Mike's Geoblog

Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

May 14th, 2010, 9:43 pm #1

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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SacoHarry
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Joined: August 22nd, 2006, 9:29 pm

May 15th, 2010, 1:21 am #2

Thanks so much for starting this blog! I'm already hooked. I think geology's fascinating, and have tried picking up bits as a hobby ever since visiting the Grand Canyon and seeing the billion years of strata there.

Never realized that mud could be "lime mud," that's pretty cool. I remember seeing somewhere that in a pinch, one can even "taste" a rock to see if saliva makes it bubble. Who knew?

Looking forward to future posts, and welcome to WeDig.
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Badger
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Badger
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May 15th, 2010, 1:00 pm #3

Great stuff, Mike. I was wondering if "mudstone" was anything like an official designation. Looking forward to the "coinstone" data.
T.Wolter
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

May 15th, 2010, 9:08 pm #4

In the interests of science, I just spat on some limestone. No fizz I'm afraid. Perhaps I'm not sufficiently acid tongued.

Mike
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

May 20th, 2010, 8:55 pm #5

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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SacoHarry
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Joined: August 22nd, 2006, 9:29 pm

May 23rd, 2010, 12:03 am #6

Random question of the day. I read that in addition to the Whin Sill, there are columns of whinstone that can be found in the area -- like little pipes of magma that squeezed up in faults and then solidified. Did the Vindolanda whinstone come from tumbled/glacial Whin Sill boulders, or from a local "pipe"? Or is there any way really to know?

Thanks again for some really interesting stuff! Amazing what can be learned from the very rocks under our feet.
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

May 23rd, 2010, 5:12 pm #7

True, odd intrusions of the same material as the Whin Sill crop up in various places. For the most part they are sheets, either horizontal(ish) in which case it's called a sill or vertical(ish) in which case it's called a dyke. For example, the geological map shows a whin dyke which is conjectured to run along the S.Tyne Valley in the Bardon Mill / Haydon Bridge area. It's shown as running along the hillside below the house we're renting above Haydon Bridge for this season, but no sign of it can be seen. The map doesn't show anything of this type for the Vindolanda area but this doesn't mean there aren't any.

However, if an intrusion were there it would be below many metres of glacial till (which can currently been seen exposed down to the underlying limestone where the builders have dug out for the new Study Centre). Applying the principle of Occam's Razor, I think it's more likely the pebbles have worked their way to the surface from within the till rather than from below it.

Mike
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

May 30th, 2010, 11:12 am #8

The rocks and landscape around Vindolanda are the product of two ice ages, one in which the rocks were laid down and one in which they are being worn away.

An ice age is an extended period of time (millions of years) during which there are large ice sheets on land. At present there are ice sheets on Antarctica and Greenland, so we are currently in an ice age which has already lasted for over two and a half million years. The previous ice age was much longer, from perhaps 330 to 260 million years ago during the later Carboniferous and early Permian periods.

It’s a characteristic of ice ages that the amount of ice on land, and hence the amount of water in the oceans, varies cyclically over periods of 10s to 100s of thousands of years. For the past 15,000 years we have been in an interglacial, with relatively small ice sheets and high sea levels. But for nearly 100,000 years before that ice covered large parts of the northern continents and sea levels were about 100 metres lower.

During the Carboniferous and Permian periods nearly all the land masses of the Earth were gathered together in a “supercontinent” geologists call Pangea. The south pole was well within Pangea, and in the ice age large ice sheets waxed and waned around it, but the north pole was within the surrounding ocean named Panthalassa. Britain was also within Pangea, close to the equator, but much of it was near the edge of a seaway connected to Panthalassa, with mountains to the north and northeast.

When sea level was high, a wealth of sea creatures lived in the warm, clear water. When they died their shells and skeletons collected on the sea floor and over time were converted into limestone. As sea levels started to fall, the water became muddy and most of the sea creatures died out. Great depths of mud accumulated which became compacted into shales. Eventually the water became very shallow and sand was deposited in a beach, delta or estuary environment. Finally, in some cycles, marshes developed with large plants and trees which fell into the water when they died and were slowly converted into coal. These cycles of limestone, shale, sandstone and coal occur over much of northern England and are called Yoredale Cycles after the old name for Wensleydale where they were first described. In the Vindolanda area they became tilted and dip at an angle of about 12 degrees to the south southeast.

Over the succeeding millions of years the Earth gradually resumed its more normal warmer state and many layers of other rocks accumulated over the Carboniferous ones. But once the ice sheets of our present ice age started to flow over the area the moving ice ground all the rocks away and the Yoredale cycles are once again exposed. During the later part of the last glaciation the ice movement was from west to east, along the grain of the dipping rocks. This left the harder rocks, the limestones and especially the sandstones, sticking up as north facing escarpments and the shales were worn down further to leave small valleys partly filled with boulder clay. The largest escarpment is that of the Whin Sill (see last week’s blog). There are places, particularly along the military road, where up to a dozen of these escarpments can be seen apparently marching across the landscape like waves approaching shore. As the last ices sheet retreated, perhaps 14,500 years ago, huge amounts of meltwater cut spillways through the escarpments as well as valleys like those which surround Vindolanda on three sides.

