The division of geologic time smaller than an era is a period.

Here's the hierarchy:

* Eon: The largest division of geologic time.

* Era: A subdivision of an eon.

* Period: A subdivision of an era.

* Epoch: A subdivision of a period.

* Age: A subdivision of an epoch (though not always used).

Let me know if you'd like to know more about specific eras, periods, or epochs!