The marker for the First Army Corps is on the north side of Mansfield Avenue, north of the North Woods (Mansfield Avenue tour map)

From the marker:

U.S.A.

First Army Corps

Maj. Gen. Joseph Hooker, Commanding.

September 16, 1862.

The First Army Corps moved from its bivouac near Keedysville at 2 P.M. on September 16. Doubleday’s Division crossed the Antietam at Pry’s Ford; Ricketts’ and Meade’s Divisions at the Upper Bridge.

The Divisions advanced in parallel columns, in a westerly direction, until 6 P.M., when Meade’s Division encountered the enemy near the East Woods.

The Corps was then formed in line of battle, facing south and southwest, Meade’s Division in the center, somewhat advanced, Doubleday’s Division on the right and Ricketts’ Division on the left in supporting distance.

The fighting ceased on account of darkness at about 6:30 P.M.

No. 1.