Historical clue formulations

• Natural dividing line on a U.S. map, as suggested by this puzzle's circled letters — 2025-06-11
• United States divider — 2011-03-18
• It runs through the middle of the 66-Across — 2010-08-05
• Volunteer State and Show Me State stream — 2007-10-14
• See 18-Across — 2000-05-24