South Mountain is the northern extension of the Blue Ridge Mountain range in Maryland and Pennsylvania. From the Potomac River near Knoxville, Maryland in the south to Dillsburg, Pennsylvania in York County, Pennsylvania in the north, the 70-mile-long (110 km) range separates the Hagerstown and Cumberland valleys from the Piedmont regions of the two states.

The Blue Ridge Mountains are a physiographic province of the larger Appalachian Highlands range. The mountain range is located in the Eastern United States and extends 550 miles southwest from southern Pennsylvania through Maryland, West Virginia, Virginia, North Carolina, South Carolina, Tennessee, and Georgia. The province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. To the west of the Blue Ridge, between it and the bulk of the Appalachians, lies the Great Appalachian Valley, bordered on the west by the Ridge and Valley province of the Appalachian range.