Tree-Ring Dating (dendroclimatology) is the method of scientific dating based on the analysis of tree-ring growth patterns.
For the entire period of a trees life, a year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. … Trees from the same region will tend to develop the same patterns of ring widths for a given period. … Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions, and for sub-regions of the world.
Fully anchored chronologies which extend back more than 10,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers). Another fully anchored chronology which extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California). Furthermore, the mutual consistency of these two independent dendrochronological sequences has been confirmed by comparing their radiocarbon and dendrochronological ages.
How it works:
First, find some Bristlecone Pine trees.
Bristlecone pine of the Great Basin region of western North America are the oldest known living trees, up to 5,000 years old.
Bristlecone pine (Pinus longaeva), White Mountains, California
Then analyze the tree rings in trees whose life spans overlapped:
Below is a page from ONE website on Dendrochronology. The point is to show how much research is applied to these issues that we know nothing about but which end up validating each others findings.
See Wikipedia's entry on Dendrochronology http://en.wikipedia.org/wiki/Dendrochronology for more on Tree Ring Dating.