Epochs are significant for many different reasons and apply to any number of various subjects which would have a need to be broken down into time frames. Epochs can be any number or length of years or even ages, but most of the time epochs connote a very long time period.
Not as commonly known but also found are religious epochs. If something is of religious epochal importance then it is something that has happened within an historical segment of time within a religion's history or within the socio-cultural era in which the religion experienced the event. It can be called epochal when it is something which is so monumental that lasting changes occur as a result.
Evolutionary epochs are also significant for the long lasting effects of the changes that occur during each epoch. Epochally significant events can refer to geology, animals, plants, human beings both physically and socially, philosophically, scientifically, mathematically, industrially, religiously and more. Events which are of epochal importance generally have not only notably immediate as well as long lasting effects but also far reaching implications in time.