Welcome Guest [Log In] [Register]

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!

Welcome to We Dig Vindolanda!

Username:   Password:
Add Reply
  • Pages:
  • 1
Mike's Geoblog; Geological aspects of the Archaeology
Topic Started: May 14 2010, 04:43 PM (1,742 Views)
Mike McGuire
Member
[ *  * ]
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
Attached to this post:
Attachments: The_big_kidz.jpg (154.7 KB)
Attachments: The_big_fizz.jpg (166.48 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
In the interests of science, I just spat on some limestone. No fizz I'm afraid. Perhaps I'm not sufficiently acid tongued.

Mike
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
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
Attached to this post:
Attachments: don_ttrythisathome.jpg (177.25 KB)
Attachments: Broken_Pebbles.jpg (59.38 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
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
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
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.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
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.
Attached to this post:
Attachments: Chinely_Burn_valley.jpg (185.6 KB)
Attachments: Dry_burn_panorama.jpg (199.08 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
I’ve alluded previously to the fact, which nowadays I think most people know, that the continents move very slowly around the face of the globe. “About the rate your fingernails grow” is a common analogy. What is less well understood is how this is possible. Quite commonly it is suggested (including in a recent BBC TV science series which should have known better) that there is a layer of molten rock below the crust. NO – THERE IS’NT. So how can the continents move? Here’s the simplest explanation I can give of what really happens.

The Earth has four layers. At the centre is the solid inner core which is mostly iron. Outside that, reaching out to just over half the Earth’s diameter of just under 14,000 km, is the liquid outer core, again mostly iron, which produces the Earth’s magnetic field. Nearly all the rest, making up about 80% of the Earth’s volume, is the mantle which is made of what we would recognise as “rock” – mostly silicate minerals. The crust is just a thin layer on the outside, typically 35-40km thick for the continents and only about 7 km thick for oceanic crust.

The layer we’re concerned with here is the mantle, which is mostly made of a rock type called peridotite. Much the most common mineral in this is olivine – the jewellers’ name for olivine is peridot. Olivine is green, so yes folks the vast majority of the Earth’s bulk really is green, not just the outer surface we live on (and, boy, is that green in the Vindolanda area at this time of year, especially now it’s back to the rainy season). Like all rocks which contain more than one mineral, peridotite melts over a range of temperature. At the surface, it starts to melt at about 1,100 degrees C, but this temperature increases as the pressure increases with depth into the Earth. By about 350 km deep the melting temperature is over 2,000 degrees C. The temperature increases as you go into the Earth and so there is a sort of competition between the increasing temperature trying to make the peridotite melt and the increasing pressure trying to stop it. In normal circumstances the pressure wins at all depths, though in a few special cases a bit of melting does occur – hence volcanoes and all igneous rocks including the Whin Sill (of which more later). However, there is a depth between about 100 and 200 km where the mantle is close enough to melting that it gets ever so slightly “squidgy” and can flow very slowly by a mechanism called solid state creep. Geologists call this layer the asthenosphere and the “non-squidgy” mantle above it together with the crust are called the lithosphere. In fact it’s sections of the lithosphere, referred to as plates, which move, carrying the continents with them, driven by complex combinations of forces which are not completely understood.

The existence of the asthenosphere was first discovered by seismologists studying how vibrations from earthquakes propagate through the Earth. They do so slightly more slowly through the asthenosphere than through the surrounding mantle. There are a few isolated instances where bits of peridotite have been “coughed up”, so to speak, at the Earth’s surface and laboratory experiments on these confirmed the seismologists’ discovery. This discovery had far-reaching implications. It provided a way in which the theory of continental drift, which had been pooh-poohed for many years as impossible, could be possible and thus could explain much else which had been a puzzle. And it also explained why vertical movements of the crust occur. Sections of the lithosphere can be thought of as “floating” in the asthenosphere and so Archimedes’ principle can be applied. Geologists call this phenomenon isostacy.

