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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.

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Viewing Single Post From: Mike's Geoblog
Mike McGuire
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As promised,an answer to the conundrum many diggers have experienced - what are the green pebbles which keep cropping up and which are occasionally mistaken for something interesting? Several members of Andy's crew this week provided some examples of various different shapes, sizes and appearance. So at lunchtime today, in front of several witnesses, I took a big hammer and smashed them to bits. They turn out to be quite hard, and are generally dark grey inside, although some carry a greenish tinge all the way through. Viewed through a hand lens this grey material is not made up of separate individual grains (like the sand grains in the sandstone which forms most of the Vindolanda stones) but is a single dense mass with, in some cases, pale coloured crystals visible in it. Of the rock types in the area this could only be limestone or whinstone, and a negative test with some acid later at home confirmed it is not limestone. So the green pebbles are whinstone.

Whinstone is the local name for the rock which forms the Great Whin Sill, the massive outcrop which the central section of Hadrian's Wall runs along in the vicinity of Vindolanda. The rock is of a type called quartz-dolerite, a form of basalt, which was intruded as molten magma into the layers of Carboniferous rocks about 295 million years ago (so about 30 million years after the rocks were laid down). Large cobbles of whinstone are quite common on the site at Vindolanda, where they were often used by the Romans in the rubble fill of walls but rarely as facing stones as they are so hard. Usually these stones are rusty brown in colour and many of them have layers of rusty material flaking off their surface - a phenomenon known as "onion-skin" weathering.

So why are the pebbles you dig up green, whereas the building stones are rusty brown like the Sill itself? My speculative answer is as follows. Dolerite contains a large proportion of minerals which include iron and magnesium in their structure. In prolonged contact with air and water, these tend to become chemically altered and the iron can be oxidised in one of two ways.

When the alteration happens in the air, the iron is strongly oxidised and forms minerals which are orange, red, brown or even purple and which we generally refer to as rust. I think the whinstone cobbles used for building had originally been plucked out of the Sill by the ice during the last glacial period (up to 15,000 years ago) and carried down the little valleys created by the meltwater as the ice melted. The Romans just picked them up along with all the other useful cobbles in the valley bottoms. The valley of the Cockton Burn to the north of the site still contains vast numbers of them. Because they have been wet and in contact with the air for nearly 15,000 years (albeit shallowly buried for the past 2,000) they are rusty.

Smaller fragments of whinstone got mixed up with the finely ground clay which was produced by the ice and which is deposited all over the area as what is called boulder clay or glacial till. Deep in the wet till, out of direct contact with the air, the iron in these whinstone pebbles was less strongly oxidised, to a state which commonly forms yellow or green minerals. The commonest of these is a clay mineral called chlorite (not because it contains chlorine but because, like chlorine, it is green and chloros is the Greek word for green). Another possibility is a mineral called glauconite but I think this is much less likely. So the pebbles became coated with a soft coating of green chlorite, and in some cases the alteration penetrated into them giving a greenish tinge to the grey whinstone. There is quite a thick layer of till underlying the Vindolanda site and I think the green pebbles you find must have worked their way up from this both by natural priocesses such as freeze/thaw and by human agencies such as Roman occupation and subsequent ploughing.

So sadly these rather attractive green finds are actually just bits of stone, but I find the story of how they may have been formed interesting and I hope some of you do too. And do look carefully before discarding one, it might really be a precious bronze object!

Next week, a bit about ice ages and how the landscape around Vindolanda was formed. Meantime, all comments wlecome.

Mike
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