The marker for the Horse Batteries and Reserve Artillery of the Army of the Potomac is on the north side of Boonsboro Pike east of Rodman Avenue. (Boonsboro Pike tour map)

From the marker:

U.S.A.

Horse Batteries and Reserve Artillery

Army of the Potomac

(September 17, 1862.)

Horse Batteries A, B and L (consolidated) and M, 2d U.S. Artillery, C and G (consolidated) of the 3d U.S. Artillery were established on the crown of the ridge to the right and left of this point on the morning of September 17. Shortly after noon, their ammunition having been exhausted, they were relieved by Batteries E and G (consolidated), 1st U.S. Artillery, and Battery K, 5th U.S. Artillery, and recrossed the Antietam for ammunition. Batteries C and G (consolidated), 3d U.S. Artillery, took position east of the Antietam on the ridge north of the Boonsboro Pike; the three other Batteries returned, at intervals, between 3 and 5 P.M., and resumed their positions, remaining in battery until the close of the day.

No. 30.