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The Geology of Vindolanda, Part I

The following notes have been put together by Mike McGuire, an amateur geologist and long-time “Friend of Vindolanda”, who is leading an on-going investigation into the sources of Roman building stones at Vindolanda. The notes are a collation, with some corrections, of information from Mike’s 2010 “Geoblog” forum on wedigvindolanda.

This text is divided into two parts, with a total of 12 sections. Part I contains Sections 1-6. Section 1: Ice Ages sets the geology in the context of the two ice ages which have been primarily responsible for the landscape of the area. Section 2: Yoredale Cycles discusses the cyclic pattern of the rock strata. Then come sections describing each of the four main rock types -- Section 3: Dolerite, Section 4: Limestone, Section 5: Shale, and Section 6: Sandstone. Sections 7-12 can be found in Part II.

Section 1 -- Ice Ages

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.

Image source: http://upload.wikimedia.org/wikipedia/ commons/5/53/Pangaea_continents.png

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 below). 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. In many places the bottoms of these valleys are packed with water-rounded cobbles which the Romans evidently used for cobbled roadways and in the rubble fill of some walls.

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, paving, etc, etc
• Sandstone and dolerite cobbles for paving and wall-fill
• Limestone for mortar
• Clay for wall bonding, tiles and pottery
• Coal for fuel
• Metal ores, particularly iron and lead
• A defensible site at Vindolanda

Section 2 -- Yoredale Cycles

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 amount 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 by rain or broken by 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; let’s 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; follow the other which 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.)

Ancient worked stone (photo by author)
(Click photo for larger version)

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. 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 they break up into nasty sharp splinters. But you can imagine 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.

Dark shale below, lighter sandstone above (photo by author)
(Click photo for larger version)

Northumberland’s weather can be memorable for its downpours but some periods, such as the spring of 2010, can have much lower than average rainfall and, as a result, the burns run at very low levels. At these times the Chineley Burn becomes very sluggish and scummy through the museum grounds 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 forms the stream bed. 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.

Section 3 -- The Whin Sill

I’ve alluded earlier 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 that there is a layer of molten rock below the crust. But there isn’t. 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 the green mineral olivine – the jewellers’ name for olivine is peridot. 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. 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 (seismic waves) from earthquakes and underground nuclear tests propagate through the Earth. They do so slightly more slowly through the asthenosphere than through the surrounding mantle and a lower seismic velocity is an indication of a softer material. Seismologists often refer to the asthenosphere as the ’low velocity layer’. But all types of seismic waves do pass through it, which some of them wouldn’t if it were liquid.

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.

At the time the Whin Sill was intruded, the hot magma caused superheated water to be expelled from the deep rocks under high pressure. Under these conditions a wide range of minerals was dissolved from the surrounding rocks. The water gradually percolated upwards to regions of successively lower pressure and temperature. Each mineral precipitated out under very specific conditions and therefore became concentrated in particular parts of the rocks in the fissures through which the water flowed. These thus became veins of concentrated mineral ores, many of which have been worked in ancient, historic and industrial times, particularly for lead.

Although the Roman masons avoided wherever possible having to work this hard material, 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 chiselled 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'tThe 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. 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.


"Limestone Corner"
(photo by author)

Section 4 -- Limestone

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, as was revealed during the excavations for building the new education centre.

Four Fathom Limestone at Vindolanda's education centre
(photo: Malise McGuire)

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.

Section 5 -- Shale and Mud (and Concrete and Clay)

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 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. 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.

Section 6 -- Sandstones

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 grains. (Quartz is the common form of the mineral silica, a compound of silicon and oxygen.) Generally these grains 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 some of the sandstones, conditions during or after deposition were such as to concentrate iron minerals in particular layers.

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.

In the Carboniferous period Britain was close to the equator. 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.

Sand grains in a sandstone at
Chesters fort; area of 2cm X 2cm
(photo by author)

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.

To see Sections 7-12, please continue on to Part II

Page created by Mike McGuire, March 2011