What is one of the main criteria for defining geological time periods?

The presence of specific fossil types is a fundamental criterion for defining geological time periods, as it allows geologists to correlate rock layers across different locations and determine their age. Fossils serve as indicators of the biological and environmental conditions that existed during particular intervals in Earth's history. By identifying distinctive fossils, known as index fossils, scientists can establish timelines for different eras, periods, and epochs, which serve as a framework for the geological time scale. This method relies on the fact that certain organisms thrived during specific time frames, making them reliable markers for dating rock strata and understanding the evolution of life.

The other criteria listed, while relevant in certain contexts, do not serve as primary indicators for defining geological time periods. The thickness of rock layers can vary widely and does not consistently reflect the time periods represented. The location where fossils are found can provide additional context but is not itself a defining factor. Types of plants may indicate environmental changes but are not as universally applicable as fossil types in establishing geological timelines.