The monument at Antietam to the 21st Massachusetts Volunteer Infantry Regiment is on the east side of Antietam Creek at Burnside Bridge. (Burnside’s Bridge tour map; 39.450782° N, 77.731583° W; map location) It was dedicated on September 17, 1898.

The 21st Massachusetts was commanded at the battle of Antietam by Colonel William S. Clark, a professor of chemistry at Amherst College.

Monument to the 39th Massachusetts Volunteer Infantry Regiment on the Antietam battlefield

For many years the monument, along with those of the 51st Pennsylvania and the 35th Massachusetts, was mounted on the end parapets of the Burnside Bridge. In the early 1960s auto traffic was moved to a new bridge upstream, the Burnside Bridge was restored, and the monuments were moved to their current location on the east bank.

From the monument:

21st Mass. Vol. Inf’y
2nd Brig. 2nd Div.
9th A.C.

The following Comrades were killed near this bridge:

2nd Lieut. Henry C. Holbrook
Co. D  Priv. Wm. B. Boynton
  ”     ”     “ Amos S. Eastman
Co. E  Corp. Henry K. Buss
  ”    ”   Priv. Geo. T. Bigelow
Co. G  Corp. Jas. S. Stratton
  ”    ”   Priv. Daniel Daley
  ”     I  Priv. Chas. Leonard
Co. K 1st Sergt. Geo. W. Davis
  ”    ”   Priv. Chas. S. Brigham

and 35 other wounded

Erected by
21st Regt. Mass. Vol.

See more about the history of the 21st Massachusetts Volunteer Infantry Regiment in the Civil War