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 descend through either large water gaps (West to east: Susquehanna, Schuylkill, Lehigh and Delaware River valleys) or wind gaps their smaller tributary gaps cut by creeks, brooks, and lesser rivers. The descent into the southern lowlands are ramplike and before southbound travelers oft lays a scenic panorama spread from horizon to horizon. The barrier ridge forms a distinct boundary between a number of Pennsylvania's geographical and cultural regions.