Welcome Guest [Log In] [Register]

From Aug 2006 - Nov 2013 WeDig provided a live forum for diggers & fans of Vindolanda. It has now been mothballed and will be maintained as a live archive.

Here you will find preserved 7 years of conversation, photos, & knowledge about a site many people love. Vindolanda gets under the skin. (Figuratively and literally as a volunteer excavator!) It's a place you remember, filled with people you remember!

Thanks for 7 great years!

Welcome to We Dig Vindolanda!

Username:   Password:
Add Reply
  • Pages:
  • 1
  • 2
Mike's Geoblog; Geological aspects of the Archaeology
Topic Started: May 14 2010, 04:43 PM (1,801 Views)
Mike McGuire
Member
[ *  * ]
A lot of terms in geology have been adapted from everyday words and so tend to have rather imprecise, often overlapping meanings. This is particularly true for terms applied to very fine particles and the rocks they can be compacted into. So the terms clay, silt, shale and mud mean different things to different people and are often used interchangeably. You’ll never get full agreement on a fixed terminology, but the following seem to me to be useful descriptions of what these terms mean.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Sadly, I have to be away from the site for much of August but I’ll try to make at least one entry in this blog before then and to put in a couple of good ones at the end of the season.
Offline Profile Quote Post Goto Top
 
SacoHarry
Member Avatar
Administrator
[ *  * ]
Mike, again this is really fascinating stuff. I had no idea one could polarize light and determine minerals by how they rotate that light. Love it! Sounds like another cool tool to use in learning not just about a site, but about how the landscape was used in creating it.

Here's one I've been wondering for a while. Are there examples of the transition from sand to stone -- like a sand deposit that's just starting to become sandstone? I always either see sand, or proper sandstone; I can't recall seeing an outcrop anywhere of a sand that has started to cement together but hasn't finished yet.
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
When I first read your question, Harry, I thought the answer seemed straightforward and sat down to post it. But the more I thought about it the more I realised things are not quite as simple as they seem. So I’ll use your question as a subject for this week’s blog about lithification, or how rocks, particularly sandstones, get turned into stone. Or not, as the case may be.

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

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

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

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

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

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

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

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

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

This week I’ve joined the ranks of the excavators and am helping to dig away the packing of the 3rd century Via Principalis to expose the floor of a 2nd century barracks. The following two weeks I shall be back in Derby supervising the re-decoration of our house. So it may be a few weeks before I can post another episode of this blog. But fear not, dear reader, I shall return before the end of the season!
Attached to this post:
Attachments: Yellow_Sands.jpg (267.61 KB)
Attachments: Sandrock.jpg (236.73 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
The decorating work at our house in Derby was completed ahead of time but involved a lot of work and effort on my part moving our belongings from room to room ahead of the painters. So it was a great relief to be back on site at Vindolanda at the start of this week on a beautiful Sunday morning. Since then we’ve had quite a lot of rain but some brilliant sunshine as well. Tuesday afternoon I risked a trip up Barcombe Hill and got wet, but a return visit yesterday morning rewarded me with a brilliant view over Vindolanda and the surrounding countryside.

I guess I’m like most geologists in being most interested in the ancient rocks forming the skeleton which underlies the landscape. But looking at the view I realised more strongly than ever 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. 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 here, 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.
Attached to this post:
Attachments: Barcombe_Pan_2.jpg (306.94 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
Last Wednesday we were visited on site by Dr David Lawrence the county geologist for Northumberland. He and some of his colleagues from the British Geological Survey in Edinburgh have been giving us great assistance with the stone sources project. I’m pleased to say he was in agreement with many of the ideas I’ve been developing during this season. I’ve identified two sites in addition to those on Barcombe Hill which David agrees with me are probable quarry sites. As yet I don’t have any specific evidence they are Roman but this does seem the most likely story.

I’ve also identified from the thin sections a possible way of distingushing stone from these quarries from the Barcombe Hill stone. But there’s still a lot more work to do on this and on Thursday, after careful discssion with Andy and Justin, I took 8 stones from the site from which, along with some of my quarry samples, we shall have some more thin sectons made.

There was one subject on which David was able to correct my ideas, which is about the famous green stones which crop up all the time in the excavations. As I said in an earlier blog, these come out of the boulder clay which underlies the site and I had correctly identified the green mineral as chlorite. However, David doesn’t think the stones come from the Whin Sill; the dolerite rock which makes up the sill doesn’t generally alter in such a way as to produce chlorite. The nearest widespread source of rocks which do 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 found in the area.

