In order to carry out his ‘scientific method’ in the forthcoming Somme battle, Foch had to complete three tasks successfully. First he had to ensure that the artillery was employed effectively with good observation by his aviation services and efficient liaison between aviation, artillery and infantry. The different types of guns would have precisely delimited roles in order to destroy the enemy’s front-line defences, before the infantry launched their assault. As for the heavy artillery, he himself kept control within the army group, with Fayolle’s agreement. Second, he had to coordinate the successive operations against enemy lines, which entailed getting all units, French and British, to re-start operations in a concerted and synchronised manner against succeeding German lines. Finally, he had to divide his attention between the north and south banks of the Somme. To the north, one of Fayolle’s Sixth Army corps (XX Corps) was in line, but the majority of troops were British to whom he could send only advice, not commands. To the south, the two remaining Sixth Army corps were in line, and another army (Tenth Army under General Alfred Micheler) was gradually formed as units were freed from Verdun. The geography of the two sectors on either side of the river was quite different. In the south lay a plateau, bounded by the river to the north and east, and by a line of villages to the south. Because the Somme upstream of Péronne flows south–north, any French penetration into the enemy front in this sector would soon reach the river, which could be crossed easily only if the Germans did not defend it – but this was unlikely. Downstream of Péronne the river flows from the east seawards and, except for communications with the southern bank, it did not affect the action of the BEF and the single French corps in the northern sector. Here the terrain rises to the watershed between the Somme and the Ancre rivers – the Thiepval Ridge, where so many British were to become casualties – with the heights crowned by fortified villages. The German lines on the far side of the ridge were situated on the reverse slope, hence invisible to British gunners whenever weather conditions prevented aerial observation.