So the geology of two ice ages provided the Romans with:-
• the Whin Sill escarpment to build their wall on
• sandstone for building their walls, forts, vici, bathhouses, etc, etc
• limestone for mortar
• clay for wall bonding, tiles and pottery
• coal for fuel
• a defensible site at Vindolanda.

Remember, oh digger, when the icy east wind blows across the site, do not complain about the cold because without cold and ice in the past the Romans would not have built Vindolanda for you to excavate.
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

June 5th, 2010, 7:32 pm #9

The four rock types in the Yoredale cycles each represent one of the main classes of sedimentary rocks which geologists recognise, and to each of which they give names based on Latin words. Limestone is said the be calcareous after the Latin word for lime, and/or the German word kalk which also means lime. The shales contain a lot of clay and so are said to be argillaceous. Sandstones are classified as arenaceous – the arena was so named because the surface was generally covered in sand, to mop up the blood it is said. The coal is composed largely of carbon and so is carbonaceous.

When they are exposed at the surface, each of these rock types weathers in a different way. Limestone dissolves in rain or river water which is slightly acid because, as it falls, rain dissolves a small amont of carbon dioxide from the atmosphere. In air, limestone tends to develop a pitted surface because it dissolves more where the rain collects in any tiny hollow. The shales and mudstones soften and the tiny clay particles are carried away by the water. The silica grains which make up sand are very resistant to weathering but in sandstone they are held together by a “cement” – iron oxide, calcium carbonate or more silica – which can be dissolved or broken by rain or the freeze/thaw action of frost. Sandstones tend to develop a rounded shape as the grains are more easily broken away from corners and edges. Eventually the rock turns back into sand; diggers will have noticed that many of the stones they excavate are well on the way to this fate! The coal slowly oxidises to carbon dioxide gas.

A good place to see the first three of these rock types is in the left bank of the burn below the museum at Vindolanda. This burn is marked on OS maps as “Chainley Burn” but locally it is called the Chineley Burn and I have also seen Charnley Burn; lets stick to the local name. You can no longer get out into its little valley from the bottom of the museum grounds. Instead, go out of the main door of the museum and a few yards up the slope you will see some steps (clearly of reused Roman stone) leading to the right up into the trees. Turn right again at the top of the steps and in a few more yards the path divides. One path leads up into the field above the valley, the other goes down beside the laboratory building to a little footbridge over the burn. (Incidentally, please do stick to one of the footpaths as marked on the map – OS 1:25,000 sheet OL43 is recommended – as there are some quite hazardous areas off the paths.)

Once you’ve crossed the footbridge onto the west (right) bank of the Chineley Burn, the very attractive valley contains a number of features of interest. Most immediately obvious is a collection of about a dozen large sandstone boulders, on one of which the centre of the footbridge rests. Look for the one with a number of wedge holes on its upper surface. Someone, one would like to think the Romans but there may be no way to be certain, has clearly been attempting to work it. The faces of many of them are remarkably flat and at right angles to each other. This could be natural but equally could be evidence of human activity.

Looking across the burn, the other side of the valley is nearly vertical. Above about 5 metres of shale is another 5 metres or so of sandstone. At least another 5 metres of sandstone is out of sight above that. The first picture below shows the valley side, with one of the big boulders in the stream bed. Notice how there are gradual transitions from the limestone below into the shale and from the shale into the sandstone. The lower of these two transitions contains some very hard calcareous mudstones. The Romans probably didn’t work these because, as my geological hammer revealed, they break up into nasty sharp splinters. But you can imaginge that they would think any loose blocks they found would make good building stones. Only after nearly two millennia in the soil do we discover just how soft they can become.

This spring’s weather in Northumberland has had two main characteristics – some remarkably sudden switches between cold and hot and much lower than average rainfall. As a result of the latter, all the burns are running at very low levels. Through the museum grounds the Chineley Burn is very sluggish and scummy and below the weir the flow vanishes completely. For around 100 yds downstream, nearly to the second footbridge, there are just some stagnant pools. Then suddenly the flow emerges again, bubbling up sparkling clear. In the dry section it is running below the surface through channels dissolved over the millennia in the “Four Fathom” limestone which which forms the stream bed and is now completely exposed. The panorama below shows the bottom half of this dry section and the point where the flow re-emerges. The apparent curvature of the stream bed in the picture is an artefact of the way five photos combine into a panorama; actually this section is almost straight. The “limestone pavement” thus revealed is typical of many such in the northern Pennines. In Yorkshire the blocks of limestone are called “clints and the fissures between them are “grikes”. The clints are rounded here because they are being dissolved by the flowing stream rather than by rain.

Over the coming weeks, except when there’s anything more immediate to discuss, I’ll write a bit more about each of these rock types and how the Romans used them. But first I’ll talk some more about the Whinstone, the story of which includes one of the most fundamental geological discoveries of the past half century.
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Joined: December 2nd, 2006, 8:48 am

June 5th, 2010, 8:10 pm #10

Just like to add 2 more photos to Mike's piece. One is taken from the footbridge below the museum gardens, the other shows the slots in the rock he mentions.

Malise
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