Some historians think for a few hundred years Hadrian's Wall was a megastructure where the locals had no solid understanding of who built it, how they managed it, or why.

http://www.bbc.co.uk/programmes/b01kkr42

It's admittedly debated, but even if just plausible, still fascinating to contemplate.

If not for the wall, there were probably periods in the local history around the pyramids that would qualify. Julius Caesar is closer to our time than he was to the time when they were built. There's a lot of time for people to forget other civilizations in there.