McClellan's army, and about three miles to the east of north from Sharpsburg was the stone bridge, on the Williamsport road, by which Hooker crossed his two advanced corps.
Lee, before the coming of Jackson, posted his men with Longstreet on the right and D. H. Hill on the left, in order to cover the approaches from the Burnside and the Boonsboro bridges, having excellent positions for his artillery to cover these. Hood's two brigades were transferred to the woods near the Dunker church, to defend the approaches from Hagerstown, while D. H. Hill's five brigades extended Hood's right to the vicinity of the Boonsboro turnpike, and Longstreet's men prolonged the line to the right to the front of the Burnside bridge. On Jackson's arrival his command was posted to extend Hood's line farther to the left, to the vicinity of the old toll-gate, while Stuart occupied the commanding Nicodemus ridge, north of Nicodemus run, from which his artillery swept the roads by which Hooker and Mansfield must advance.
Lee, Longstreet and Jackson were in conference, with a map spread before them, in a house in Sharpsburg, when Stuart reported McClellan's advance, by the Williamsport road, late in the afternoon of the 16th. Jackson was promptly sent to take charge of the left wing and meet the threatened engagement. The turnpike road from Sharpsburg to Hagerstown runs nearly north and south, and, for at least a mile, between somewhat parallel and rather bold limestone ridges. At the Dunker church, a little more than a mile north of Sharpsburg, the Smoketown road enters this turnpike at an acute angle. By this latter road the Federal army advanced, having turned to the left, soon after crossing the Antietam. About half a mile above the junction of these roads there were patches and fringes of rocky woods on each side of the Smoketown road. These are known as the "East woods;" and to the northern edge of these, the Federal skirmishers came late in the evening of the 16th, while the Confederate skirmishers held the southern edges of the same. Quite a body of open oak forest surrounded the Dunker church and extended northward for some little distance along the west side of the Hagerstown turnpike; thence a narrow field extended, for a half mile or more, between that road and the skirt of forest
