they are useful by showing where the fossil is buried then on that certain layer, it will be studied to know the age.
relative age
Index fossils
mantle
In order to be useful in determining the relative ages of rocks, an indicator must provide different indications for rocks of different ages, right? So how could something which doesn't change over time be useful?
the outer most layer of earth is CRUST
The Asthenosphere.
The crust because that is the outer layer and we live on the crust so... there are rocks on the layer we live on! The answer is Crust.
crust or lithosphere
LITHOSPHERE.
crust and mantle
rocks
rocks