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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!

Mike's Geoblog
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.

Mike's Geoblog
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.

Mike's Geoblog
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.