and there is no saying that some unknown ancient civilization did not use glass and copper as structural materials, both of which would have long disappeared, or been re-purposed, especially if that civilization existed before the glaciation which covered the northern continents.
BTW have you ever seen the story on the Map of the Creator?
I know it’s just because of a strong silly streak, but that map reminds me of the one Gilligan’s Island episode where they find a stone map made by people they at first thought were trying to leave the island, not get there.