At first glance this detailed and dense map looks foreboding and somehow off-putting--at least for me, and that was before I understood what the numbers represented.
The blue numbers on this section of a larger map refer to soldiers killed on the battlefield of the 1916 Somme battlefield. It is the work of Lieutenant Colonel Arthur Messer (Assistant Director of Graves Registration and Enquiries in France), who undertook to record the crosses of the Fallen on the battlefield and register their location, and then to re-inter the bodies together in larger cemeteries.
What we are seeing here are pieces of four 6x6 grids (one complete and three partial) numbered (in red) 1-36, each one of these squares further subdivided into four section. Each larger square composed of 36 squares is 1000x1000 yards total, meaning that each one of the 36 subdivisions is about 166x166 yards, and each of the four segments of one smaller square is 83 yards. The blue numbers indicate a soldier killed on that field of battle which means that in the large 36-square "M" subdivision #18 that there were 210+29+372+17 fatalities, or 628 on a 166x166 yard field, or in one case 372 killed on a 83x83 yard plain. The deaths were even more intense on other areas of the field--in Square S #11 there were 749+207+234+126, or 1,416 deaths in that 166x166 yard field, and 749 on the 83x83 yard field. It is hard to visualise such loss.
According to the Imperial War Museum, temporary markers for fallen soldiers looked like this:
After the war the Imperial War Graves Commission replaced the wooden crosses with stone markers.
The wooden markers would then returned to the family.