In the hours before dusk of September 13, Pleasonton's cavalrymen drove Confederate cavalry from Hagan's Gap in Catoctin Mountain west of Frederick and then likewise secured Middletown from the gray horsemen. Pleasonton requested support from Ambrose Burnside in his rear. In the meantime, he pushed his troopers to the base of Turner's Gap in South Mountain itself.
At the crossroads town of Bolivar, Pleasonton dismounted some of his cavalry and pushed them up South Mountain north of the National Road. This movement produced minor skirmishing. Standing at the base of Turner's Gap, where the National Road crossed South Mountain, Pleasonton recognized the strength of the position and the difficulty any Federal force would have seizing it. Thus, Pleasonton also authorized reconnaissance efforts while in the area, from which he learned that two roads, one north of and one south of Turner's Gap, reentered the National Road on the west side of South Mountain behind the gap. Pleasonton believed these roads "would assist us materially in turning the enemy's position on both flanks."
Once the sun rose on September 14, Pleasonton renewed his intelligence-gathering efforts at the eastern base of South Mountain. Pleasonton placed two artillery batteries on the high ground west of Bolivar. They immediately commenced shelling Turner's Gap and the Confederate artillery posted there. The response this drew from the enemy guns in the gap allowed Pleasonton to gain an understanding of enemy dispositions.
Shortly after the artillery exchange began, the van of Burnside's infantry support reached Pleasonton, who rode back towards Catoctin Creek looking for it. He found Eliakim Scammon's Ohioans in the lead about one mile west of the stream. Pleasonton, knowing the ground "fully," according to Burnside, ordered Scammon to take his men south of the National Road and use the Old Sharpsburg Road to ascertain the enemy strength south of Turner's Gap. This movement precipitated the action at Fox's Gap, which grew into the left hook of the Federal effort to turn the enemy's position at Turner's Gap.
By the afternoon of September 14, Joseph Hooker's First Corps arrived in the vicinity of Bolivar. Ambrose Burnside, then imposing tactical oversight of the fight for Turner's Gap, sent Hooker's men north of the National Road to outflank the Confederate left while Jesse Reno's Ninth Corps pressed against the Confederate right at Fox's Gap. Burnside's knowledge of the road network and terrain north of Turner's Gap may have been learned from Pleasonton.
|This early map of the Battle of South Mountain shows the road networks and terrain scouted by Pleasonton's cavalry before the battle began and the subsequent Union effort, begun by Pleasonton, to turn rather than directly assault Turner's Gap.|
The information that Alfred Pleasonton gathered on the evening of September 13 and the morning of September 14 materially aided the Federal effort in the struggle for Turner's Gap. Pleasonton's accurate knowledge of the local terrain and roads spared the Army of the Potomac from making headlong assaults into Turner's Gap. Instead, only one brigade, John Gibbon's Black Hat Brigade, directly assaulted the gap on September 14. The Ninth Corps fought to turn the enemy's right while Hooker's First Corps endeavored to do the same to the enemy's left. Pleasonton began this effort on the morning of the battle by dispatching Scammon's brigade to Fox's Gap. The trend of turning rather than storming the enemy's position continued during the rest of the Battle of South Mountain.