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The Devils Punchbowl near to Croxton, Norfolk. The Devils Punchbowl is a dowline http://en.wikipedia.org/wiki/Sinkhole a circular depression formed by ground subsidence or collapse in areas with limestone bedrock consists of chalk a cover of unconsolidated sandy sediment, called glacial tilt, deposited during the Ice Age. The chalk is easily attacked by acidic underground water causing caverns and fissures to form this then leads to subsidence in the landscape. For other view see http://www.geograph.org.uk/photo/1759627