You may have wondered how it was possible for all the Yoredale Cycles to keep piling up on top of each other and yet still stay at about sea level. Isostacy provides the answer. Partly it was the weight of the accumulated sediments pressing the whole pile down into the asthenosphere. But this would not have been sufficient on its own. The lateral movements of the plates can cause the crust to become compressed in some places, as they did about 100 million years earlier when England and Scotland were welded together, and to become stretched in other places. It seems that, in the mid-Carboniferous when the Yoredale Cycles were deposited, this area of crust was being stretched and the whole Northumberland basin sagged down. The combination of these two effects caused the crust to subside at just the rate which was needed to create the repeated deposition cycles we now observe.

On a smaller scale, isostatic movements have occurred in Britain since the end of the last glaciation. Since the weight of all that ice was removed, the northern part of our island has been rising up – hence phenomena such as raised beaches in Scotland. In compensation, the southern part of Britain has been subsiding. The Romans knew what we now call the Scilly Isles as a single quite substantial land mass.

The stretching in Northumberland had one other important effect. Because the crust became thinner, the load on the top of the mantle was reduced to such an extent that a very small amount of melting was eventually able to occur. When such partial melting happens, some minerals in the peridotite melt sooner than others and the resultant liquid has a different composition called basalt. Liquid basalt is less dense than the rocks of the lower crust so it rose up through fissures until it reached a level where its density matched that of the rocks. Then it spread out sideways over a vast area to form what we now see as the Whin Sill. Outcrops of the Sill are found all the way from the Farne Islands in the north east to the Pennine ridge overlooking Penrith in the south west as well as down Teesdale at places such as High and Low Force.

Such “decompression” melting is one, but not the only one, of the main ways in which partial melting of the mantle occurs to produce the igneous rocks which eventually become the Earth’s crust.

Sorry if some of the terminology is a bit confusing. In common with most scientists, geologists tend to use Greek and Latin words to generate names for the new concepts they discover. This contrasts with my own former full-time profession computing (yes, I confess it, I am an ancient Geek) where new ideas are named by taking the American word for something completely different.

Next week, something a bit less abstruse – limestone.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
One other thing I meant to add about the Whin Sill. Although the Roman masons avoided having to work this hard material as far as possible, in one place they had no option. To the east of Vindolanda along the military road, just beyond the fort of Brocolitia, is a location rather oddly called "Limestone Corner" where the wall ditch intersects the Sill. Dutifully the Roman soldiers started hacking away at the Whinstone to create a ditch through it. Amazingly they nearly completed the job. Large boulders were broken loose and pulled up out of the ditch, probably by passing ropes under them and "rolling" them up the sides. Many of these boulders are still where they were left. But one very famous boulder is still in place. This has a number of slots chisled into its surface. There is a myth commonly repeated that these are Lewis holes which the Romans intended to use to lift the boulder out of the ditch. NO - THEY AREN'T.

For a description of what a Lewis is, go to http://en.wikipedia.org/wiki/Lewis_(lifting_appliance). The Romans used the version called a "three-legged Lewis" which requires a slot to be cut in the rock in which the end faces slope away from each other as you go into the rock. There is a good example in a sandstone slab just outside the north gate of the fort at Vindolanda. But the slots in the boulder at Limestone Corner are quite different, as the the inset in the second picture below shows. In these, the short faces slope inwards. In any case, they wouldn't have cut so many Lewis holes, all at different angles and aligned along small fissures. These are quite clearly wedge holes which were to be used to split the rock into more manageable pieces. In this case they would probably have used iron wedges and some lusty blows with big hammers, although another common technique for accurate splitting of building stones was to hammer in wooden wedges and then wet them so the expansion would split the stone.

But before they got round to doing the splitting, they must have got called away to other duties, or someone said "Nobody's looking, let's not bother". In any case, they left us this fascinating insight into Roman stone working techniques. And you can now put straight anyone who says these particular examples are Lewis holes.
Attached to this post:
Attachments: rock.jpg (206.65 KB)
Attachments: rocktop.jpg (204.41 KB)
Edited by Mike McGuire, Jun 15 2010, 07:29 AM.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
After the Earth formed four and a half billion years ago, its first atmosphere was much denser than now and was made up mainly of carbon dioxide. At that time the sun was only about 70% of its current brightness, but the “greenhouse” effect of all that carbon dioxide meant the surface temperature was much the same as at present. Over the eons which have passed between then and now, as the sun has got brighter, the amount of carbon dioxide has decreased in such a way that the surface temperature has always been amenable for life. And it is life itself which has caused this decrease, in a way which some people see as evidence for the idea that the Earth itself can be thought of as an organism. Most scientists agree that there is indeed such a “feedback” mechanism involving life, but don’t think this effect is comparable with the feedbacks within organic life forms. Whichever view you take, it’s clear that all that carbon has gone somewhere, so where is it now? Two of the rock types we find around Vindolanda provide the answer.

