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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
Member
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The four rock types in the Yoredale cycles each represent one of the main classes of sedimentary rocks which geologists recognise, and to each of which they give names based on Latin words. Limestone is said the be calcareous after the Latin word for lime, and/or the German word kalk which also means lime. The shales contain a lot of clay and so are said to be argillaceous. Sandstones are classified as arenaceous – the arena was so named because the surface was generally covered in sand, to mop up the blood it is said. The coal is composed largely of carbon and so is carbonaceous.

When they are exposed at the surface, each of these rock types weathers in a different way. Limestone dissolves in rain or river water which is slightly acid because, as it falls, rain dissolves a small amont of carbon dioxide from the atmosphere. In air, limestone tends to develop a pitted surface because it dissolves more where the rain collects in any tiny hollow. The shales and mudstones soften and the tiny clay particles are carried away by the water. The silica grains which make up sand are very resistant to weathering but in sandstone they are held together by a “cement” – iron oxide, calcium carbonate or more silica – which can be dissolved or broken by rain or the freeze/thaw action of frost. Sandstones tend to develop a rounded shape as the grains are more easily broken away from corners and edges. Eventually the rock turns back into sand; diggers will have noticed that many of the stones they excavate are well on the way to this fate! The coal slowly oxidises to carbon dioxide gas.

A good place to see the first three of these rock types is in the left bank of the burn below the museum at Vindolanda. This burn is marked on OS maps as “Chainley Burn” but locally it is called the Chineley Burn and I have also seen Charnley Burn; lets stick to the local name. You can no longer get out into its little valley from the bottom of the museum grounds. Instead, go out of the main door of the museum and a few yards up the slope you will see some steps (clearly of reused Roman stone) leading to the right up into the trees. Turn right again at the top of the steps and in a few more yards the path divides. One path leads up into the field above the valley, the other goes down beside the laboratory building to a little footbridge over the burn. (Incidentally, please do stick to one of the footpaths as marked on the map – OS 1:25,000 sheet OL43 is recommended – as there are some quite hazardous areas off the paths.)

Once you’ve crossed the footbridge onto the west (right) bank of the Chineley Burn, the very attractive valley contains a number of features of interest. Most immediately obvious is a collection of about a dozen large sandstone boulders, on one of which the centre of the footbridge rests. Look for the one with a number of wedge holes on its upper surface. Someone, one would like to think the Romans but there may be no way to be certain, has clearly been attempting to work it. The faces of many of them are remarkably flat and at right angles to each other. This could be natural but equally could be evidence of human activity.

Looking across the burn, the other side of the valley is nearly vertical. Above about 5 metres of shale is another 5 metres or so of sandstone. At least another 5 metres of sandstone is out of sight above that. The first picture below shows the valley side, with one of the big boulders in the stream bed. Notice how there are gradual transitions from the limestone below into the shale and from the shale into the sandstone. The lower of these two transitions contains some very hard calcareous mudstones. The Romans probably didn’t work these because, as my geological hammer revealed, they break up into nasty sharp splinters. But you can imaginge that they would think any loose blocks they found would make good building stones. Only after nearly two millennia in the soil do we discover just how soft they can become.

This spring’s weather in Northumberland has had two main characteristics – some remarkably sudden switches between cold and hot and much lower than average rainfall. As a result of the latter, all the burns are running at very low levels. Through the museum grounds the Chineley Burn is very sluggish and scummy and below the weir the flow vanishes completely. For around 100 yds downstream, nearly to the second footbridge, there are just some stagnant pools. Then suddenly the flow emerges again, bubbling up sparkling clear. In the dry section it is running below the surface through channels dissolved over the millennia in the “Four Fathom” limestone which which forms the stream bed and is now completely exposed. The panorama below shows the bottom half of this dry section and the point where the flow re-emerges. The apparent curvature of the stream bed in the picture is an artefact of the way five photos combine into a panorama; actually this section is almost straight. The “limestone pavement” thus revealed is typical of many such in the northern Pennines. In Yorkshire the blocks of limestone are called “clints and the fissures between them are “grikes”. The clints are rounded here because they are being dissolved by the flowing stream rather than by rain.

Over the coming weeks, except when there’s anything more immediate to discuss, I’ll write a bit more about each of these rock types and how the Romans used them. But first I’ll talk some more about the Whinstone, the story of which includes one of the most fundamental geological discoveries of the past half century.
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