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 Appalachian National Scenic Trail follows the crest of the mountain through Maryland and a portion of Pennsylvania.