It is probably a Confederate bayonet since, though 1853 Enfield rifles were used by units on both sides during the American Civil War, the Enfield was much more widely used by Southern forces and represented the primary Confederate long arm.
From its position, it had to have been dropped by a soldier positioned behind the wall looking east, which means that, most likely, the bayonet was dropped by a Southerner defending the pass as the Union Sixth Corps under Horatio Wright, July 16-17, 1864, pursued Jubal Early‘s Army of the Valley District in its retreat through the pass following its victory at Monocacy on July 9th and unsuccessful probe of the defenses of Washington on July 11-12.
Its owner probably drew the bayonet, and not wanting to make his 55 inch (1.397 m.) long rifle even longer and more unwieldy in a brushy wooded location until necessary, placed it ready for rapid use on the wall by his firing position. But Northern infantry or Duffie‘s cavalry advanced faster and in greater numbers than he had anticipated, and the Confederate was forced to make a run for it so quickly that he did not have time to bother trying to pick up his bayonet.
His pursuers clambered over the wall, knocking the abandoned bayonet to the ground and dislodging several of the upper stones which fell down and covered it. Those fallen rocks protected it from the elements and significantly reduced the amount of oxidation that might have been expected over the 140 year interval before it was recovered.