Worms and dirt

[Mark]

Maybe this is stupid but...

worms are supposed to be the reason archaelogists always have to dig down, but I don't get where all the dirt comes from. Why are ancient settlements always underground, but old ruins that are still standing are still at the surface.

What's going on?

[NaturalMystik]

I've always wondered about this myself. Fungus also does a great job of making dirt, but even after so much time I think there has got to be a bigger factor.  I'm thinking large scale geologic disruptions. Depending on location, massive flooding, landslides, earthquakes, volcanic eruption.