When Little Toller Books wondered if I would like to write a monograph on any aspect of our islands’ landscape, Silbury Hill in Wiltshire immediately reared into view. The largest prehistoric mound in Europe, mysteriously empty of anything but chalk and earth, this grassed-over Neolithic alp has long attracted both archaeologists and the mystic-minded. I guess I fall somewhere between the two. Now that the book – my first effort at non-fiction – is finished, there is no longer any excuse not to start the next novel. Inevitably, I’ve been reflecting on the difference between sticking to the facts and making things up.

Where Silbury’s concerned, this difference is tested by our complete ignorance as to why the hill exists. An estimated million hours spent on construction rather than herding or cooking or stitching must have had a point, but we don’t get it. Is conjecture a species of fiction? To muddy the difference further, Silbury insisted on being called “she”. I obeyed, not out of New Age winsomeness but from the influence of country dialect, in which neuter pronouns are as alien as robot leaf blowers.

Otherwise, strapping on my archaeologist’s helmet, I rode into the thick of fact and data-led theory. Take the simple question of Silbury’s age: she rose in several distinct phases. The field calculations have gone on evolving. Yet precision had to be the watchword: poetic licence was invalid in this foreign country. Qualifiers like “probably” and “it is surmised” began to spread like ice over the prose, which I had to keep pinching to avoid academic bloodlessness.

This was all further complicated by an involuntary flood of emotional memoir, unexpectedly triggered by the project. My boarding school was three miles up the road. The memories came unbidden, and I struggled to confine myself to what was strictly relevant. The opening pages drifted into the first time I was thrashed by the headmaster of my previous school, nowhere near Silbury, and had to be excised. Trauma has a way of clamouring to be noticed once the light streaks through the trapdoor. I might have written hundreds of pages about freezing cold baths or Corps or the minutiae of bullying, each strand fanning out into sub-strands rather like a map of the brain: Silbury lost in the neurons. How does anyone ever finish a memoir? I am used to characters doing their own thing, with the author trotting along behind, but here it was the author chasing himself. As Karl Ove Knausgaard puts it: “What happens if you do tell, really try to tell everything, and don’t give a damn about subtext?” You write six massive volumes called My Struggle, that’s what.

The imagination, however, did intermittently bubble up into purely novelistic cameos. Almost every attempt to depict Neolithic life in northern Europe is mere guesswork, anyway. Despite Paul Eluard’s memorable comment that “words describe things in everyday language, while in authentically poetic language it’s the things, the objects, which generate the words”, prehistoric artefacts are voiceless. However “eloquent” they are claimed to be, they are loud with silence.

Silbury is eloquent, but as a screen is: a blankness onto which researchers project their own shadowy hopes. Confidently solid theories about her purpose have piled up like tomb offerings (each a fascinating reflection of their times), and I duly added mine, arrived at through observation rather than inspiration – which makes it no more convincing, of course. The book ended up like the small, primary mound discovered at the heart of Silbury: a marble cake of different soils. Memoir, data, theory, streaks of poetry, swirls of fiction.

There are similarly autobiographical works that are not marbled so much as thoroughly mixed: V. S. Naipaul’s superb The Enigma of Arrival – also set in Wiltshire – is a memoir disguised as fiction, the disguise being merely the appellation “a novel” on the cover. The effect on the reader is like one of those old stereoscopic photos: heightened realism and increased artifice at the same time. Naipaul’s voice, or just a writer’s voice?

I am aware in my own effort that the voice is very much mine, but slightly exalted, perhaps even febrile at times. It may turn out to be embarrassing in a way that a novel could never be, although I dropped the potentially libellous bits about my school in case they fired up the tabloids (if you search for images of “Marlborough College” on the web these days, they are headed by two old Marlburiennes called Kate and Pippa Middleton). I feel now that I have shown my hand, displayed my unshelled part. W. G. Sebald, in his trilogy of novels Vertigo, The Emigrants and The Rings of Saturn, manages to expose those softer parts through the bullet-proof glass of detachment – fiction disguised as memoir, perhaps. As his narrator meanders down historical trails that cloud into memory to break out again into sunny clearings of fact which feel just as subjective and dream-like (his citations have no quotation marks), Sebald escapes the old categorizations, tempts us not to care about the difference. Into which category do dreams fall?

Memoir can be so much false memory, of course: my first sighting of Silbury is missing because I cannot remember it – not even the first climb up. Invention was tempting. Still, I did tighten the focus where everything remained too hazy. I was convinced the sudden existential recognition of my minuscule place in the cosmic order at the age of fifteen happened deep inside the unexcavated East Kennet Long Barrow, but the entrance has always been blocked. The memory of that moment is clear: I just misread the stony decor. The impressionistic nature of memory clamours to be channelled into a story: our schoolboy sortie to the long barrow had to have a beginning, middle and end, whereas all I’d retained were spots from the middle. There is no path to the barrow, which is on private land; so, assuming that we had left our bikes before crossing the field and that we must have been trespassers, I wrote: “We left our bicycles on the nearby bridleway and struck off across the furrows, expecting an angry farmer’s shout at any moment”. This image of me and my two friends struggling over the clods has now replaced any vestige of real memory.

The final chapters were more straightforward to write, more like reportage: they recount a visit last year, on All Hallow’s Eve, to Silbury and its neighbouring stone circle at Avebury. With the coming monograph in mind, I took photos, scribbled notes. My sound recording of the firelit neo-pagan ceremony that took place among the monolithic sarsen stones meant that I could directly quote the crouched and hooded gatekeeper of the “labyrinth” (delineated in table salt) as he shook his wand, snarling judgement at each candidate. “Selfish! You’ve jumped the queue! Next! You may walk around, but not come in!” A Green Man told us that you could only enter if the gatekeeper, like an ultimate editor, was convinced by your negative experience – already written out on a piece of paper, ready to be cast into the flaming cauldron at the centre.

No need to call on imagination to fill the gaps. In this case, anyway, with the shadows dancing on our faces, imagination would never have caught up with the reality. The mild English day and now moon-streaked night were resplendent with facts that left fiction struggling far behind.

© Adam Thorpe & The Times Literary Supplement