How did geologists decide where one division of the geologic time scale ends and the next begins

Heyi am new to are you all americans? ( just want to know , i don't mean any offence ) ?

The states that the process that have been occurring today have been occurring since earth formed. what is it?

Which of the following situations is an example of an internal migration? a. moving to mexico city, mexico b. moving to san diego, califormia c. moving to toronto, canada d. moving to london, england