Question

What appears to be one criterion that scientist use when defining the timing of geological periods?

Answer 1

Geologists and other scientists use the geologic time scale to map the timing and relationships between events which have occurred during the history of the Earth. It combines the estimates of the age of geological formations obtained by radiometric dating techniques and the direct evidence of sequences and events in the rock record collected by the geologists. 

Different spans of time on the time scale are delimited by major geologic or paleontologic events, like mass extinctions. For instance, the boundary between the Cretaceous period and the Paleogene period is marked by the extinction event which marked the demise of the dinosaurs and many marine species.