The Situation on 13 September: Early on the 13th Confederate cavalry commander Stuart pulls the brigades of Hampton and Munford west to the line of Catoctin ("Kittoctan") Mountain at Hagan's Gap on the National Road and at Jefferson, respectively. He plans only to delay the approaching Federal Cavalry, but by the end of the day he is pushed back by brigade-strength Federal infantry to the South Mountain gaps.
The three Confederate columns approaching Harpers Ferry nearly close the ring on this Saturday: McLaws pursuing the Federal defenders south along Maryland Heights, Walker approaching the Loudoun Heights from the south, and Jackson nearly completing the encirclement from the west. Longstreet's command remains in the vicinity of Hagerstown, and DH Hill's Division deploys in an arc centered on the village of Boonsboro. In response to a call from Stuart, howver, he detaches Colquitt's Brigade to Turner's Gap late in the afternoon.
Late in the evening, alarmed by the rapidity with which McClellan's Army of the Potomac is appearing near Crampton's and Turner's gaps, General Lee orders Longstreet's command (less the trains, reserve artillery, and Toomb's Brigade) to march for Boonsboro next morning. He also directs DH Hill personally to Turner's Gap, and warns McLaws of the threat on his flank and rear.
During this day, the the right and center wings of the Union Army of the Potomac (I, II, XII Army Corps, Sykes regular division) consolidate and enter Frederick . The left (VI Corps, Couch's Division) advance north and west to cover the Army's flank.
The Federal Cavalry, supported by IX Corps infantry, push up the National Road and, by day's end, drive the Confederate cavalry through Middletown to the base of Turner's Gap. Other of Pleasonton's brigades approach Crampton's Gap, and skirmish with Stuart's rearguard there.
Also on this day, McClellan has handed to him the orders of his opponent, and a clear picture of General Lee's plans to clear Harpers Ferry.
A. P. Hill's Light Division
Army of Northern Virginia
D. H. Hill's Division
Jeff Davis (MS) Legion (Cavalry)
Stuart's Cavalry Division