Clarks KnobClarks Knob is a summit in Franklin [County, Pennsylvania]. It is the highest point on Blue Mountain, the eastern front range of Pennsylvania's Ridge and Valley Appalachians region.GeographyClarks Knob's northeast-southwest running summit ridge forms the boundary of the Susquehanna River and Potomac River drainages. It stands over above the town of Chambersburg and the Great [Appalachian Valley]. This mountain is protected within the Buchanan State Forest.