12th Pennsylvania Cavalry

Organized between December 1861 and April 1862, at Philadelphia. "It was past the middle of July [1862] before the command was mounted, and little progress had been made in training and discipline, before active operations commenced."
(from Bates)

Major Titus, in command on the Manassas Campaign, was captured by the enemy on 26 August 1862, and Major Congdon assumed command.

At South Mountain on 14 September, the regiment was assigned to Sumner's 2nd Corps, and was in reserve. On the 17th at Sharpsburg the Regiment was on independent duty to aid the Provost Marshall in handling Confederate prisoners and Federal stragglers in the rear of the I and 12th Corps on the right of the Federal position. On the 18th they were on reconaissance to Harpers Ferry.

Tablet #122: Army of the Potomac - 17 Sep, 10 AM to 17 Sep, 6 PM
Tablet #21: Cavalry Division - 17 Sep, 6 AM to 17 Sep, 8 PM
Tablet #121: Army of the Potomac - 17 Sep, 8 AM to 17 Sep, 6 PM

See a very fine site about the Regiment by James Stump, including a page with great detail on their role on the Maryland Campaign.

Source: Bates, Samuel P., History of the Pennsylvania Volunteers, 1861-65, Harrisburg: 1868-1871, extracted online at Pennsylvania in the Civil War by Alice J. Gayley.