LIKE many who returned, my grandfather refused to speak about the Somme. My mother recalls only that he loathed jam because the troops in the trenches had used it to sweeten their tea. And since he suffered life-long chest complaints, she believed that he had been gassed.

All we have then are his two medals and the fact that he survived, which is the reason that my son, Joe, and I are alive 90 years later, standing deep in Picardy mud, remembering those who did not.

I thought I understood the First World War from its poets and a thousand fictions about lost youth from Journey's End to Regeneration via Blackadder. But until I stood in the sunken lane outside the village of Beaumont Hamel, I knew only clichés about the madness of war, not its cold and brutal logic.

“Up there,” says our Holt Tour's guide, John Grimwood, a retired RAF warrant officer, pointing 100 metres up a sloped field where cows graze, “were the German trenches.”

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In this place 800 soldiers of the Lancashire Fusiliers gathered, emboldened with rum, at 7am on July 1, 1916, the first day of the Somme. All they had to do was cross that cow field. It seems nothing really on a bright autumnal late afternoon. Until Grimwood points up around the ridges on every skyline, the German machinegun posts that within an hour had brought down 700 of those young men.

Neither had I considered the topography of war until I wandered this seemingly random tract of French farmland. The Germans dug into their high redoubts; the British and Allies below, picked off, exposed, forever slogging uphill.

War runs deep in this land: chalky zig-zags on brown earth are trench scars, farmers plough up live ordnance and bodies are still found. Each anonymous copse of trees, each dip in a field was significant enough for the Tommies to christen it: Sausage Valley next to Mash Valley obviously, Lager Trench and Beer Trench, four woods named after the Apostles.

Here I grasp that Rupert Brooke's “some corner of a foreign field” was no romantic metaphor but a stark prediction, since beside each crop of sugar beet is a neat wall girdling slabs of white Portland stone. I had expected many graves, but not so many graveyards: 240-odd in the Somme alone. It feels more poignant that these men were buried with comrades close to where they fell than gathered at some huge and impersonal mass necropolis.

In our two days here, my son and I became cemetery connoisseurs. We compared the solitude of Railway Hollow with the sadness of Queen's in Serres. We saw graves that began as trenches. We compared the British style (rustic, light, romantic almost) with French (pebble-dashed concrete graves) and German (stark iron crosses shaded by oak trees). We saw a father and son buried side by side, a man of 67, a boy of 15, only three years older than Joe.

It might seem a grim holiday for children, this tour of death, but the young are the Western Front's new pilgrims now that the old soldiers have gone. Eighteen coachloads had just visited Newfoundland Park, where you can still walk through a trench, albeit one lined with wooden decking. This war is tangible: you can pick shrapnel from the ploughed furrows, see the vast mine crater at Lochnagar, feel the bone-chilling cold seep through the Somme mud into your bones.

And it was a youthful war. The lads who joined the Pals Regiments, excited by the notion that they could fight beside their mates, arrived as boisterous and larky as these schoolkids. The faces in the formal battalion photographs are trying not to laugh, as if someone has cracked a dirty gag. At Montauban, Captain Billie Nevill led his men over the top kicking footballs, a prize for whoever booted one into German lines: he died as he left his trench.

But the question that haunts the Somme is why the commanders kept sending wave after wave to their deaths. Grimwood gives a bracing military man's answer, more concerned with strategy than breast-beating. “Because just a few needed to get through,” he says. “And if they sent enough, some would.” So many didn't, though, and their sacrifice still intrigues us, fills the First World War battlefields with amateur genealogists searching for a long-dead relative's grave or, like us, some small trace.

And yet, beyond the thronging memorial sites of Thiepval and Newfoundland Park, it is possible to find peace and contemplation. By the Ulster Tower, a memorial to the 36th Division, we are shown the remains of a German machinegun post and, beside it in the winter wheat, a few summer poppies survive. Joe picks one - I press it in my notebook and, back home, send it to my mother. I'll never know what horrors my grandfather locked away all his life, but perhaps I have a better idea.

NEED TO KNOW

Holts Tours (0845 3750405, www.holts.co.uk) has a variety of First World War battlefields tours in 2009, including “Somme Walks” departing on June 11 for four days from £650pp; “The Somme 1916” departing on September 11 for four days; and three four-day “It's a Long Way to Tipperary” tours from £495pp. Prices include three-star hotel accommodation, most meals, and coach travel from London or Dover.

Times readers exclusive

Holts is offering a Family History Tour on the Somme for relations of soldiers who fought in the First World War there. The itinerary will incorporate requests from group members to visit particular battlefield areas, graves or memorials. Departure is on September 3 for four days. Call for further details or go to www.holts.co.uk/times

‘I will never forget'

Joe Preston, aged 12

“WHERE is your destination?” the Customs man asked. “The Somme,” John, our tour guide said. I wondered how many young men had said that less than a hundred years ago.

It didn't take long to find out. Before we went to our B&B we looked around some cemeteries. First the memorial to the Tank Corps at Pozières - this was the first war to feature tanks - where a huge stone is dotted with poppies, crosses and wreaths.

Then we went to the Thiepval Memorial, a huge arch, red-bricked with Portland stone on the inside. There are names of all the people who died, but whose bodies were never found.

In the German cemetery there are hundreds of black steel crosses - each one has two, three or even four names on it. There is also a mass grave containing 11,000 bodies. We go to even more cemeteries; the death count just gets bigger.

On the first day of the Somme there were 57,000 casualties. A casualty is, as our guide puts it, “anyone who doesn't turn up next day for breakfast”, so they could be wounded, dead, a POW or just missing.

I had read a lot about the First World War, but until I went on the tour I didn't know what it was really like. It shows you how horrible and what scale that war was.

If we don't see this, then surely it could be forgotten, and if that happens, could there be another war like it?

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