Probably stupid. But perhaps some of you archaeology types can answer it for me.

In my incessant reading on Rome, I’ve become particularly fascinated with what’s underground, as you can probably tell. I was reading last night in the quite wonderful A Traveller in Rome in which Morton is describing his experience of what we today would call the Scavi tour – not then open to the general public. As he recounts what he saw – the street lined with tombs, and so on, thirty feet under the present floor of St. Peter’s, the question that pops up every time I read things like this re-emerged.

I just don’t understand these layers of building – if it were just foundations that were excavated, upon which subsequent layers were built, that would make sense, but this existence of entire structures underground puzzles me. Even though we are talking about hundreds of years of construction, I still don’t understand how this happens. Were areas – streets and neighborhoods =  purposefully filled in and then new levels constructed on top? I just can’t picture how this could gradually, naturally happen.

Enlighten me!