Fortifications

Fort Marcy is approximately 1/2 mile south of the Potomac River on the south side of the Chain Bridge Road leading from Chain Bridge to Langley and McLean, Virginia.

The hill on which the fort is located was known as Prospect Hill. Originally the fort was called Fort Baldy Smith, after General William Farrar Smith, the troops of whose division began construction of the work. His division crossed Chain Bridge on the night of September 24, 1861, and immediately commenced construction of Fort Marcy and Fort Ethan Allen. The 79th New York Highlanders, the 141st Pennsylvania and the Iron Brigade also helped complete the work in the fall of 1862. A force of about 500 contrabands were also employed and the 152nd New York worked on the entrenchments, which are still in a very good state of preservation. 

The fort was not completed until the fall of 1862. It was named in honor of a native of Massachusetts, Randolph B. Marcy, a distinguished soldier, father-in-law, and chief of staff to General George B. McClellan. Detachments of the 4th New York and 3rd Pennsylvania Heavy Artillery and the 130th Pennsylvania Infantry were among the troops that performed garrison duty here during the war. An interesting incident of history is that the body of troops which afterwards became famous as the "Iron Brigade" was originated at this fort in the summer of 1861, and was composed of the 19th Indiana, the 2nd, 6th and 7th Wisconsin Regiments. The 24th Michigan was added to the brigade soon after the Battle of South Mountain. These units remained together until the close of the war.  ( Source - Wikipedia )