Wednesday, 18 February 2009

Hadrian's Wall geology and a break from gardening

The plants are changing so little at the moment, the snowdrops two weeks behind, the aconites barely opening because of lack of sun, so I thought I would post something about a day course I went on yesterday. Run by Hadrian's Wall Heritage, it was put on to give tourism businesses some insight into the area, so we met on a dull, low cloud day but at least it wasn't raining or snowing and it was relatively warm!

Led by geologist, Richard Holmes, we spent the day in a small area around the spectacular Walltown Crags, scene of past quarrying of the hard rock of the Whin Sill and a place where Hadrian's Wall does those dramatic undulations that make it so widely photographed. Here's my own photo to add to those legions (what an appropriate word) but of course it was a very dull, uninspiring day photographically. We learnt much about the geology of this wonderful part of Northumberland, the effect of the ice sheet, the glacial erratics it left behind, the sandwiched layers of sedimentary rocks and the use that they have been put to locally over the years. But the highlights for me were often the smaller things; the tiny holes, no larger than a 5p piece, in the Whin Sill that show were gases had made their way towards its top edge - and their word, vesicles, a good one for crosswords.

Particularly fascinating was being able to see the point at which the intrusive rock of the Whin Sill met the layer of sandstone in a shallow quarry feature on the north side of the Wall. This whole area below the crags is Access Land and a great
viewpoint from which to look up at the uplifting mass of columnar dolerite. Another good detail was the current bedding - as in the photograph - which shows where the deposits that created the sandstone where moved this way and that by an ancient sea. The connection with the garden is still there in some of the plants that I spotted as we walked about, wildflowers that have herbal properties or that I grow because I like them, but it was great to have someone else do the talking and impart their knowledge for once!

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