About 20% of the carbon has gone into the coal, oil and gas which collectively we call fossil fuels. Such deposits are the organic remains of living things – land plants in the case of coal and microscopic sea creatures in the case of oil. Most fossil carbon is in such low concentrations that it will never be economic to extract it but concentrated deposits have been, and still are, the basis of industrial society. Coal was extensively mined in the area around Vindolanda well into the 20th century and was also an important resource for the Romans. Lumps of it regularly turn up during excavations.

The remaining 80% or so of the carbon became locked up in the carbonate rocks classified as limestones. Although there are some special circumstances in which limestones are produced by chemical precipitation from water, the vast majority of such rocks are derived from the “hard parts” of once-living organisms. The oldest limestones known are at least three and a half billion years old and are part of the evidence that life, albeit just in the form of bacteria, got started very early in the Earth’s history. But the story of limestone really got under way with the so-called “Cambrian Explosion” around 550 million years ago when big creatures with calcium carbonate shells and skeletons first evolved.

Limestones of various types are amongst the most characteristic rocks of England and include (getting older down the list):-
•The Chalk, as in the White Cliffs of Dover, formed in the Cretaceous period from sub-microscopic platelets produced by single-celled algae
•Jurassic limestones from which many of our best-known buildings are constructed, for example in the city of Bath
•Permian dolostones, so-called because they are formed of a mineral called dolomite which is calcium magnesium carbonate
•Carboniferous limestones, for example in the Mendips, in the Peak District and in the Pennines from the Yorkshire Dales to Northumberland.

The Carboniferous limestones of Northumberland are composed of the remains of shelly sea creatures, mostly ground into a fine powder by the action of the sea but sometimes as identifiable fossil remains. The commonest and most characteristic fossils are those of crinoids. There are still some of these remarkable creatures around today, but much reduced in number from their heyday in the Carboniferous. You can find many examples of picture of crinoids and crinoid fossils on the web, for example at http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Crinoidea. Usually the fossils take the form of a short, thick-walled cylinder, anything from a millimetre to a centimetre or more in diameter. Each cylinder is just one segment of a crinoid stem or arm. Corals, in a wide variety of forms, are also common and shells of a great variety of molluscs can be found as well as fossil burrows.

Limestone outcrops can be seen at many places in the area. They can often be distinguished from sandstone outcrops because, in the limestones, horizontal joints between the beds are often wavy, on a scale of a few centimetres, as a result of the way the rock slowly dissolves as water runs through it. Each of the limestones was formed at a time of high global sea level, so they can be correlated over a wide area by the details of the fossils in them and by their position in the sequence of rocks. Often, the quarrymen gave the limestones names which reflect their typical thickness. The five key ones in the Vindolanda area are, from bottom to top, the Five Yard Limestone, the Three Yard Limestone, the Four Fathom Limestone, the Great Limestone and the Little Limestone. The Vindolanda Museum is sited on the Four Fathom Limestone. A very good quarry exposure of the Great Limestone can be seen next to the restored lime kiln at Crindledykes which is visible from Vindolanda on the hillside to the north east. The most extensively worked coal seam is just below the Little Limestone.

These limestones rarely make good building materials. They are hard to quarry and shape, they weather quite rapidly and they are often dark grey as they contain amounts of mud and/or organic material. The Romans were well aware of this and very few limestone blocks come up in the excavations. However, limestone is, and was to the Romans, an essential material for making lime, which is used for mortar and for agriculture. Heating the calcium carbonate in a kiln drives off carbon dioxide to leave calcium oxide (quick lime). This reacts vigorously with water to form calcium hydroxide (slaked lime) which sets very hard and makes a very serviceable mortar if mixed with sand.

Even stronger than lime mortar is cement, which the Romans pioneered, but this also involves shale which I’ll write a bit about next week.

