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.
"Historical" and "historic" can both relate to something important in history. For example, one might find a historical marker posted at a major battle site. It would be easier to understand the difference if we explained that "historic" refers to something that "makes history" while "historical" refers to something that is "from history" or something that reminds us of a point in history.