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

June 13th, 2010, 1:49 pm #11

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

June 15th, 2010, 12:26 pm #12

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.
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Last edited by Mike McGuire on June 15th, 2010, 12:29 pm, edited 1 time in total.
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

June 20th, 2010, 7:57 pm #13

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/Palaeofile ... /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.
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Last edited by Mike McGuire on June 20th, 2010, 8:06 pm, edited 3 times in total.
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Averil&Ian
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Joined: June 22nd, 2007, 8:36 am

June 27th, 2010, 5:27 pm #14

Many thanks, Mike, for the geological explanations.
As regards the large rock in the Chinley Burn (near the footbridge), you say: "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."
I've always imagined that this block was quarried at the top of the hill and rolled down into the burn, with a Roman quarryman shouting "Quattuor" or the equivalent of "Fore"!
Do you think these stones come from that quarry?
Best Wishes,
Ian McHaffie
mchaffie@tesco.net
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Mike McGuire
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June 28th, 2010, 5:41 pm #15

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
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Mike McGuire
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June 29th, 2010, 9:47 pm #16

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.
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SacoHarry
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July 2nd, 2010, 12:07 pm #17

Loving this as always Mike. Thanks for the latest post -- and glad to see recovery from dogbites and back strains!

I was curious about shale used as mortar. Reading Eric Birley's excavation reports from the 30s, he seems to have come across Roman mortar with a lot of shale in it around the north gate of the stone fort (Stone Fort II). I had never heard of that anywhere else. (But that may just be me showing my ignorance.) In your travels have you seen shale mortar? Was Prof. Birley right about what he found? I just wonder what it is about shale that makes a good mortar.
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Mike McGuire
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Joined: May 14th, 2010, 7:41 pm

July 3rd, 2010, 8:36 am #18

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.

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Mike McGuire
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July 8th, 2010, 6:55 pm #19

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.
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Mike McGuire
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July 23rd, 2010, 11:59 am #20

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.
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