Periods are subdivided into units of geologic time called eras.

Scientists chose where units of geologic time began and ended based on major changes in ___________________ at certain times.

The long span of time that begins geologic time is called _____________.

Geologist divided the time between Precambrian Time and the present into three long units of time called _____________.

The Triassic Period, the Jurassic Period, and the Cretaceous Period occurred in the ___________________ Era.

____________________ for many of the geologic periods come from places around the world where geologists first described the rocks and fossils of that period.