The marker for Batteries C and G, 3rd U.S. Artillery is on the north side of the Boonsboro Pike, east of Rodman Avenue. (Boonsboro Pike tour map)

From the marker:

U.S.A.

Batteries C and G, 3rd U.S. Artillery 

Captain Horatio G. Gibson, U.S.A., Commanding.

(September 17, 1862.)

Horse Batteries C and G (consolidated), 3d U.S. Artillery, crossed the Antietam in the forenoon of September 17, and went into position a short distance south of this point, on the left of Battery M, 2d U.S. Artillery, and the right of Batteries B and L (consolidated), 2d U.S. Artillery. After an engagement of nearly two hours it was relieved by Battery E, 1st U.S. Artillery, and withdrawn east of the Antietam, taking up its second position on the ridge north of the Boonsboro Pike.

No. 95.

See more on the history of the 3rd United States Artillery, Batteries C & G in the Civil War