Leaf litter accumulates and breaks down and becomes new soil, mountains and hills erode and that sediment accumulates in lower areas, wind move lots of sand and fine sediment around, All of that piles up in places until things once on top of the soil are now buried.

It's a little more complicated than that, but that's part of the process.

Another aspect is that cities used to literally rise as they deposited waste in situ and had to build up as 1st floors became basements. This is partially the source of mounds.

Neat reddit discussion: https://www.reddit.com/r/AskHistorians/comments/17je7i/arche...

It's not that complicated. Humans tend to build in flat areas. Flat areas tend to be flood plains. When flood plains flood they leave behind sediment. There are additional ways that sediment builds up such as airborne deposits and importantly from growing plants leaving behind biomatter, but in a lot of cases flooding explains the substantial depth of sediment deposited.

It’s one of those overlooked facets of Nature. Much of the Great Plains sits on the Ogallala Aquifer. This is an underlying terrain, to depths of hundreds of meters, filled in by sediment from the Rockies (https://en.m.wikipedia.org/wiki/Ogallala_Aquifer#General_cha...).

The greater part of the LA Basin is a floodplain which consists of hundreds of feet of sediment from the San Gabriel mountains.

Yeah, same here. And some ancient cities and settlements have _layers_ below them[1]. How?

[1]https://en.wikipedia.org/wiki/Troy#Historical_Troy_uncovered

Archaeologists have characterized various layers, the depth and thickness will vary.

So a layer there might just consist of evidence of some particular activity that one settlement engaged in that earlier and later ones did not.

There are lots of depositional (and erosive) processes happening. They're all subject to local conditions. But the most common (in no particular order) are: accumulation of plant matter, flood deposits, human-generated organic and building debris, and earthworm casts. You can also see other things, like volcanic activity and wind-blown material, in fewer places.