Question

What information did scientists use when developing the geologic timescale?

Answer 1

Scientists studied the fossil record. They found four major changes in life forms. These four major changes were used to mark when one geologic era ended and the next began.

Answer 2

Scientists first developed the geologic time scale by studying rock layers and index fossils through out the whole world. With this information, scientists placed Earth's rock layers in order by relative age. Later, radioactive dating helped determine the absolute age of the divisions in the geologic time scale.