It has always seemed to me that it’s careless, to mislay a Roman road or a villa or a city. Look at this picture — there’s a big pavement under there, and they completely mislaid it for thousands of years. These are large items! How can they become lost to all human knowledge?
But if you stay here for a while you can see it happening. Stone pavements do indeed last forever, barring earthquake or bulldozers. But have a look at this one. This street is in Sarlat-le-Canede, and although it looks old it was laid in the 1960s or 70s as part of the restoration of the town. You can see the square 20th century portals for conduit and sewage lines.
And the black dots? Those are moss. We had a heavy rainstorm with hail, and the rounds of thick puffy moss which have taken root on the tile roofs of the buildings to either side were scoured off and fell onto the pavement. There were quite a lot of them, squashy and wet, and eventually the municipal street sweepers cleared them away.
A nice thick layer of moss would obscure all the stones. If feet and wheels didn’t wear it away — if the town’s population plummeted, for instance — then the stone surface would be hidden by growth. A couple of millennia and you’d have a nice layer of soil and grass, maybe trees over your stonework. And then it’s a dirt road, or maybe nothing at all.
And similarly, the roofs, the walls, everything we build. In a region with healthy rainfall and vigorous vegetation, the government spends all its time trimming the bushes, clearing the ditches, rebuilding the roads, and sandblasting growth off of roofs. Everything looks nice if it’s kept nice — if it’s important to the residents, if there are any residents. If not, it all goes to seed.
Civilization is something that has to be maintained, continually kept up. It slips away fast.