battalion. Dorsey Pender's division of Hill's Corps stepped through the exhausted lines of Heth's men at about 4:00 p.m. to finish off the I Corps survivors. The brigade of Brig. Gen. Alfred M. Scales attacked first, on the northern flank. His five regiments of 1,400 North Carolinians were virtually annihilated in one of the fiercest artillery barrages of the war, rivaling Pickett's Charge to come, but on a more concentrated scale. Twenty guns spaced only apart fired spherical case, explosive shells, canister, and double canister rounds into the approaching brigade, which emerged from the fight with only 500 men standing and a single lieutenant in command. Scales wrote afterwards that he found "only a squad here and there marked the place where regiments had rested." The attack continued in the southern-central area, where Col. Abner M. Perrin ordered his South Carolina brigade (four regiments of 1,500 men) to advance rapidly without pausing to fire. Perrin was prominently on horseback leading his men but miraculously was untouched. He directed his men to a weak point in the breastworks on the Union left, a gap between Biddle's left-hand regiment, the 121st Pennsylvania, and Gamble's cavalrymen, attempting to guard the flank. They broke through, enveloping the Union line and rolling it up to the north as Scales's men continued to pin down the right flank. By 4:30 p.m., the Union position was untenable, and the men could see the XI Corps retreating from the northern battle, pursued by masses of Confederates. Doubleday ordered a withdrawal east to Cemetery Hill. On the southern flank, the North Carolina brigade of Brig. Gen. James H. Lane contributed little to the assault; he was kept busy by a clash with Union cavalry on the Hagerstown Road. Brig. Gen. Edward L. Thomas's Georgia Brigade was in reserve well to the rear, not summoned by Pender or Hill to assist or exploit the breakthrough. Union retreat The sequence of retreating units remains unclear. Each of the two corps cast blame on the other. There are three main versions of events extant. The first, most prevalent, version is that the fiasco on Barlow's Knoll triggered a collapse that ran counterclockwise around the line. The second is that both Barlow's line and the Seminary defense collapsed at about the same time. The third is that Robinson's division in the center gave way and that spread both left and right. Gen. Howard