As planned, the main framework of the defense in depth was based upon machine-gun positions, protected by belts of wire entanglements so placed, in relation to the field of fire of the machine guns, that they were enfiladed over their entire length. The whole area was compartmented in such a way that the loss of ground at any one point could be localized, and would not cause a forced retirement from adjoining areas. Machine-gun emplacements of the Champagne type were constructed, and dugout accommodation for the machine-gun detachments was provided in the deep tunnels of these emplacements. This framework was completed as rapidly as possible by trenches and by defended localities organized for all-round defense. A great many dugouts were made to accommodate the garrisons of these localities, and for dressing stations and battle headquarters. Advantage was taken of the possibility of utilizing the subways tunneled in 1916-17 for the attack on Vimy Ridge, and in addition steps were taken to create an obstacle on the southern flank of Vimy Ridge by the construction of dams to enable the valley of the Scarpe to be flooded as required. Trial inundations were made to insure the smooth working of these arrangements. A great deal of care was given to the distribution of the artillery in relation to the policy of defense. Three systems of battery positions were built so as to distribute the guns in depth and sited so as to cover the ground to the northeast, east, and south, in case the flanks of the corps should be turned. These batteries were protected with barbed-wire entanglements and machine-gun positions against a sudden penetration of the enemy, and they were designed to become the natural rallying points of infantry in this eventuality. Successive lines of retirement were also prepared, battery positions were selected, organized, and marked, cross-country tracks were opened up, and observation posts, echeloned in depth, were located and wired in. On Vimy Ridge alone seventy-two new battery positions were built and stacked with ammunition; these positions could be used either for the distribution of the corps artillery in depth, or as positions which reenforcing artillery could immediately take up in the event of a heavy attack. The weather being much finer during the months of January, February, and March, 1918, than is generally the case, very good progress was made, and the following defensive works were completed in rear of the main front-line defensive system: 250 miles of trench; 300 miles of barbed-wire entanglements; 200 tunneled machine-gun emplacements. In addition to the above, existing trench systems, dugouts, gun positions, and machine-gun emplacements were strengthened and repaired. Each trench system was plentifully marked with signboards and many open machine-gun positions were sited and marked.