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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!

Viewing Single Post From: Mike's Geoblog
Mike McGuire
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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.
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