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The Geology of Vindolanda, Part II

The following is a continuation of The Geology of Vindolanda, a compilation of notes put together by Mike McGuire, amateur geologist and long-time “Friend of Vindolanda." Mike 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.

Part II contains the final six sections. Section 7: Lithification explains how rocks lithify. Section 8: Petrological Microscopy describes how rock structures can be unravelled using a microscope. Section 9: The Present Ice Age discusses in more detail the effects of the current ice age on the landscape. Section 10: Mudstones and Section 11: Green Stones explain a couple of the more unusual types of stone excavators are likely to encounter. Finally, Section 12: Bibliography gives a brief list of some excellent choices for further reading.

Section 7 -- Lithification

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

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

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

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

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

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

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

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

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

Section 8 -- Petrological Microscopy

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.

Thin sections have now been made from 40 stone samples from the Roman masonry at Vindolanda and from possible quarry sites in the vicinity. 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 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.

Section 9 -- The Present Ice Age

I guess I’m like most geologists in being most interested in the ancient rocks forming the skeleton which underlies the landscape. But the view from the top of Barcombe Hill makes me realise the importance of the part which the more recent ice cover has played in determining the shape and texture of the land surface.

For a start, without the erosive power of repeated glaciations during the present ice age much younger rocks would still be at the surface and the present surface rocks would still be deeply buried. And although the harder and softer rock layers do influence the highs and lows of the landscape they are by no means the whole story. Why, for example, is Barcombe Hill there at all? To east and west of it the corresponding strata have all gone. Some quirk of the way the ice flowed must have been responsible, but we may never fully understand what.

After the peak of the glaciation about 20,000 years ago conditions seem to have become slightly less extreme so probably the ice became a bit less thick and flowed a bit more slowly. The ice’s erosive power declined and instead of wearing the rocks away it started in places to smear it’s burden of eroded material across the landscape as what we call glacial till, a form of boulder clay. Again, although there is some correlation between the landforms and where the till is thickest (generally in the valleys), it is also partly dependent on the vagaries of the ice flow. In some places mounds of till called drumlins were left behind.

After about 15,000 years ago the climate became mild enough that the ice started to melt away. As the ice front retreated past the Vindolanda area, meltwater streams carried huge amounts of water across the landscape, generally southwards towards the South Tyne valley. The force of the water was enough to erode out the small valleys we see today. Mostly it was the boulder clay which was washed away but in some places the streams cut into the underlying rocks creating near-vertical valley sides. Since these meltwaters subsided, there has never again been a sufficient flow to change the shape and size of the valleys significantly. Even flash floods such as the one a couple of years ago, though some big boulders were moved, didn’t cause any major erosion.

About 12,500 years ago there was a marked deterioration in the climate which lasted about 1,000 years. The ice didn’t return but conditions were freezing for much of the time. Repeated freezing and thawing caused surface rocks to split and was even capable of moving smaller stones gradually downhill in a process with the delightful name of gelifluction. On Thorngrafton Common, behind Barcombe Hill, are a number of lines of boulder scree produced by this process.

Finally the climate warmed again and has remained relatively mild for over 10,000 years. Thick woodland developed over all but the highest ground. However, as settled agriculture started to develop, and particularly as metal tools became available in the bronze age, increasing numbers of trees were cut down to create fields to feed the growing population. When the Romans arrived there was already an established pattern of woods and fields much as there is today, although the proportion of woodland was probably considerably greater. The Romans themselves, of course, contributed to the deforestation and, although there may have been some recovery in the early Middle Ages, recent agricultural practices have reduced the tree cover still further.

So I think the view from Barcombe Hill over Vindolanda has probably changed remarkably little since Roman times. On a fine day it’s one of the best views in England and one of the world’s great archaeological panoramas. We’ve all seen lots of photographs of it, I’ve contributed a panorama of my own below, but none of them really do it justice. Next time you’re at Vindolanda, and if you’re sufficiently able bodied, do try to make the time to climb the hill to the Long Stone and see for yourself.

(photo by author)

Section 10 -- Mudstones

(photo: Malise McGuire)

Those of you who were present at the diggers' hut one lunchtime in May 2010 may well remember my rather amateurish bit of geo-conjuring involving a pot-washing bowl, four nondescript bits of stone and a bottle of mild acid. The four stones, all taken from the 2009 excavation area, were -

1) some hard, grey, slimy mudstone with bits of fossil in it,
2) some of the hard, darker rind often found on the grey mudstones,
3) a small piece of a very brown mudstone,
4) some very brown sandstone with fossily bits in it.

When I poured a few drops of acid onto each stone the results were that samples 2 to 4 showed no reaction at all (well, 2 did a little bit after a while) but sample 1 reacted as if I'd taken the lid off a tiny, well-shaken Coke bottle.

