The major divisions of geological time are categorized into eons, eras, periods, and epochs. These divisions help scientists understand and study the Earth's history, including the evolution of life forms and geological events. The syllabus of UPSC covers the following major divisions:

1. Eons:
- The Precambrian eon: This is the longest eon, spanning from the formation of Earth around 4.6 billion years ago to the beginning of the Phanerozoic eon. It is further divided into three eras: Hadean, Archean, and Proterozoic.

2. Eras:
- The Paleozoic era: This era signifies the rise of complex life forms and is divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

- The Mesozoic era: Also known as the "Age of Reptiles," this era witnessed the dominance of dinosaurs and the emergence of birds and flowering plants. It includes three periods: Triassic, Jurassic, and Cretaceous.

- The Cenozoic era: This era is often referred to as the "Age of Mammals" and is further divided into two periods: Tertiary and Quaternary. The Tertiary period is divided into five epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene. The Quaternary period includes the Pleistocene and the Holocene epochs.

3. Periods:
- The Cambrian period: This period marks the appearance of a diverse range of complex organisms, including the first appearance of hard-shelled organisms.

- The Jurassic period: This period is known for the dominance of dinosaurs, including the giant sauropods and theropods like Tyrannosaurus rex.

- The Carboniferous period: This period witnessed the formation of extensive coal deposits due to the abundance of plant life, including giant ferns and early trees.

- The Pleistocene period: This period is characterized by repeated glaciations, with vast ice sheets covering large parts of the Earth's surface.

4. Epochs:
- The Holocene epoch: This epoch represents the current geological time, starting around 11,700 years ago after the last major ice age. It is characterized by the development of human civilizations and significant climate fluctuations.

Understanding these divisions of geological time helps scientists study the Earth's past and predict future geological and biological changes. It provides valuable insights into the evolution of life and the Earth's dynamic history.