Of course, odd bits of limestone do get excavated. The first picture is something a digger thought was bone; only the tiny crinoid fragments told Malise when she cleaned it this afternoon that it was limestone. The second picture shows the glacially eroded top of the Four Fathom Limestone where the contractors briefly uncovered it below many metres of boulder clay during the building work on the new study centre.
Attached to this post:
Attachments: Bone_or_stone.jpg (206.58 KB)
Attachments: Limestone_uncovered.jpg (218.85 KB)
Edited by Mike McGuire, Jun 20 2010, 03:06 PM.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
Ian

Depends what you mean by "the top of the hill". For a while I very much liked the idea that they had been rolled down from the top of Barcombe Hill. But, alas, I don't think even blocks of that size would have rolled so far. So I think they came from the top of the cliff which you are facing when you stand by the blocks on the footpath and look across the stream. You can see to what appears to be the top of the sandstone in this cliff, but actually there is at least as much thickness of sandstone again above that, but set a bit back so you can't see most of it from the footpath. I think it's in this area the blocks came from.

Yes, I think you may well be right about the quarrying method. They probably used somthing like crow-bars to loosen the blocks, shouted if they were feeling kind, and pushed them down. It may well be that even though many of the faces look very flat and square, these are actually the natural joint faces. My guess is they pushed plenty down for the job they were doing and then split them into the shapes they needed until they had enough. What the job was, and at what era in the life of the fort it happened, is very much open to speculation. Early on or late on are two popular guesses. Suggestions on a post card please!

My next blog is a bit delayed due to some slight health complications (bitten by a farm dog and, separately, strained my back) but I hope to write something as promised about the shales in a day or two.

Mike
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
A lot of terms in geology have been adapted from everyday words and so tend to have rather imprecise, often overlapping meanings. This is particularly true for terms applied to very fine particles and the rocks they can be compacted into. So the terms clay, silt, shale and mud mean different things to different people and are often used interchangeably. You’ll never get full agreement on a fixed terminology, but the following seem to me to be useful descriptions of what these terms mean.

Clay – extremely fine particles less than four thousandths of a millimetre across. Clay, as in the stuff that sticks to your boots and makes the garden hard to work, usually contains a high proportion of such particles. The grinding action of ice produces vast quantities of them – hence boulder clay. The term “clay minerals” refers to a group of silicate minerals with layered structures which are usually found as very fine particles and are the main components of clay. Clay is fired, now as in Roman times, to make pots and tiles and all sorts of other things.

Silt – fine particles between four thousandths of a millimetre and one sixteenth of a millimetre across. Above this size, particles are considered to be “sand”. Silt is often associated with sediments deposited by flowing water but this isn’t always the case.

Shale – rock made of clay and/or silt and which is fissile, i.e. it breaks easily along parallel, usually horizontal planes. The old-fashioned geologists’ way of finding out whether a shale contains silt-sized particles is to grind a bit gently between the teeth. If it feels what the Scots would call “a wee bit gritty”, then it contains silt; if not, it’s just clay. Nowadays, of course, health and safety abhors such a practice.

Mud – if you’ve dug at Vindolanda, especially in Justin’s area, and you don’t know what mud is you’re extremely lucky. There seems to be no formal definition of mud as a term on its own, but lots of geological terms contain the word mud. One such is mudstone, which is a rock made of clay- or silt-sized particles which is massive rather than fissile, i.e. it doesn’t break along parallel planes.

In the Yoredale cycles there is often a great depth of shale and mudstone between the limestone and the sandstone. This was deposited over a long period, probably many tens of thousands of years in most cases, from fine material carried out into deep water by the diminishing flow of rivers as they entered the sea. The shales are generally dark grey or black partly because many of the minerals are dark but also because the organic remains of innumerable sea creatures were incorporated into them. In some parts of the world there are “oil shales” which contain so much organic matter that oil can be distilled from them.

