Does anyone know how to cite a historic marker or plaque?

You should not have to site it formally. If you are quoting something it says directly, just put the quote in quotation marks and state where the quote came from. Examples: No author marker: The such and such plaque says, "Something worthy of putting in your paper goes here." Marker or plaque with an author: On the historical marker at such and such place, John Doe states, "Something that your reader needs to know goes here." Marker or plaque that quotes a famous figure: Thomas Jefferson is quoted as saying, "Something poignant or thought provoking goes here," on the such and such plaque. If you are paraphrasing the same rule basically applies. For example: According to the such and such marker, something very interesting and relevant to your paper would go here. There is no need to do a formal APA or MLA citation with the quote or include it in your reference page.