Question

what did geologist use to mark where one unit of geologic time ends and next begins
GPT 3.5
Geologists use various markers, including changes in fossil assemblages, geological events such as volcanic eruptions or asteroid impacts, sedimentary layers, and isotopic dating techniques, to mark where one unit of geologic time ends and the next begins. These markers are known as "boundary stratotypes" or "golden spikes" and help to define and distinguish different periods and epochs in the geologic time scale.