South Mountain is the northern extension of the Blue Ridge Mountains in Maryland and Pennsylvania, stretching 70 miles from Knoxville, Maryland on the Potomac River to Dillsburg, Pennsylvania. The Province of Maryland's border with the Susquehannock Indians was demarcated along South Mountain during the 1730s, but Maryland expanded beyond the mountain starting in 1744. Following John Brown's 1859 raid on Harpers Ferry, seven raiders escaped to Pennsylvania via South Mountain. Crampton's Gap, Fox's Gap, and Turner's Gap were the sites of the 1862 Battle of South Mountain amid the Maryland campaign, while the 1863 fight at Monterey Pass occurred during Robert E. Lee's retreat from the Battle of Gettysburg.