Most clay minerals in shale originate from the chemical weathering of silicate minerals in igneous rocks such as feldspars and minerals containing iron and magnesium. Sand, on the other hand, is mostly the grains of quartz (silica) which were released from the igneous rocks as the other minerals weathered away. As well as silicon, the clay minerals also contain substantial amounts of aluminium, some iron and small amounts of various other metals. When shale is heated together with limestone, the result is a mixture of the oxides of calcium, silicon, aluminium and iron; we call this mixture cement. When cement is mixed with water, it slowly forms a number of very complex compounds, many of which form hard needles which interlock to give a solid of great strength. Mixed with sand, this is called mortar; mixed with sand and some form of aggregate it is called concrete. Concrete was first invented in modern times by John Smeaton who used it to build the Eddystone Lighthouse in 1756.

The Romans also used a type of cement which was made by heating limestone with a volcanic ash called pozzolano. This seems first to have been invented by the inhabitants of Campania, perhaps in Pompeii, in the 4th and 3rd centuries BC. The pozzolano comes from an area on the north side of the Bay of Naples. This cement has a very similar composition to its modern equivalent and was mixed with sand and water to form mortar. The Romans developed their use of it in a wide variety of ways, combining it with various forms of stones, rubble, brick and tiles to create a variety of types of concrete with names such as opus incertum, opus reticulatum and opus testaceum. The high point of Roman concrete construction must surely be the dome of Hadrian’s Pantheon in Rome, still one of the most remarkable structures ever built.

In Roman Britain pozzolano was not available and I don’t think there is any evidence the Romans knew how to use shale to make concrete. So for most building purposes they used lime mortar or sometimes clay. But they did have an alternative known as opus signinum which was used particularly for floors in important buildings such as bath houses, mainly in the first to second centuries. In this material, lime mortar was mixed with broken up tiles. To some extent, the pieces of tile simply form an aggregate which gives some additional strength and a decorative, red-coloured appearance. However, the tiles are themselves fired clay and so, if sufficient of the tile material is finely powdered, it can form similar minerals to those found in concrete and thus give much greater strength.

I should add that in the previous two paragraphs I’m straying well away from geology into the fields of materials science and archaeology. As it’s 33 years since I was a materials scientist and I’ve never been an archaeologist, I’ve culled much of the above from some of the excellent books on Malise’s bookshelves. But I wouldn’t be at all surprised if I’ve got some of it wrong and if any of you have better information than me, please share it with us all to correct my errors.

Next week we finally get on to that most familiar material to all of us – sandstone.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
For proper cement, the clay minerals in the shale need to be heated to high temperature to release the oxides of silicon and aluminium (known as silica and alumina). It is these which react with the calcium oxide (lime) from the limestone to create the hard cement mortar or concrete. I assume Prof. Eric's shale mortar had shale instead of sand as a filler in a basic lime mortar, possibly because although there's plenty of sandstone in the area there's not a lot of loose sand. It's possible that the minerals in the shale get involved in the setting process and create a slightly harder mortar, but you would need to be an expert to know about this.

Glad you're still enjoying the blog, Harry. Sorry we didn't see you here this last week. I had also planned to do a week's excavating then but I didn't want to risk the back, which is only improving slowly (the dog bite is healing well). Andy still has hopes of getting me into the trenches before the end of the season.

Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
For many of us in Britain sandstones are a familiar, everyday sight. Even if we don’t live in an area to which it’s “native”, there are older buildings and pavements made of sandstones in most towns and cities. In many other parts of the world sandstones occur commonly in both the natural and built environments. From Uluru (Ayers Rock) to New York Brownstones, sandstones are a ubiquitous part of our lives, whether in the “flesh” or in images. Almost everyone in the world must have seen a Western set amongst the buttes of Monument Valley. And yet very few people would be able to identify a sandstone when they see it in a building, quarry or natural outcrop. I recently asked a very knowledgeable amateur archaeologist what type of stone they thought Hadrian’s Wall was mostly made of. “Er, limestone I suppose”. Oh dear!

Sandstones consist mostly of quartz (silica) grains. Generally these will have come originally from weathered igneous rocks such as granites, but they may have been reworked from sand to rock and back to sand several times over geological history. The grains are deposited from a moving fluid when its speed decreases, the fluid being either water or air. Water lain sandstones are commoner and may result from a wide variety of environments – lake, river, estuary, delta, beach or off-shore. Air-borne or “aeolian” sands are deposited in deserts or back-shore environments – any situation where we would find sand-dunes today.

