On the night of the 28th-29th the units of the Fifty-sixth Division which had been most heavily engaged were relieved by these five Canadian battalions, which came under orders of the Third Canadian Division.
It was not until about 10.00 p. m., on the night of the 28th-29th, that the leading troops of the Forty-sixth Division arrived and began to relieve the Fourth Canadian Division.
In view of the seriousness of the situation, units of the Fourth Canadian Division were moved, as the relief progressed, by lorry and light railway to Neuville St.-Vaast, and marched quickly into the line to relieve the elements of the Fifty-sixth Division.
The situation of the Canadian divisions at noon, March 30, 1918, after some other readjustments had been carried into effect, was as follows:
Third Army. Under Sixth Corps—Second Canadian Division: Neuville-Vitasse sector. Under Seventeenth Corps—First Canadian Division: Telegraph Hill sector.
First Army. Under Canadian Corps—Third Canadian Division: Acheville-Mericourt-Avion sector. Under Canadian Corps—Fourth Canadian Division: Gavrelle-Oppy sector.
On April 7, 1918, the First Canadian Division relieved the Fourth British Division astride the Scarpe and came under orders of Canadian Corps; the army boundaries being altered so as to include the sector taken over by the First Canadian Division in the First Army front.
In the meantime, on the night of March 28th-29th, 1918, owing to operations astride the river Scarpe, the front-line system had been abandoned under orders of the Thirteenth Corps and the troops withdrawn to the Blue line in front of the Bailleul-Willer-val-Chaudière-Hirondelle line, as far north as the Mericourt sector.
This Blue line was originally sited and constructed as an intermediate position, and consisted in most parts of a single trench none too plentifully supplied with dugouts. This meant that until a support line was dug and made continuous the troops had to be kept in strength in the front line, subject to heavy casualties from hostile shelling and to probable annihilation in case of an organized attack.
Any advance beyond the Blue line on the Fourth Canadian Division front would have brought the Germans within assaulting distance of the weakest part of the Vimy Ridge, and the severity of the shelling seemed to indicate that a renewal of their attacks was probable.