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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!

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Mike's Geoblog
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.

Mike's Geoblog
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!