Absolute dating techniques were not available to him (radiocarbon dating wasn't invented until the 1940s); and since they were separately excavated graves, stratigraphy was no use either.
Petrie knew that styles of pottery seemed to come and go over time--in his case, he noted that some ceramic urns from the graves had handles and others had just stylized ridges in the same location on similarly shaped urns.
Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.
The majority of the time fossils are dated using relative dating techniques.
He assumed that the change in styles was an evolutionary one, and, if you could quantify that change, he surmised it might be used to indicate which cemeteries were older than others.
Petrie's notions about Egyptology, and archaeology in general, were revolutionary.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
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We'll explore both relative and numerical dating on our quest to understand the process of geological dating.
The seriation method works because object styles change over time; they always have and always will.
A good example of a change in artifact type is the development of hand-held PDAs from those first enormous cell phones. As an example of how change through time works, consider the different music recording methods that were used in the 20th century.