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Ok, somebody fill me in on how we can get tree ring data from 207BC.
Just a layman's view.. from my perspective..
Start with existing trees, and their tree ring structure.
Then start digging up old tree stumps, remnants of ancient footings of buildings, piers, docks, etc..
Cross reference same to build up a picture of corresponding tree ring structures... Each set of older rings overlapping with sets of younger rings..
Collate your library to show continuous ring growth patterns going back to earlier and earlier eras..
Index by regions..
A fairly accurate rendition of growth patterns of trees in a given geographical area, running back thousands of years..
Additional data can be gotten from fossilized wood, giving estimates in certain general age groups, going back millions of years..
Add to this core samples of sediments from local lake beds, and other geological strata, and one can build up a pretty good picture of what sort of weather there was, and even what kind of plants were growing in the area, as well as much of the insect life, bones and teeth of small animals, fish, mollusks, and occasionally, large animals, including man or his ancestors...
More than you ever wanted to know about tree ring dating of archeological / geological sites..