I entrained at London Bridge, my fingers sticky with the residue of an almond Danish, and eager, as ever, for the flatlands of the Thames estuary. This interzone was where my mission began in the late 1980s: to untangle human and physical geography. I've never wavered from my conviction that there's something bizarre about all those millions of Londoners who have never seen – or even seen represented – the point where the River Thames flows into the North Sea. That there can be a location so nearby and yet so under-imagined – that alone tells us that the world remains strange to us.
Conrad's Heart of Darkness takes its readers on a hellish voyage up "a mighty big river ... resembling an immense snake uncoiled ... its tail lost in the depths of the land", but it begins opposite Gravesend in Kent, as Marlowe and his transfixed listeners wait on the Nellie, a cruising yawl, for the ebbing tide. The conceit is nice: the most famous tale of the dark heart of Africa is told in a place, that while well comprehended in Conrad's day, has since become a curiously blank spot on the mental map.
Not that you'd be wise to say this to the people of Gravesend, who presumably know full well where they are: pottering up and down the Regency terraces and occasionally glancing across the ecru river to the container port of Tilbury on the far shore. I had time to kill: I'd missed my companion for the day Antony Gormley at London Bridge, and he'd then missed his stop and gone on to the Medway. We oriented ourselves with mobile phones, echo-locating bats with the faces of middle-aged men, flapping around an estuarine cave. We met eventually in the covered market. Digital watches and AAA batteries, Scottie's Liver Treats and Doggie Chews – all of it being sold from under the bulging eyes of a Queen Victoria who's made – Antony established by tapping her breast – out of terracotta.
Maglorian, the Jack Russell puppy, led us out of town between metal-bashing sheds and the redoubts of the old batteries, while Antony talked to someone on the phone about a plan to climb Mont Blanc. His energy was frankly preposterous: his latest show had opened the previous evening at the White Cube in town, its centrepiece Firmament a single "expanded field" constructed from 1,770 steel elements and 1,019 steel balls. The catalogue copy described it as: "an assembled matrix of volumes that map a celestial constellation while also implying the form of a body that is lost within it".
To me it looked like a big steel girder bloke fighting to escape the gallery.
The river embankment skirted Eastcourt, Shorne and Higham marshes – we dutifully plodded on. Ahead we could see the burn-offs of the oil refinery at Canvey Island flaring in the dullness. Why – it bothered me – is this place so overlooked? Certainly there are plastic shells on the shore – but there are also oyster catchers dibbing in the mud. Besides, this is one of the most celebrated literary landscapes in England: the Cliffe marshes, where the young Pip is discovered by Magwitch at the beginning of Great Expectations.
And now, at the fort where the escaped convict took refuge, there's a small flotilla of dinghies drawn up on the bank of a flooded gravel pit, a two-kilometre-long conveyor belt rattling aggregate across the land to a cement works. Instead of a prison hulk, there was a huge Chinese freighter beating downstream, the containers stacked on its deck presumably jam-packed with dormant VDUs, PCs and plastic pianos, all headed for Shanghai, to be reawakened and played upon. Conrad's novel was an early assault upon the relentless logic of globalisation: his Kurtz would have understood only too well the silent horror of this ghostly ship loaded with detritus.
On we plodded and came upon Redham Marsh, a demented Tellytubby landscape of green hummocks and greener dells: the deathly playground of Second World War anti-aircraft batteries. Travellers' ponies cropped the perfect sward in between long rows of identical reinforced concrete ammunition stores, each one a three-inch-thick shell. I explained to Antony that the individual hepatitis C virus was an icosahedron-shaped blob only 50 nanometres across – so small that it cannot reflect visible light, and so is technically colourless. It seemed the right sort of conversation to be having, here in the whispering, once malarial marsh.
We turned inland for Cliffe, Pip's native village. The puppy trotted at my heels: a tiny dog on a great plain. We were heading back towards the city now: "The water shone pacifically; the sky, without a speck, was a benign immensity of unstained light; the very mist on the Essex marsh was like a gauzy and radiant fabric, hung from the wooded rises inland, and draping the low shores in diaphanous folds. Only the gloom to the west, brooding over the upper reaches, became more sombre every minute, as if angered by the approaching sun."