1917 takes place around the real towns of Écoust-Saint-Mein and Croisilles, about six miles north of Bapaume, a British objective during the Battle of the Somme the previous year (never reached during the battle itself; the battle ended with the front three miles short and it was only captured after the German withdrew to the Hindenburg Line in the spring of 1917). The soil of the Somme was predominately chalk, which in contrast to the sodden clay of Flanders further north, made it excellent for establishing trenches and conducting mining: 19 mines were tunnelled under the German trenches and blown up on the First Day on the Somme. The "white stone" we see in 1917 is therefore most likely chalky soil, and indicates that the trenches have been hastily-dug as improvised defences (which is consistent with the fact that the Devonshires are said to have only recently advanced quickly). Given more time, the trenches would have been substantially improved with revetting, including wriggly tin, wooden boards, and wire obstacles (as seen earlier in the film in the older, more-developed trenches Schofield and Blake deploy from). The chalky spoil would also most likely have been concealed to reduce the visibility of the trenches from the air.

Sources:

Gordon Corrigan, Mud, Blood and Poppycock