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Wednesday, 9 January 2013

Black clay, green clay, red clay...

I had a fantastic day near Worcester today, being shown around a geological dig by Natalie from Herefordshire and Worcestershire Earth Heritage Trust, who was extremely generous with her time and knowledge. This image shows the site of the dig, where trenches have been cut into a field to expose the geology below. I was expecting clay as the surface is so sticky but it was like a potters (or geologists) Aladdin's cave down there.
At the centre of the dig is a dome of the Precambrian Malverns complex - igneous intrusions of granite and dolorite that have been variably metamorphosed. In layers away from this are Cambrian sandstones, various Carboniferous sediments (including the clays in this image), Silurian mudstones and Triassic deposits. The clay seams are quite thin and interlain with sands and sandstone and have incredibly varied colours - red, purple, green and black. They were deposited in freshwater and some are quite pale, so I'm hopeful they may be refractory enough to glaze fire (eternal optimism of the potter!)
The area is a goldmine for me as there is such a varied local geology. This sandstone outcrop is overlooking the excavation. It is beautiful red Triassic sandstone. There are also limestone ridges nearby littered with beautiful old overgrown quarries (such as in the last image). These are Silurian nodular limestones, interbedded with shales and (beleive it or not) bentonite! It certainly helps having a geologist to show me around.

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