Of course, as with any such sediments, the vast majority are washed away or re-worked again and again without becoming rock. But a few get covered up by more layers of sediments and eventually are buried deep enough for the process of “lithification” (making into rock) to start. This involves the sand grains being stuck together by some sort of cement – iron oxides, calcium carbonate or silica. Because the sand grains are very hard, sandy sediments are porous and water flows through them easily. Usually the cement forms from minerals dissolved in the water being precipitated on the sand grains. These stick the grains together and reduce the porosity. If, once the sandstone has formed, the overlying strata are eroded away, the sandstone is exposed as rock at the surface.

In Britain, four geological periods are particularly noted for producing sandstones. These four tie up quite neatly with the periods when continental drift carried this bit of continent though the tropical regions from south to north. So in Devonian times, at a latitude similar to today’s Atacama, Kalahari and Australian deserts, the Old Red Sandstone, a typical desert sandstone, was laid down in places as far apart as Orkney and the Brecon Beacons. In Devon itself, the Devonian rocks include mainly off-shore sandstones. Large areas of variously buff-coloured sandstones in Central Scotland, Northern England and South Wales are mainly near-shore, delta and estuary deposits from the Carboniferous, when the climate and was similar to today’s Amazon, Congo and South-East Asian equatorial regions. Carboniferous York stone from West Yorkshire is perhaps Britain’s finest and most durable building and paving stone. During Permian and Triassic times, Britain was at about the latitude of the present Sahara and Arabian deserts. In the west of Britain, the Permian New Red Sandstones, which occur from the Isle of Arran to the South Devon coast and including particularly the Vale of Eden, are again desert sands. In the north east there are yellow back-shore sands underlying the Permian dolostones. Britain’s Triassic rocks, covering large parts of Lancashire, the Vale of York and the Midlands, were mostly formed in semi-desert conditions and include very easily worked and uniform sandstones of the Sherwood Sandstone group. The best place to visit these is at Britain’s oldest pub, the Trip to Jerusalem in Nottingham.

It’s apparent from this description that sandstones formed in desert environments tend to be red. On the other hand marine and alluvial sandstones tend to be anything from almost white through all shades of buff, yellow and orange to brown. These colours represent the state of oxidation and amount of any iron coating the grains. But this can change as water and/or air flow through the stone after exposure, quarrying, use and, in the case of archaeological material, shallow burial. Colour is, sadly, a rather poor indicator of a stone’s origins and history.

Of more interest are the sizes and shapes of the sand grains when looked at through a hand lens. Desert sand grains tend to be very well rounded and uniform and have a frosted surface from all those impacts as the winds blew them around. Water-lain sandstones tend to have glassy surfaces and vary from fine, well-rounded grains to the coarse, angular grains in what are often called gritstones. Other variables are the content of non-silica grains such as shiny mica or opaque feldspar, the nature of the cement (often hard to determine without a microscope) and the porosity. Large stones or rock outcrops may also show signs of the structures – dunes, ripples and many others – characteristic of the way the sand was laid down which tell geologists much about conditions and how they varied all those millions of years ago.

The first picture below shows coarse sand grains in a stone at Chesters Fort. The area pictured is 2cm x 2cm. The second picture is of a natural sandstone outcrop, around 5m high, just south of the wall to the west of Housesteads. These complex structures may well have been laid down in a delta environment where distributary channels intersect and overlie each other as the delta develops.

The durability of sandstones varies enormously, and often for reasons that are far from obvious. If you go to Chesters Fort, which was mostly built during a single period and so probably sourced from a single quarry, you will notice that hardly any of the stones’ surfaces have started to flake off. At Vindolanda we are much less fortunate and a significant proportion of the stones have become very friable. There is some correlation between this and the period of the buildings concerned but all periods seem to include at least some stones which are not doing well. One factor is the porosity; porous stones allow in water which can freeze and expand in winter forcing the surface layers off. The solubility of the cement holding the grains together is also a significant factor. But the, often unknown, history of the stone from when it was first exposed at the surface to the present can also have significant effects which are hard to unravel.

