South Mountain isn't a single peak, it's the name given to the part of the Blue Ridge Mountains that extends into Maryland. The AT follows the ridgeline on its way to the Mason-Dixon Line and Pennsylvania.
The AT in Maryland is a long walk into Civil War history. Confederate forces operated in and around the western side of South Mountain in September of 1862, leading up to the battle of Antietam (I took this picture at the Antietam battlefield - South Mountain is in the background), which still holds the infamous record of being the single bloodiest day in all of U.S. military history. The Rebs were back in Maryland again in the spring of 1863 before the battle of Gettysburg.
You can visit the Maryland state park site here: South Mountain State Park