The name has nothing to do with the Battle of Gettysburg, but everything to do with a very large black snake that lived in the rocks.
Union troops had cover this side of the low stone wall as three divisions of Rebels advanced across nearly a mile of open farmland in Pickett's Charge on July 3, 1863. Lee's attempt to break the center of the federal line failed, and the Confederates who made it as far as this side of the wall were routed.