I hope this gives a useful introduction to the stones you hard-working diggers spend so much time uncovering. My original intention of a weekly blog is slipping a bit, and will slip a bit more now as I’m away this weekend. But I hope in 10 days’ time or so I shall have some more detailed information from a visit to our geological advisors in Edinburgh and I shall be able to tell you something about the techniques we’re using to try to identify Vindolanda’s stone sources and the periods in which they were used.
Attached to this post:
Attachments: Coarse_grains.jpg (413.66 KB)
Attachments: Big_structures.jpg (360.96 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
Although we can tell quite a lot about a piece of rock by looking at the outside, especially with a hand lens, a way of looking inside it is needed to really learn about it. We need to know what minerals make it up and in what proportions, what sizes and shapes the grains are and how they are related to each other. The most widely used technique for this is to prepare what’s called a thin section.

A polished section of the rock is stuck to a glass slide and ground down to a thickness of just three hundredths of a millimetre. Polarised light is passed through it and examined with a microscope. At this thickness, most minerals are transparent, or at least translucent. The sizes, shapes and relationships of the grains can be seen and many of the minerals can be identified.

But the really clever bit is then to pass the light coming out of the microscope through a polarising filter set at right angles to the original light beam; this is called crossed polars. This arrangement would normally be expected to cut out all the light, but many minerals rotate the light in a way which is characteristic of the mineral and its orientation. As the thin section is rotated between the crossed polars, these mineral grains go black and then become visible again.

The light which passes through the quartz grains which make up most of a sandstone is rather plain shades of grey under crossed polars, but other minerals which may be present have more dramatic effects. Micas glow with brilliant blue and pink colours and feldspars have dark and light bands, sometimes just one of each per crystal, sometimes many parallel bands and sometimes bands at right angles in what’s called a tartan pattern. Iron oxide looks brown in transmitted light but goes black under crossed polars. Sometimes the rock from which the sand was derived contains rare minerals, such as zircon or tourmaline, which can be identified in the sandstone thin sections. Examining the thin section can also enable us to determine what the cement material is which binds the sand grains together, to see how porous the stone is and to see how much clay there is in the pores.

The geologists from the British Geological Survey in Edinburgh who came to Vindolanda a while ago took away samples both from some of the quarry sites and from a number of stones on the site which Andy and his team had selected. The site samples represent a good spread of phases of building, both in the forts and in the vicus. Thin sections were made but the BGS have not yet had time to examine them carefully. So we have arranged that I should borrow them for a few months and I have now had a week to examine them carefully. Although my microscope is not nearly as sophisticated as the ones at the BGS, I can get useful information about the size and shape of the quartz grains and have identified numerous examples of the different types of feldspar and mica and have even found a few tourmalines. The porosities of the samples vary quite a bit, although I don’t have an easy way of putting a value to this, and I can see clumps of very tiny clay minerals in many of the pore spaces.

What I’m doing now is trying to see if there are any consistent differences in any of these characteristics between the different quarries and the different phases of building. There does seem to be a reasonable chance of getting some archaeologically useful information from this approach but it will be some time before we can come to any definite conclusions.

My microscope doesn’t have a camera attachment so I can’t show you any pictures of my own, but there are lots of examples of thin sections on the internet – put ‘sandstone thin sections’ into your search engine and you’ll get lots of interesting hits, mostly from university sites. The ones from Oxford (earth.ox.ac.uk) seem to be quite good.

Sadly, I have to be away from the site for much of August but I’ll try to make at least one entry in this blog before then and to put in a couple of good ones at the end of the season.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
When I first read your question, Harry, I thought the answer seemed straightforward and sat down to post it. But the more I thought about it the more I realised things are not quite as simple as they seem. So I’ll use your question as a subject for this week’s blog about lithification, or how rocks, particularly sandstones, get turned into stone. Or not, as the case may be.

To start with the simple answer. The Carboniferous age sandstones such as those around Vindolanda were covered over after they were laid down by more and more layers of sediments, eventually many kilometres thick. As with most cases of lithification it’s the pressure of these overlying layers which, over millions of years, caused the initially loose sediments to turn to hard stone. So of course you don’t see the intermediate stages because they happen while the rocks are deeply buried. Eventually, erosion over many more millions of years has re-exposed at the surface (“exhumed”, as geologists say) the sandstones we see today.

