Franklin's VI Corps attacked a small, hastily assembled Confederate force at Crampton's Gap in South Mountain that sought to protect the rear of Confederate Maj. Gen. Lafayette McLaws, who was across Pleasant Valley on Maryland Heights taking part in the siege of Harpers Ferry. Despite inferior numbers, the Confederate force held out throughout the day, taking heavy casualties. By the evening the VI Corps broke the Confederate line and proceeded through the gap into Pleasant Valley. Franklin, however, failed to follow up on his success and did not attack McLaws on Maryland Heights.