General Burnside was now separated from the main army. Longstreet held the hills east of the town, and from his batteries there, could partly enfilade Richardson on the one hand, and Burnside on the other. His cannon swept the bridge on the Boonesboro' pike. None of McClellan's troops had crossed there. It was nearly two miles from Richardson to Burnside. General McClellan was fearful that Lee would cross the middle bridge to the east side of the Antietam and cut off Burnside; therefore General Porter's corps was held in reserve east of the river by the heavy guns.[72] But Lee would have found it a difficult task, for Porter's heavy guns commanded the approach to the bridge from the west. If McClellan could not cross the bridge because Longstreet's guns swept it, neither could Lee have crossed under the fire of Taft, Langner, Von Kleizer, Weaver, Weed, and Benjamin. [Footnote 72: McClellan's Report, p. 207.] The Antietam, a half-mile below Burnside's bridge, makes a sudden curve toward the west. It is crossed by one other bridge, at Antietam Iron-works, and then joins the Potomac. By throwing General Burnside across the Antietam, General McClellan designed not to turn the right of Lee and gain possession of his only line of retreat to Shepardstown, but to carry the heights, then pass along the crest towards the right.[73] But this movement isolated General Burnside from the army. He must hold the bridge or be cut off. He would be in a _cul de sac_, a bag with only one place of escape, at the Antietam Iron-works. [Footnote 73: McClellan's Report, p. 201.] When General Lee saw the preparations of Burnside to advance, after having carried the bridge, he weakened his left to strengthen his right. Hood, who was lying in reserve behind Jackson, was sent down. Longstreet moved some of his brigades. Jackson made a demonstration at Poffenberger's, already noticed, to make McClellan fear an attack at that point. General Lee intended to do more than merely hold his line against Burnside.[74] By massing his troops at Sharpsburg, when Burnside was far enough advanced, Lee intended to seize the bridge and cut off Burnside's retreat. [Footnote 74: Statement of a Rebel officer after the battle,--a prisoner.] Burnside's divisions crossed the stream at the bridge and at the ford, and formed for an advance upon the heights near the town. Wilcox was on the right, supported by Rodman in the center, Scammon's brigade on the left, and Sturgis in rear of Rodman. While the troops were crossing and forming, Longstreet's and A. P. Hill's batteries kept up a constant fire of shells. Clark's, Durrell's, Cook's, and Simmons's batteries went across the bridge, gained the crest of the hill beyond, came into position, and opened fire in reply.