So how does all this pressure cause the lithification? Well, in the case of the sandstones the mechanism is well understood, if a little complicated, and I can see the evidence of it in all the thin sections I’ve been looking at. The silica which makes up most of the sand is normally only very very slightly soluble in water. But under high pressure, particularly at the points where a corner of one grain presses into another, the solubility increases dramatically. Water flowing through the sandstones carries silica away from the points of contact and then deposits it again in the spaces between the grains where it acts as a cement, gluing the grains together. This process is called pressure solution.

But there are circumstances where you can indeed see sandstones which are partially lithified. Here are some examples – sorry they’re all from Britain but I’m sure similar cases exist widely in the USA and most other countries.

Sometimes we see the very early stages of the lithification process in sands which have not yet been buried. Just a couple of miles from where we live to the south of Derby, sand and gravel deposits from the past 15,000 years or so cover large parts of the valley of the River Trent. Of course, most of these deposits will be washed away, or nowadays dug up and carried away, but in time some will be covered by more deposits and eventually perhaps be buried deep enough to be turned to stone.

Even with sands which have been buried, the extent of lithification can vary considerably. During the first geology summer school of my OU studies, in 2001, we visited the aptly named Quarrington quarry in County Durham. In the “yellows sands” picture below, behind the three disparately sized individuals (I’m the middle sized one), is an apparently normal yellow sandstone with what appears to be a cross-section of a big sand dune in it. However, when the quarrymen start to dig at it, this “stone” just disintegrates into piles of yellow sand. At exactly the same geological time, in the Permian period about 260 million years ago, only 50 miles to the west of Quarrington, the red Penrith Sandstone was being deposited which has become well lithified to a very durable building stone. Both the yellow and red sands were deposited in desert environments, but the yellow ones were close to the sea shore and the differences in the chemistry of the material surrounding the sand grains has produced this great difference in coherence.

Many sandstones have been buried less deeply and for a shorter time than the Carboniferous ones and in consequence are much softer. The Sherwood Sandstone under Nottingham is easily carved away and over the centuries an extensive network of caves has been created. In England the rocks generally get younger towards the south east and most sandstones from this part of the country are quite soft and rarely make good building stone, which perhaps explains why people from those parts think of limestones, such as Bath or Portland Stone, rather than sandstones as the building stone of choice.

Occasionally chemical conditions allow lithification to happen on the surface without the need for high pressure. Such surface-hardened rock is called duricrust and comes in a number of varieties depending on what type of cement binds the grains together. One version is called calcrete and consists of sand grains cemented by calcium carbonate. Extensive deposits of this were formed about 200,000 years ago along the north coast of Cornwall. These deposits are very friable and are themselves now being eroded but their remains, known locally as sandrock, can still be seen, for example at Godrevy Point which is at the east end of St Ives Bay. In the picture, two blocks of sandrock can be seen lying over the eroded surface of folded Devonian rocks.

When a duricrust consists of sand grains cemented by silica it is called silcrete. Extensive deposits of silcrete are thought to have been formed over parts of southern England around 50 million years ago. These are now nearly all gone but in a few places large blocks called sarsens are still found. These are extremely hard and include, as I’m sure you archaeologists know, the stones used to build the great trilithons of Stonehenge.

So we usually don’t see lithification in progress because it happens at great depth, but various stages can be seen where the sands have never been buried or where the burial was not deep enough and/or long enough to form a good hard stone like our Northumbrian sandstone.

This week I’ve joined the ranks of the excavators and am helping to dig away the packing of the 3rd century Via Principalis to expose the floor of a 2nd century barracks. The following two weeks I shall be back in Derby supervising the re-decoration of our house. So it may be a few weeks before I can post another episode of this blog. But fear not, dear reader, I shall return before the end of the season!
Attached to this post:
Attachments: Yellow_Sands.jpg (267.61 KB)
Attachments: Sandrock.jpg (236.73 KB)
Offline Profile Quote Post Goto Top
 
ZetaBoards - Free Forum Hosting
Create a free forum in seconds.
Learn More · Register for Free
Go to Next Page
« Previous Topic · Excavation & General Archaeology Discussions - Open to All! · Next Topic »
Add Reply
  • Pages:
  • 1