I am trying to identify the source book of a specific scene which I can't recall well. This was in an American science fiction book sometime probably at least 20 years ago. I remember that a not-very-technological male and female protagonist were exploring (flying somehow?) in a future, post-apocalyptic world, and in their travels they came upon a giant building, which contained a huge seated statue. I remember wondering if the building was meant to represent the Lincoln Memorial. They did not understand what the building was for, and wondered about it. I don't believe they were able to enter it. Also, the building seemed to be emitting smoke into the sky, which somehow spelled out cryptic messages one letter at a time, which could not be understood. They wondered at it a bit and moved on. The overall effect was a bit eerie and mysterious as I recall.
I remember the style of the story having been in the general vein of Philip Jose Farmer's "Dark is the Sun", or perhaps Bryan Aldiss' "Hothouse", but I have looked at both of those and haven't been able to identify the scene. Clifford Simak had some things in a similar style, but I didn't find it there either.