What this experiment demonstrated is that there are two types of mud and the Vindolanda mudstones are mixtures of these two types in very varying proportions. The first type of mud is just that, mud, mostly clay minerals, which has no reaction with the acid. The second type is lime mud, the finely ground (by the sea) remains of the shells and hard parts of sea creatures which were present when the Carboniferous rocks around Vindolanda were laid down about 325 million years ago. The visible bits of fossil are where the grinding was not so fine. This second type of mud is the mineral calcium carbonate which reacts with acid by giving off carbon dioxide - yet another way of putting fossil carbon back into the atmosphere. The grey mudstones (sample 1) are mostly lime mud - hence the big fizz - but they have some ordinary mud in them and after a couple of thousand years in the soil the lime mud has dissolved away from the surface layers, to leave a hard crust of the ordinary mud (sample 2). Sample 3 was ordinary mud, hence no reaction. In sample 4, even the fossily bits did not react with the acid, which shows that in sandstone the fossils have been turned to silica whereas in the lime mud the fossils remain as calcium carbonate.

Section 11 -- Green Stones

One question diggers often ask is what are the green stones which keep cropping up in their trenches and which are occasionally mistaken for something interesting? The answer turns out to be less interesting archaeologically than people hope, but is of interest geologically.

If they are broken with a hammer they turn out to be quite hard, and are generally dark grey inside, although some carry a greenish tinge all the way through. Viewed through a hand lens this grey material is not made up of separate individual grains but is a single dense mass with, in some cases, pale coloured crystals visible in it. This marks them out as igneous (like the whinstone) or metamorphic rocks not sedimentary like all the other rocks in the area. The green on the outside often becomes much less intense as the surface dries out in the air and in a matter of days may turn to a pale yellow colour.

The stones actually come out of the boulder clay and so have been brought to the site by ice movements 15,000 and more years ago. There is quite a thick layer of boulder clay underlying Vindolanda and the green stones work their way up from this both by natural processes such as freeze/thaw and by human agencies such as Roman occupation and subsequent ploughing.

The green mineral is chlorite which is commonly produced by chemical alteration of some of the minerals in certain types of rocks. The nearest widespread source of rocks which chloritise, including igneous rocks such as andesites and also some metamorphic rocks, is the Lake District. This is consistent with the fact that the last movement of the ice during the last glaciation was from west to east and rocks from that direction such as Shap Granites are also found in the area.

So sadly these rather attractive green finds are generally actually just bits of stone, transported by ice from 50 or more miles away, But do look carefully before discarding one, it might really be a precious bronze object!

Section 12 -- Bibliography

Here’s my answer to the question “If you were to be marooned on a Roman Frontier, which 8 geology books would you chose to have with you?”

Ancient Frontiers (British Geological Survey, 2006, ISBN 085272541-8) is a very accessible, informative, concise and well-illustrated account of the rocks and landscape of the Hadrian’s Wall area – what they’re like, how they formed and how they have influenced the people of the area. One of the authors is David Lawrence, the county geologist for Northumberland, who has helped a great deal with the Vindolanda stone sources project.

Geology of Hadrian’s Wall by G.A.L. Johnson (Geologists’ Association, 1997, ISBN 090071749-1) is a slightly more detailed geology, though still concise and readable, which describes the bedrock geology for the whole Wall area from coast to coast.

Northumbrian Rocks and Landscape edited by Colin Scrutton (Yorkshire Geological Society, 2004, ISBN 095016564-6) describes a series of 17 walks in various parts of The Borders, Northumberland and Durham showing where features of geological interest can be seen. Walk 11 is the most specific to Hadrian’s Wall. My favourites are walks 1 (Siccar Point – everyone interested in geology should go there), 2 (Burnmouth) and 7(Howick Bay).

Earth Story by Simon Lamb and David Sington (BBC, 1998, ISBN 056338799-8) is the book of the TV series which was, alas, never brought out on DVD. It can occasionally be seen on Freeview channels and was to my mind the best TV documentary series ever. Aubrey Manning was a brilliant presenter. This book, and the next two below, were the ones which most influenced my decision to study Earth Science.

The Hidden Landscape by Richard Fortey (Pimlico, 1994, ISBN 071266040-2). Richard Fortey has written many brilliant books about geology and is the best popular author on the subject. I can recommend any of his books. This one presents a very personal and vivid survey of how each of the geological periods is represented in the rocks of Great Britain.

Stepping Stones by Stephen Drury (OUP, 1999, ISBN 019850271-0). The last three books are a bit more demanding than the first five but are all well worth the effort. Steve Drury presents a comprehensive and coherent account of how the Earth came to be the way it is, how it works, the story of life and mankind’s place and influence in the scheme of things. If you want to understand enough of the basic science of how the Earth system works to follow important issues such as global warming, this book is a very good source.

British Regional Geology: Northern England (5th Ed, British Geological Survey, 2010, ISBN 095272652-5) The Regional Geology series has always been good value; this latest edition for Northumberland, Durham and Cumbria is the best yet. Period by period it gives all the most up to date information and ideas.

Geological History of Britain and Ireland edited by Nigel Woodcock and Rob Strachan (Blackwell, 2000, ISBN 063203656-7) For a decade now “Woodcock and Strachan” has been the definitive textbook of all the latest ideas as to how the British Isles were “assembled” through geological time.

Acknowledgements: The above information has been compiled partly from my own observations but mostly from sources including the books listed above, a dozen OU courses and the wise guidance of professional geologists and archaeologists too numerous to mention. Any good stuff has come from them; all the mistakes are entirely my own.

Page created by Mike McGuire, March 2011