How can fossils be used to determine the relative ages of rock layers?
The deeper a fossil is found, the older the rock layer it is found in.
deposition has been occurring on earth for a very long time. as time goes by, more deposits form layers on the earth, including the remains of animals and plants that would later become fossils, burying the previous deposits in another layer of newer deposits. therefore it follows that layers which are found deeper are older than those found in layers near the surface