What stretches about 1200 miles and borders the Blue Ridge Mountains?
Blue Ridge Mountains:  To the west of the Blue Ridge, between it and the bulk of the Appalachians, lies the Great Appalachian Valley, bordered on the west by the Ridge and Valley province of the Appalachian range. Great Appalachian Valley:  The trough stretches about 1200 mi from Quebec to Alabama and has been an important north-south route of travel since prehistoric times.
Great Appalachian Valley