Army of the Potomac

   On the morning of September 15, 1862, the Army of the Potomac pursued the retreating Confederates from South Mountain: Pleasonton's Cavalry, the Second, First and Twelfth Corps by Turner's Pass, Boonsboro and Keedysville; Sykes' Division of the Fifth Corps, the Reserve Artillery and Ninth Corps by Fox's Pass and the Old Sharpsburg Road. The Sixth Corps and Couch's Division (attached to the 6th Corps) remained near Crampton's Pass. Pleasonton overtook the Confederate Cavalry rear guard at Boonsboro, attacked and cut it off from the main body and pursued it in the direction of Hagerstown. Richardson's Division, Second Corps, in the advance, followed closely and skirmished with the retreating Infantry, until it reached the ridge bordering the Antietam, behind which it formed line, north of the Boonsboro Pike. Tidball's Battery (A 2nd U.S.) and Petit's Battery (B 1st N.Y.), from the crest of the ridge, engaged the Confederate Artillery posted at and south of this point. French's and Sedgwick's Divisions, Second Corps, halted on either side of the Pike, midway between Keedysville and the Antietam. The First Corps took position east of the Upper Bridge; the Twelfth Corps halted near Keedysville. Sykes' Division formed south of the Boonsboro Pike, on Richardson's left. Late in the day the Ninth Corps encamped on Geeting's farm at the west base of Elk Ridge. Army Headquarters were established at Pry House.