A million men died at the Somme but their memories live on in stone. Emma Cullinan reports

The fields of the Somme are calm beneath the bright Picardy sky. A place of peace where, 90 years ago, thousands of soldiers walked into walls of machine-gun fire in attempts to gain control of small sections of land. Even before the war had ended, the question arose of how to commemorate the dead: both those whose bodies were found and the more than half a million who fought on the British and French sides who disappeared with scant trace.

Commemorating the dead is hugely important to families, says John Horne, professor of Modern European History at Trinity. "Memories of a homeland forged, by death, in a foreign field," he said in a lecture in the war museum at the French town of Péronne, two weeks ago.

Delegates had come from the north and south of Ireland to remember battles by their forebears. A group that included members of the Royal Dublin Fusiliers visited the little known cross commemorating the 16th Irish division, in the village of Guillemont.

It had been 90 years, to the day, since a significant battle. Nearby is the more ostentatious Ulster Tower in Scots Baronial style, a copy of a Victorian folly near Bangor.

The constructions reflect how the two communities feel about their role in the war: one a simple cross and the other a prominent structure.

The Ulster Tower is in Thiepval - a town flattened by battle - near an exquisite memorial designed by architect Edwin Lutyens in 1932, that commemorates 73,357 missing from the British Commonwealth (from all reaches of the former empire) and France.

From some angles Lutyens' memorial seems a heavy building, embedded into the very earth that holds the dead and carrying the weight of the tragedy, and from others it is lightweight, showing the sky through a majestic Roman arch, perhaps symbolising an empty tomb, the spirit of the dead and a reach towards the heavens.

This brick building sliced by Portland stone is founded on a number of architectural styles and an extraordinary amount of calculations. It carries them all lightly and with dignity. The brick interspersed with white bands alludes to Edwardian architecture, which largely disappeared after the war.

The stepping-back as the building rises gives it an Egyptian-pyramid outline and a ziggurat form. This style is also contemporary: similar to the recessing of skyscrapers, that planners were asking for in US cities at the time. Then there are the Classic Roman-style arches, which play an extraordinary role in this building.

The memorial needed the space on which to write 73,357 names and Lutyens created a remarkable structure that has allowed for an astonishing number of these in a relatively small space. As Gavin Stamp writes in his book The Memorial of the Missing of the Somme, "Lutyens adopted the triple Roman triumphal arch model... where smaller side arches flank a central arch. By adopting this type of elevation on all four sides rather than two, he no doubt realised that the intersections of three arched tunnels on two axes would create 16 separate piers and thus provide large areas of internal wall space: 48 panels on which to carve those 'intolerably nameless names'. Out of this concept he created a form entirely without precedent."

The interlocking arches could have worked in a solid geometric form but in this building each sits in its own cubic surround which, like the arches, climb up the building with ever increasing scale. All of the arches, despite differing sizes, are to exactly the same proportions: their height is two and a half times their width.

This memorial is a perfect example of the use of constituent elements which are smaller versions of the main event: in this case the monumental arch at its centre. Each of these growing elements is knitted together by a pattern of string courses (horizontal bands): above the keystone on each arch the string course turns a corner to meet the springing of the next arch (the point where the arch begins).

There is something reassuring about a building that has been composed with such harmony. This dependable and serene structure offers both a calm and monumental memory to a chaotic and murderous event.

Such serenity is evident in the much newer (1992) building by Peru-born architect Henri Ciriani that houses the L'Historial de la Grande Guerre museum at Péronne - just a few miles from Lutyens' memorial. As with Lutyens' structure, this building has a heavy side - at its entrance where it is embedded in medieval and 17th century buildings - and a light side, where it stands on piloti, some of which plunge into a lake at the rear of the building.

Piloti are, of course, a trademark of Le Corbusier's and Ciriani was a former student of the Swiss architect. Le Corbusier thought buildings should enable journeys through architecture and the Péronne building offers an exploration of the Great War.

The journey begins awkwardly. The steps into the building aren't uniform so you can't gain a rhythm as you ascend. This stop-and-think aspect of the building is continued in the entrance corridor which is in white concrete on one side (coloured through the use of white cement) and polished Carrera marble on the other, to encourage reflection, both actual and intellectual.

The museum is infused with a milky white light designed to distinguish it from the external Picardy sky. "Light in an interior space is what penetrates it from nature," said Ciriani. "What situates us ever so slightly in the cosmos. The cosmos is absent from underground parking lots." Like Lutyens' memorial, this too looks heavenwards.

The plan of the building is shaped like a head in a helmet - or a parachute. The structure pivots around one central room, with the surrounding rooms offering a journey though war - through French, British and German eyes. Life before the war is depicted in a timber-floored room to denote warmth, while the war room is reached via a ramp designed to give a feeling of the inevitable and unstoppable descent into conflict.

Models of soldiers in the room aren't standing, dressed for a fight; instead their defensive clothing and armoury lies beside their decapitated bodies in white ditches - designed to evoke the chalky soil in the area, marble tombs and the trenches. Even rowdy teenagers become quiet when they enter this room, says the curator.

This calm, white building - with its thin slits in the ceiling - has some resemblance to the National Gallery extension in Dublin (by Benson and Forsyth) and, where you can walk beneath it, there are similarities to the Glucksman Gallery in Cork (by O'Donnell and Tuomey).

While Lutyens' and Ciriani's buildings are different in appearance - and reflect their times - both architects have achieved masterpieces infused with humanity and spirituality derived from a careful consideration of function.

There is a lot to try and understand about the First World War. No one wants their loved ones to have died in vain but death on this scale for so little point is questionable. Professor Horne, member of comité directeur of the Centre de Recherche (an international group of experts who regularly meet in the Peronne Historial museum to investigate and share information on the war), invited people to tell their personal stories after his talk.

One told how he had lost one relative in the Somme in April 1916 on the same day as another relative was killed in O'Connell Street during the Rising.

As historians such as Professor Horne will tell you, our task is not to judge people by what we know today but to try and think back to what they knew at the time. Many went to war convinced that they were defending freedom (where have we heard that more recently?).

The museum at Péronne aims to help us understand the war while the Thiepval monument preserves the names of those who would have otherwise disappeared from memory; wiped into the face of the earth on the muddy battlefields.