write a scientific explanation about how index fossils aid geologists in determining the age of rock layers

Index fossils are really useful for geologist because they help them match rock layers.
Index fossils aid geologists because when they are looking into rock layers they see if the index fossils match to a rock layer and if they do it means they the rock layer was created at the period of time the index fossil was.

Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed for a brief time period. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.