Answer: The correct answer for the fill in the blank is Epochs.
Geological time scale is a system of chronological dating that represents different events that occurred from the beginning of earth till present.
The primary division of time is Eons that are divided into different eras, which can be further divided into periods, epochs, and age.
Periods are divided into epochs. For instance, Quaternary period- 1.8 mya to present ( second period of cenozoic era) is divided into two epochs that are Holocene ( 8000 years ago to present) and Pleistocene ( 1.8 mya to 8000 years ago).