A walk can be a dangerous thing. As Bilbo Baggins observed, paths flow into roads that can lead anywhere, to Rivendell and to Mordor. But they can also lead to anywhen.

I’ve always been fascinated by how the geography of Britain has changed, with generations of farmers nibbling at the sea, while the sea gnaws the coast. Mersea, a tidal island in the Colne Estuary, demonstrates this vividly. And who wouldn’t want to walk around an island?

To avoid the July heat, I crossed The Strood, the causeway linking Mersea to the mainland, at dawn. Now tarmac, and regularly inundated at high spring tides, The Strood is itself a link to the past, for the causeway was first laid around 700 AD, when an Anglo-Saxon magnate ordered three to five thousand oak pilings to be sunk into the underlying clay. Oak pilings don’t talk, and later Viking invaders destroyed pretty well all written records in East Anglia and Essex, but one candidate as builder was the monk-king, Sæbbi of Essex, who abdicated to devote himself to prayer.

The rising sun drew a morning mist from the ground and sea. The tide was coming in, and in the dawn silence I heard it slow swirl through the channels of the mud flats. Continuing across the island to West Mersea, I set off clockwise around the island. Walking away from the harbour, seaweed shaggy pilings rulered out into the slowly filling channel. When, many hours later, I reached the causeway again the lunarscape of mud flats had been replaced by flat sheets of grey water.

From here, a short detour inland made for a long walk into the past. At the top of the rise overlooking the causeway is a barrow dating to the start of the second century AD. Now topped with an oak tree, it would once have been the most visible feature of the landscape for people crossing the causeway. When the mound was excavated early in the twentieth century, archaeologists found, at the heart of the barrow, a lead box containing cremated bones, creating a conundrum under the hill. For the Romans did not raise barrows, and the Britons did not cremate the dead. But here were both.

Returning to island circumnavigation, I followed the Pyefleet Channel that runs between the island and the mainland. The sun had burned off the morning mist and the water sparkled in the early light. Saxon invaders, in their shallow-drafted boats, used these channels as highways into the country’s heart. Not far up the coast, at Sutton Hoo, an Anglo-Saxon king was buried in one of these boats, accompanied into the next life by some of the most magnificent jewellery and armour ever made. It was not hard to imagine the creak of oarlocks and the hiss of oars as the dragon-prowed boats moved stealthily upstream. Indeed, the settlers and invaders of 1,500 years ago were accompanied by much the same soundtrack as I was: the harsh croak of seagulls, the piping whistles of curlews, and the hiss of water and wind. For a few miles I walked in a soundscape unaltered for a thousand years.

The north shore of Mersea Island is quiet. I saw a handful of people, mostly on boats, but many swifts, the birds of eternal summer, jinking over the salt marshes, and, along a thistle-lined stretch of path, a cortege of butterflies accompanied me on my way.

Reaching the tip of the island, the North Sea opened out, unusually blue and tranquil. To the south, the edge of the island was marked by shallow orange cliffs. The sea is hard gnawing the land here. Tree roots jut out into empty space, clawing against the inevitable, before they finally tumble down upon the beach. The cliffs were laid down 300,000 years ago, when elephants, rhinos and bear roamed the area, and fossil hunters still turn up remains.

The sun was up and I was thirsty and hungry. An advantage of this walk was ending it at one of the excellent seafood shacks in West Mersea, eating the wildlife – oysters, cockles, shellfish – that had, unseen, underwater, accompanied me around the island. As walks go, making my way around Mersea had proved somewhat less difficult than Bilbo had warned but it had revealed unsuspected depths of time as well as providing views over great expanses of sea space.