Some distant view of "the Blue Mountain" dominates the southern tier of most eastern and central Pennsylvanian counties, providing an ever-visible backdrop cutting across the northern or western horizon. Most transport corridors and road beds piercing the barrier necessarily pass through either large water gaps (west to east: the Susquehanna, Schuylkill, Lehigh and Delaware River valleys) or wind gaps, low gaps in the ridge caused by ancient watercourses. The barrier ridge forms a distinct boundary between a number of Pennsylvania's geographical and cultural regions.