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.