What determines the divisions of eras and periods on the geologic time scale?

Each era is subdivided into a number of periods. The periods of the Cenozoic, the present era, are further divided into epochs. Many divisions of the geologic time scale are based on events in Earth's geologic history. Some divisions are based entirely on the fossil record.

Answer after wwii, communist countries in eastern europe formed the warsaw pack, a military alliance, and comecon, an economic alliance;