This beautiful little packhorse bridge carries the footpath over Thorsgill beck beside Egglestone abbey in Teesdale. It probably dates from the 17th. century and countless feet must have passed over it since it was built. But when you stand on the top there is evidence of something else that slithered over its stones, around 300 million years ago.
One of the capstones carries this strange serpentine marking. Some long-extinct invertebrate left its trail when it wriggled across the tropical sea floor back in the Carboniferous, before the sediment turned to limestone.