Robert MacFarlane canoes around the Walton backwaters and spies common seals. Naturally curious creatures, they pop their heads up out of the water like periscopes and big liquid eyes watch his approach with a calm, intransitive attention. Pups less than a day old swim on their first high tide. Two large males spar with one another on the foreshore. It's easy to understand why seals feature prominently in the folklore of our coastal waters, inbetween creatures half human and half marine. The common seals that live on this stretch of Essex estuary are incredible colours, coppers, russets and burnished browns. These colours are the result of the mud on which they haul out. It's London clay which is naturally rich in iron oxide - rust basically. Wild creatures stained the colour of iron and industry.