@Setanta,
What they called "Dumps" the middens of older civilizations, were more Aerobic and would actually "compost" .Putrescible crap would be oxidized and would be mostly gone except for denser materials, ceramics, and metals . Most of the Roman and Med dumps were like this. Dumps of the sub saharan and Arabian peninsula would dessicate the stuff and lots would be preserved. Pollen is a substance that is retained after composting (even natural heating) and is a good indicator of climate and can be worked together with stuff like remnant magnetics to calc ages
LANDFILLS, our more modern "containment vessels " were designed originally with a beliefe that the crap would be gone in a few decades and that just isnt the case. LAndfills, covered and sealed like crypts and not allowing any liquids to enter, can be taken apart and read like a book.
I was in a case where one of our "clients" mine dumps had been backfilled with trash from a western city. The city sued my client for operating an illegal landfill in an area that predated when the city began using the site. (The city took over the landfill site against my clients judgement, and they did it by condemnation and eminent domain) WE excavated that area (in a plan to rebury it in a designed "Cell") and found that the dates of the newspapers therein were of the date and later that included the citiy's own operation period. The city admitted to taking over the site in 1973 and we found plenty of 1973 newspapers in the contetsed area(It was kinda hard to bury **** in the future-even though the city's crack attorneys got themselves tied up in that kind of argument).
The city lost cause we went to court and they freaked and settled in order to preserve them having to pay even bigger reparations .
Then the city helped us to recoup all of our insurance money to pay us back for what we already spent. They were really good about helping us post facto.
A landfill is like a time capsule and the stuff therein has lots of stories it can tell, including whthere the landfill was accepting haz materials or had caused contamination.