I saw a large picture accompanied by a few notes, painted on the side of a U-Haul van parked on a street near us while we were walking the dog. The subject was the Serpent Mound, which it said was in Adams county. Adams being a neighboring county to the one I live in I was surprised that I had never heard of it before so I googled it when we got home and found that this particular Adams county is in Ohio, not Colorado. Further googling took me to A Hot Cup of Joe and this post, which is a good discussion of the various interpretations of this structure by our friend: http://ahotcupofjoe.net/2008/06/the-serpent-mound/
I don't really have anything to add - just found this another interesting tidbit about pre-Columbian America.