Exquisite fossil coral unmasked in wet slabs

Bollihope, Durham Splashed with water from the burn the wet rock matrix darkens to reveal the coral's white filigree

Standing under a leaden sky on this bleak moorland, with a bitter wind blowing over Harnisha Hill, it was difficult to imagine that the Frosterley marble that we had come to find was once a coral reef in a tropical sea.

It has taken 325m years of tectonic plate movement, at a pace slower than a fingernail grows, to bring it to this valley from its equatorial origin.