So-called 'Chloris Ringstone'. The red colour in sandstone is simple iron oxide, or rust. A two stage-process: after rock formation, the iron has migrated and clumped into small blobs, called nodules or concretions; later, superheated water circulated through the rock and attacked the small blobs. Compare blackcurrants squashed into white cloth and left for a while. Ring formations up to 3cm. New Red Sandstone, Nithsdale, Dumfriesshire. (National Museum of Scotland)

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