SacoHarry
Member
Joined: August 22nd, 2006, 9:29 pm

July 29th, 2010, 12:28 am #21

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.
Quote
Like
Share

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

August 2nd, 2010, 8:35 pm #22

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!
Sandrock.jpg
Yellow_Sands.jpg
Quote
Like
Share

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

August 26th, 2010, 10:23 pm #23

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.
Barcombe_Pan_2.jpg
Quote
Like
Share

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

September 11th, 2010, 7:49 pm #24

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!
Quote
Like
Share

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

October 12th, 2010, 7:44 pm #25

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.
Calanais.jpg
Beach.jpg
Quote
Like
Share

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

October 12th, 2010, 7:47 pm #26

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.
Quote
Like
Share

Lesley
Member
Lesley
Member
Joined: October 10th, 2009, 4:30 pm

October 13th, 2010, 11:51 am #27

Gorgeous Lewissian gneiss!! Must visit.

Have got 2 of the books, Earth Story on dvd, will spend winter reading the other 5! :)
Quote
Like
Share

SacoHarry
Member
Joined: August 22nd, 2006, 9:29 pm

October 13th, 2010, 1:06 pm #28

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.
Quote
Like
Share

SacoHarry
Member
Joined: August 22nd, 2006, 9:29 pm

March 21st, 2011, 4:58 pm #29

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
Last edited by SacoHarry on March 21st, 2011, 4:59 pm, edited 1 time in total.
Quote
Like
Share