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Mike's Geoblog
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.

Mike's Geoblog

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's Geoblog
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.

Mike's Geoblog
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.

Mike's Geoblog
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.

Mike's Geoblog
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.