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From Aug 2006 - Nov 2013 WeDig provided a live forum for diggers & fans of Vindolanda. It has now been mothballed and will be maintained as a live archive.

Here you will find preserved 7 years of conversation, photos, & knowledge about a site many people love. Vindolanda gets under the skin. (Figuratively and literally as a volunteer excavator!) It's a place you remember, filled with people you remember!

Thanks for 7 great years!

Viewing Single Post From: Mike's Geoblog
Mike McGuire
Member
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As a new member of WDV, let me introduce myself. I've been a regular visitor to Vindolanda since 1997, but until now only as a spouse - husband of Archaeologist and expert pot-washer Malise whom many of you will know. So thus far I've just been known as "the pot-washer's chauffeur". But after 10 long years I have, much to Malise's relief, finally got my Open University degree in Earth Science. And as you all know, anyone noticed hanging around Vindolanda who has a skill which might be useful rapidly gets roped in. So, somewhat to my surprise, I now find myself to be lead volunteer for the exciting Stone Sources Project. Once I've got a bit further with this project I'll use this blog to let you know about it and what's going on. In the meantime, I thought I'd have a "Geo topic of the week" to answer some of the geological questions which diggers have been asking me so that I can give them more considered answers which might also be of interest to others. But please remember that, despite the grey hair, I'm still what a friend calls a "sprog geologist", so feel free to put me straight if you think I've got it wrong or to add your own comments or to ask further questions.

This week's topic, which has come up in another forum, is mudstones - or "The mystery of the acid in the washing up bowl".

Few of you who were present at the diggers' hut last Thursday lunchtime will fail to have noticed my rather amateurish bit of geo-cunjouring involving one of Malise's pot-washing bowls, four nondescript bits of stone and a bottle of mild acid. The four stones, all taken from the 2009 excavation area, were - 1) some hard, grey, slimy mudstone with bits of fossil in it, 2) some of the hard, darker rind often found on the grey mudstones, 3) a small piece of a very brown mudstone, 4) some very brown sandstone with fossily bits in it. When I poured a few drops of acid onto each stone the results were that numbers 2 to 4 showed no reaction at all (well, 2 did a little bit after a while) but 1 reacted as if I'd taken the lid off a tiny, well-shaken Coke bottle.

What this experiment demonstrated is that there are two types of mud and the Vindolanda mudstones are mixtures of these two types in very varying proportions. The first type of mud is just that, mud, which has no reaction with the acid. The second type is lime mud, the finely ground (by the sea) remains of the shells and hard parts of sea creatures which were present when the Carboniferous rocks around Vindolanda were laid down about 325 million years ago. The visible bits of fossil are where the grinding was not so fine. This second type of mud is the mineral calcium carbonate which reacts with acid by giving off carbon dioxide - yet another way of putting fossil carbon back into the atmosphere, which is probably why it's a bit warmer today after weeks of biting cold winds! The grey mudstones (sample 1) are mostly lime mud - hence the big fizz - but they have some ordinary mud in them and after a couple of thousand years in the soil the lime mud has dissolved away from the surface layers, to leave a hard crust of the ordinary mud (sample 2). Sample 3 was ordinary mud, hence no reaction. In sample 4, even the fossily bits did not react with the acid, which shows that in sandstone the fossils have been turned to silca (which is what sand is) whereas in the lime mud the fossils remain as calcium carbonate.

Hope that's all clear and of interest. Any queries or comments welcome. Next week, the intriguing case of the green pebbles. Meantime, for those who missed it, here's Thursday's big event.

Mike McGuire
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