What long, steep, narrow mountain range provides a controversial border that splits Maryland into two parts?
Western Maryland:  However, the people of the more mountainous and isolated Allegany County and Garrett County consider Sideling Hill the boundary between Western Maryland and what they refer to as "down-state." Sideling Hill: Sideling Hill (also Side Long Hill) is a long, steep, narrow mountain ridge in the Ridge-and-Valley (or Allegheny Mountains) physiographic province of the Appalachian Mountains, located in Washington County in western Maryland and adjacent West Virginia and Pennsylvania, USA.
Sideling Hill