So the 2010 excavation season is coming to an end. Next Thursday we shall all say our goodbyes and Malise and I will be moving out of the house we’ve been renting in Haydon Bridge. A great wrench to leave our fabulous view of the South Tyne valley for the boring suburbs of Derby. So my next blog will be the last of the regular season. I’ll share some of the deas I’ve been mulling over during the past 5 months and give you my suggestions for some further reading on Northumberland geology. I’ll put more enties in my blog as and when anything new crops up during the close season. Then it’ll be the 2011 season and it all starts again!
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
I promised a final entry in my blog for this year, so here it is at last. It’s a bit delayed because of the time it’s taken us to get our house in Derby straight after we moved back from Haydon Bridge, and the fact that in the middle of it all we escaped the chaos for a brilliant 10-day holiday in Scotland. Steaming across the Sea of the Hebrides into a full gale aboard the Calmac ferry ‘Clansman’ was really exhilarating. When you get to Barra the rocks are so wonderfully old and the beaches are magnificent (first picture). And on a clear evening the view back to the mainland from Point of Eye on Lewis is unsurpassed. If you aren’t already an addict for Scotland’s highlands and islands, please do go there and become one; it’s an addiction which is both legal and harmless.

While on Lewis we took the opportunity to visit again one of Britain’s greatest archaeological treasures – the ancient standing stones of Calanais (as usual in Scots Gaelic, pronounce the final ‘s’ as ‘sh’). They are ancient both in human terms, about 5000 years, and geologically as they were quarried from three billion year old Lewissian Gneiss (second picture). We both – Malise from an archaeological viewpoint and me from a geological – find them much more appealing than Stonehenge. They’re 100 times older than the Sarsens and who knows about the enigmatic, not to say controversial, ‘bluestones’.

The controversy about the transport of the Stonehenge bluestones (did they come from South Wales by human effort or by glacial transport?) is one where geology and archaeology are somewhat at odds. Which is a shame because the two disciplines have so much in common and so much to contribute to each other. After all, both involve that most basic of human instincts, digging things up and making up stories about them. I hope that as the Vindolanda stone sources project progresses I shall be able to provide more evidence of these synergies.

But there is one difference between geology and archaeology which I find both puzzling and disturbing and which you might care to ponder in the long winter nights. There are some opportunities to get a job and make reasonable money as a geologist, especially if you’re helping to locate new sources of the Earth’s mineral wealth. For archaeologists, sadly, the opposite is true; good jobs are as rare as the proverbial hen’s teeth because archaeology has no ‘economic value’. And yet the oil, iron, etc, etc to which we give such high monetary value has no intrinsic worth, it’s just a means to the end of improving our lives, whereas the understanding and excitement which comes from uncovering evidence of our past is surely a life-enriching experience in itself. Why do we accord so little value to our quality of life but so much to the purely material means of achieving it? Why, for example, do people spend loadsamoney on petrol to sit in a tin box for hours driving to Vindolanda and then gripe at a few quid to get in?

See you in 2011.
Attached to this post:
Attachments: Beach.jpg (144.06 KB)
Attachments: Calanais.jpg (227.72 KB)
Offline Profile Quote Post Goto Top
 
Mike McGuire
Member
[ *  * ]
Just remembered I promised to suggest some reading matter. 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 Landscapes 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.
Offline Profile Quote Post Goto Top
 
Lesley
Member
[ *  * ]
Gorgeous Lewissian gneiss!! Must visit.

Have got 2 of the books, Earth Story on dvd, will spend winter reading the other 5! :)
Offline Profile Quote Post Goto Top
 
SacoHarry
Member Avatar
Administrator
[ *  * ]
Thx again for all of this blog! It's been really fascinating & eye-opening. Like I said, I'd like to work with this winter on creating a Digger's Guide page from it so that it can have a permanent place of prominence.

And thanks for the reading list -- I can vouch for the "Ancient Frontiers" book. Really interesting, well-produced, great tidbits on where to find various formations (and the flora and fauna they attract) around the Wall.
Offline Profile Quote Post Goto Top
 
SacoHarry
Member Avatar
Administrator
[ *  * ]
This winter, Mike did a very big thing. He took the notes from the Geoblog and compiled them into a full text of the Geology of Vindolanda. I've just finished compiling & uploading them to the Digger's Guide. It's in 12 sections, divided into two pages: Part I and Part II.

This is an amazing amount of work, and an amazing resource. We tend to think of history in human terms. But much of our history is shaped by the resources under our feet -- the things that we had to learn to make do with. It's the same at Vindolanda, and the Wall in general. The remarkable Whin Sill -- the ridge that gives the Wall its imposing overlook. How did it get there? What kind of stone did the garrisons at Vindolanda use to create their forts? Limestone? Sandstone? How did it get there? What other resources (and obstacles) did they have to contend with? Why has some of the stone held up well, while other areas crumble?

Mike's "Geology of Vindolanda" will help answer all of this. It will also take any interested WeDig'er on a 300-million year tour of northern England. You can start to understand how ancient mountains, deserts, seas, and glaciers created the Tynedale known to Roman soldiers -- and modern diggers.

A huge Thanks! to you, Mike, for taking the time & energy to make this resource available to all.

- Harry
Offline Profile Quote Post Goto Top
 
ZetaBoards - Free Forum Hosting
Free Forums. Reliable service with over 8 years of experience.
« Previous Topic · Excavation & General Archaeology Discussions - Open to All! · Next Topic »
Add Reply
  • Pages:
  • 1
  • 2