Undoubtedly, one of the surprises of the war was the discovery that the Germans had prepared the positions just described. The preparation must have involved great labour. But it should not be forgotten that from time out of mind one of the chief industries in this part of France is represented by the chalk quarries, out of which is dug the material, known in its prepared state as plaster of Paris. All through Champagne there was, before the war, a considerable German population. Not a few of the plaster quarries had passed into the hands of Germans. The principal quarries are on the steep north slope of the ridge along the Aisne. Cut into the hillsides, these chalk pits present a labyrinth of galleries and chambers, where the quarrymen were accustomed to take their meals and even to sleep. These quarries, numbered by scores, might well form the refuge and stronghold of an army. The region is remarkable, also, for its many natural caves. Even more important, however, from a military standpoint, is the southern side of this plateau. The only means of approaching the plateau from that side is either up the clefts or side valleys, or from the western end where the level gradually falls. But an attack made up one of the side valleys could be assailed from both sides. In possession of the plateau above, the defence, while keeping its force undivided, could move that force to any point where attack was threatened, having itself no clefts or fissures to deal with. It will be seen, therefore, that the ridge formed a sort of vast ready-made castle, big enough to stretch from London to beyond Oxford, or from Liverpool to Manchester, and that the quarries and galleries made it habitable, at all events on the banditti level of existence. As Sir John French has pointed out,[19] owing to the patches of wood on the upper slopes and tops of the bluffs, only small areas of the plateau were open to view from the tops of bluffs on the south side of the river. Hence the movements of the defenders were, looked at from across the river, to no small extent concealed. Two further _military_ features of the ridge should be noted. One is the fact that its steep northern slope forms one side of the valley of the Lette, and that, therefore, it is bounded by a river on both sides; the other is, that some eight miles from its eastern end at Craonne the plateau narrows to a mere neck less than a mile wide, and that across this neck is carried the Oise and Aisne canal. Not relying, however, merely on the natural features of the place, the Germans dug along the plateau lines of entrenchments connected by galleries with other trenches in the rear where reserves, not in the firing line, were held. These back trenches formed living places. The mass of men was too large, for any save the smaller proportion, to find shelter in the quarries. It will be seen, therefore, that the business of turning the Germans out of such a fastness could be no easy matter.