In fact, I didn't really think of my novel as science fiction. I have in my library a lot of what I call speculative archaeology, dealing with phenomena that cannot be readily explained by current theories. It was believed that the Indians came to America in 10,000 B.C., passing over the land bridge, but then objects were found that carbon dated at 40,000. Unexplained discoveries have been made in this country, such as a Roman camp in Virginia. Such discoveries ruffle the feathers of archaeologists who would rather ignore or dismiss them than change their familiar theories.
Civilizations can disappear with barely a trace.
Out West, on the Gallina River, just before World War II, an authority on early American history decided to check out an Indian tradition about some towers in the area. He assumed they were cliff dwellings, but in a remote valley, now totally dry, he found signs of intensive agriculture and irrigation. Along the cliffs on both sides were four-story towers of stone, not cliff dwellings at all. They had no windows, and could only be entered through the roof, presumably with the aid of ladders. When he examined one of the towers, he found it had been burned out.