CARCI Middle School Pt .2 - Free Printable

Layers A and C each contain an index fossil.

Scientists determine the relative age of rocks by using index fossils—fossils of organisms that lived during a specific, short time period but were widespread geographically. When the same index fossil is found in rock layers at different locations, scientists can correlate those layers as being the same age. They also use the principle of superposition, which states that in undisturbed rock sequences, the oldest layers are at the bottom and each higher layer is younger. By matching index fossils across regions and observing the order of layers, scientists can construct a timeline of Earth’s history even without knowing the exact numerical ages. This method allows them to compare rock formations from distant areas and understand the sequence of geological events.