on different stones will have very different "exposure ages."

It's not that difficult to differentiate between heavily weathered and fresh faces -- for example, the differences between the "old" and the "new" faces on the big "proto-orthostat" at Rhosyfelin are very obvious.

1 comment:

chris johnson said...

Any sensible investigation of "stones for stonehenge" would have embraced the opportunity to date according the new technologies. Unfortunately there has been a blind spot regarding the geomorphological possibilities.

Personally I believe glaciers moved the stones to Whitland from whence they were pulled on sledges along the A40. I do accept this is a minority view, but one supported by an equal or even superior amount of data.