The little check was not without its importance. It meant that the rapid forward march of his vanguard had failed to force that extreme extension of the French line, which was called "The Line of the Trouille" from the name of the small river that falls into the Haine near Mons. In point of time--which is everything in defensive warfare--the success of the defence at St Ghislain meant that all action by the allies was retarded for pretty well a week. Meanwhile, the weather had turned to persistent and harassing rain, the allied army, "toiling through a sea of mud,"[6] had not invested Mons even upon the eastern side until the evening of the 7th of September. On the same day Villars took advantage of a natural feature, stronger for purposes of defence than the line of the Trouille. This feature was the belt of forest-land which lies south and a little west of Mons, between that town and Bavai. He strengthened such forces as he had on the line of the Trouille (the little posts which had checked the first advance upon Mons, as I have said), concentrated the whole army just behind and west of the forest barrier, and watching the two gaps of that barrier, whose importance will be explained in a moment, he lay, upon the morning of Sunday, September the 8th, in a line which stretched from the river Haine at Montreuil to the bridge of Athis behind the woods; keeping watch upon his right in case he should have to move the line down south suddenly to meet an attack. As Villars so lay, he was in the position of a man who may be attacked through one of two doors in a wall. Such a man would stand between the two doors, watching both, and ready to spring upon that one which might be attacked, and attempt to defend it. The wall was the wall of wood, the two doors were the opening by Boussu and the other narrow opening which is distinguished by the name of Aulnois, the principal village at its mouth. It was this last which was to prove in the event the battlefield. All this I must make plainer and elaborate in what follows, and close this section by a mere statement of the manoeuvring for position. [Illustration: Sketch Map showing the Lines of Woods behind Mons, with the two gaps of Boussu and Aulnois.] Villars lying, as I have said, with his right at Athis, his left on the river Haine at Montreuil, Marlborough countered him by bringing the main of his forces over the Trouille[7] so that they lay from Quevy to Quaregnon.