The Reserve Artillery of the Army of Northern Virginia marker is on the south side of Boonsboro Pike next to the entrance to the National Cemetery. (National cemetery tour map)
From the marker:
C.S.A.
Reserve Artillery
Army of Northern Virginia
Brig. Gen. William N. Pendleton, Commanding.
September 16-18, 1862.
A portion of the reserve Artillery took part in the Battle of September 16 – 17; the rest was posted at the fords in the vicinity of Williamsport and Shepherdstown to command the crossings of the Potomac River.
On the 16th, the Artillery was posted on the high ground on either side of the Boonsboro Pike, east of Sharpsburg, from which it engaged the long-range Artillery of the Army of the Potomac which occupied the high ground on the east side of the Antietam.
No. 355.