The Hemlock Stone is a pillar of Nottingham Castle sandstone. Such rock is normally weakly cemented, but this outcrop is firmly held together by the mineral barytes, which increases in proportion towards the top of the pillar. Over the last 20 million years the softer overlying and surrounding sediments have been eroded, and the barytes-rich 'cap' has protected the strata beneath it. There is a fanciful story about the stone featuring in ancient Celtic druidic rituals, but no evidence.
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