Richard F. answered 09/14/20
Advanced intellectual mentoring from a novelist and philosopher.
Dabian's right. Here's another way to think about it. Antietam is a historic (important) battle, so the county puts up a historical (about history) marker. Suppose that's the county's first-ever historical marker - so someone comes along decades later and says "Wow. We should put up a marker about this marker - it's historic." Everything in history is historical; the notably important stuff is historic.