Are only sedimentary rocks used for relative age determination? Why?

When it comes to determining the relative age of rocks, only sedimentary rocks are used because they are the only ones that contain fossils. This is because the relative age dating method relies on the use of index fossils and the scientific principles of evolution, as proposed by Charles Darwin. The concept of evolution states that simpler organisms are older than those that are more complex. If a fossil of a complex organism is found in a layer of rock that is lower than a layer of rock containing an index fossil (which is believed to be older based on evolutionary principles), then the layer of rock containing the complex organism fossil is determined to be younger. Since only sedimentary rocks contain the necessary fossils, only they can be used for relative age determination.