At particular times of day when the sun is low, the contours of trenches and craters from the Western Front can still be seen pock-marking the fields of Flanders and Picardy. Seen from the air in certain seasons, ghostly lines of the old front line wind across the landscape. When the fields of Thiepval are ploughed over in the winter, the trench lines of the Leipzig Redoubt show up pale against the dark soil where, ninety or so summers ago, German soldiers dug tunnels and communication channels in the chalky ground. Trench warfare transformed this landscape into a mud that sucked up corpses, releasing long-buried seeds into surreal outcrops of flowers, burying dead men's bones, weapons, military buttons and mess-tins for future archaeologists, farmers and souvenir hunters to find. Every year, they say, ploughing brings forth an 'iron harvest' of rusted military hardware. Buried shells and grenades have a habit of working their way to the surface; as the earth repeatedly freezes and thaws over decades any solid objects buried in it move upwards. Stone-age axes regularly appear here too, for the same reason. Much like the Thames valley, with which it shares certain geological and archaeological characteristics, the whole area around the Somme valley was the site of prehistoric and Roman settlement. An excavation of the front line at Serre not long ago uncovered the remains of a German soldier with a Bronze Age flint scraper in his bread bag. Men living below the surface of the earth must have become intimate with it to the point of madness.