The “ague” — malaria — had been one of the chief perils of living here; the marshes, Pip tells Magwitch, are “dreadful aguish.” When the embankment was built, standing floodwater where malaria-carrying mosquitoes bred was prevented and the disease largely eradicated.

As the Thames widens toward the sea, the towering robotic cranes of the London Gateway container terminal dominate the river’s far bank a mile north, but to the south the marshes are at their most exposed and expansive. Looking across them toward Northward Hill, two miles away, I feel my city eyes widening, adjusting to the open space.

The environmentalist Wallace Stegner wrote in 1960 that we need “wild country available to us, even if we never do more than drive to its edge and look in.” It may be hard to think of this landscape so heavily shaped by humankind as wild, but to know that it is here, just an hour or two from London, still intractable and sparsely populated 180 years after Dickens — it’s a source of comfort.

“The distance from Cliffe Creek to Egypt Bay is about three miles,” Colonel Gadd announces. It takes him an hour, “as the way is for the most part on slippery mud.” The embankment path avoids the mud, but is more like six miles, and it’s past 4 p.m. by the time I reach Egypt Bay. I arrive sunburned and windblown, my lips taut and salty.

The provenance of the bay’s name is unclear, though it has been suggested it’s because a Phoenician coin was unearthed nearby. What is known is that this sandy inlet was an ancient landing place, and favored by smugglers in the 19th century. Beyond the mudflats, says Colonel Gadd, is where the prison hulk was from which Magwitch escaped before apprehending Pip — “in the topographic sense,” at least, for the vessel once anchored here was not a prison hulk, the colonel concedes, but a Coast Guard lookout.

I rest on the sea wall and when I stand, a group of swans rises from a nearby dyke, five of them, passing over the marshes until they are no more than gleaming dashes against the dark woods of Northward Hill. Turning away from the river, I follow them south across the marsh, locating the wooden footbridges that cross the drainage ditches, and finally heading west, until the square stone steeple of St James’s Church comes into view.

According to the historian Edward Hasted, writing in the 1770s, Cooling was “an unfrequented place, the roads of which are deep and miry, and it is as unhealthy as it is unpleasant.” On this late summer’s afternoon the village has an air of tranquil prosperity, and is so still and unpeopled as to feel like a film set. I don’t see a soul.

In the warm raking light, I suppress a small shudder. A place might inspire fiction, but fiction in turn can shade your experience of that place.

“Close by the south porch,” Gadd writes of the church, ‘are the gravestones of Pip’s little brothers.’ A meter in length and torpedo-shaped, the real gravestones — thirteen in total — belong to two branches of the Comport family, victims of malaria in the late 18th century. In Dickens’s novel there are just five, but as his friend and biographer John Forster put it: “with the reserves always necessary in copying nature not to overstep her modesty by copying too closely, he makes the number that appalled little Pip not more than half the reality.” No reader would believe thirteen.

Colonel Gadd insists that the stones were imaginatively “imported” by Dickens to St Luke’s at Lower Higham, where my walk began. But for me it is St James’s, because of their presence, that will always be where Pip met Magwitch on that “memorable raw afternoon towards evening.”

Even today, in the warm raking light, I suppress a small shudder. A place might inspire fiction, but fiction in turn can shade your experience of that place.

I remember my first reading of the novel, long before I set foot here. It was Pip who knew these “death-cold flats,” but that terrifying stranger who would transform his life — part magus, part witch — seemed to be the very marshes incarnate, a shackled golem formed of salt-mud and fog. (“Keep still, you little devil, or I’ll cut your throat!”)

Once I’ve written my name in the church’s visitor book, I return “The Great Expectations Country” to my backpack, and start the four-mile walk back to Lower Higham and my train to London.

William Atkins is the author of “The Immeasurable World: Journeys in Desert Places” (Doubleday).