The marker for the First Brigade of the Cavalry Division of the Army of the Potomac is on the north side of Boonsboro Pike east of Rodman Avenue. (Boonesboro Pike tour map)

From the marker:

U.S.A.

First Brigade, Cavalry Division

Major Charles J. Whiting, 2d U.S. Cavalry, Commanding.

Organization.
5th and 6th United States Cavalry

(September 17, 1862.)

The Fifth United States Cavalry, Captain Joseph H. McArthur, commanding, crossed the Middle Bridge over the Antietam shortly before noon and took position on the right of the road, its left resting at this point, in support to the Artillery in its front. It was withdrawn across the creek late in the day.

The Sixth United States Cavalry, Captain William P. Sanders, commanding, was detached and deployed in skirmishing order, as a Provost Guard, in rear of the right wing of the Army of the Potomac.

No. 89.