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Each region has its own unique master sequence since weather patterns are not the same from one area to another.
However, the longer bristlecone pine sequence is of little value except for cross-checking the reliability of other dating techniques because logs of this species are rarely found in association with ancient humans.
give the time of an event with reference to another event that is not worldwide in scale.
One way to find the age of a xenolith or subsidence area surrounded by volcanic debris is to correlate its layers with the layers of wall or parent rocks.
Stratigraphy is the study of sedimentary rock layers.
They do not tell us when an event happened in years before the present. This usually means that there is a 2/3 probability that the actual date of the event was within the range of 300-380 B. The 2/3 probability is what statisticians refer to as one standard deviation.
For example, in comparing the cross-section of the series of geological strata illustrated below, level 4 is older than 5 because it is below it. At a higher probability, the plus or minus factor will be larger.
Layers of volcanic ash are igneous deposits, while layers of rock these deposits surround are usually sedimentary. Igneous intrusions form when magma breaks through a layer of rock from beneath, or lava flows down from above. When igneous intrusion causes newer sedimentary layers to sink into older ones, it's called subsidence.