How are geological epoch and eras defined?

Answer 1

Geological epochs and era basically denote a certain measurement of time. An epoch is longer than an era. A period of time is defined by 'epoch' if certain events have taken place that are serially significant developments. An era, conversely, is a smaller unit of time that is usually more specific to a beginning, such as the Birth of Christ (4 BCE). An example of an epoch is the Holocene, which is marked by human civilzation. Holocene epoch, alongside the Pleistocene (first humans), Philocene (first homonids), Miocene (more mammals, including horses, dogs, and bears), Oligocene (minor extinction; the emergence of new mammals, such as pigs, deer, and cats), Eocene (mammals abound, rodents appear), and Paleocene (first mammals and primitive primates appear) eepochs, are contained within the Cenozoic Era, otherwise known "The Age of Mammals".