South Mountain is the northern extension of the Blue Ridge Mountain range into 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 range separates the Hagerstown and Cumberland valleys from the Piedmont regions of th…South Mountain is the northern extension of the Blue Ridge Mountain range into 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 range separates the Hagerstown and Cumberland valleys from the Piedmont regions of the two states.