​How do major geologic events help build Earth's time line?

​sometimes major events in Earth's history can leave a distinctive layer in the rock record.

​Scientists hypothesize that a meteorite impact might have caused the mass extinction that occurred when

​ evidence for this impact is in a clay layer containing the element iridium in rocks around the world.

​Iridium is rare in Earth rocks but common in meteorites.

​key bed

​A rock or sediment layer used as a marker and help geologists correlate rock formations in different geographic areas.

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