3.1.16 (HL)—Geological epochs

Eons contain eras, eras contain periods, and periods contain epochs; boundaries mark major geological or biological change.

Use the hierarchy to locate scale, then inspect fossils and rocks for extinction, appearance or environmental disruption. It organizes evidence rather than replacing it.

A layer showing abrupt loss of many fossil groups may mark a boundary associated with major environmental change.

A dated, correlatable rock change plus a coherent fossil turnover, not memorized names alone.

The units are nested scales, not interchangeable labels or equal-length intervals.